Date: 2025-10-07T13:56:21

User’s Guide for VirtualGL 3.1.4

Intended audience: System Administrators, Graphics Programmers, Researchers, and others with knowledge of Linux or Unix operating systems, OpenGL, GLX or EGL, and the X Window System.

- 1 Legal Information
- 2 Conventions Used in This Document
  - 2.1 Terminology
- 3 Overview
- 4 System Requirements
  - 4.1 Linux and Other Un*x Operating Systems
  - 4.2 Mac/x86
  - 4.3 Windows
  - 4.4 Additional Requirements for Stereographic Rendering
- 5 Obtaining and Installing VirtualGL
  - 5.1 Installing VirtualGL on Linux
  - 5.2 Installing the VirtualGL Client on OS X/macOS
  - 5.3 Installing the VirtualGL Client on Windows (Cygwin)
  - 5.4 Installing VirtualGL from Source
  - 5.5 Uninstalling VirtualGL
- 6 Configuring a Linux or Unix Machine as a VirtualGL Server
  - 6.1 Initial System Configuration
  - 6.2 GLX Back End: Granting Access to the 3D X Server
    - 6.2.1 Sanity Check
  - 6.3 EGL Back End: Granting Access to the GPU(s)
    - 6.3.1 Sanity Check
  - 6.4 Using VirtualGL with Multiple GPUs
  - 6.5 SSH Server Configuration
  - 6.6 Un-Configuring the Server
- 7 Configuring a Windows Machine as a VGL Transport Client
- 8 Using VirtualGL with the VGL Transport
  - 8.1 VGL Transport with X11 Forwarding
    - 8.1.1 Procedure
  - 8.2 VGL Transport with X11 Forwarding and SSH Tunneling
  - 8.3 The VirtualGL Client Application: Nuts and Bolts
    - 8.3.1 The VirtualGL Client and Firewalls
- 9 Using VirtualGL with X Proxies Such as VNC
  - 9.1 Using VirtualGL with an X Proxy on the Same Server
  - 9.2 Using VirtualGL with an X Proxy on a Different Machine
- 10 Support for the X Video Extension
  - 10.1 The VGL Transport with YUV Encoding
  - 10.2 The XV Transport
- 11 Transport Plugins
- 12 Using VirtualGL with setuid/setgid Executables
- 13 Using VirtualGL with VirtualBox
- 14 Other Application Recipes
- 15 Advanced OpenGL Features
  - 15.1 Stereographic Rendering
    - 15.1.1 Quad-Buffered Stereo
    - 15.1.2 Anaglyphic Stereo
    - 15.1.3 Passive Stereo
    - 15.1.4 Selecting a Stereo Mode
  - 15.2 Troubleshooting
- 16 Performance Measurement
  - 16.1 VirtualGL’s Built-In Profiling System
  - 16.2 Frame Spoiling
  - 16.3 VirtualGL Diagnostic Tools
- 17 The VirtualGL Configuration Dialog
- 18 Advanced Configuration
  - 18.1 Faker Settings
  - 18.2 Client Settings

# 1 Legal Information

This document and all associated illustrations are licensed under the Creative Commons Attribution 2.5 License. Any works that contain material derived from this document must cite The VirtualGL Project as the source of the material and list the current URL for the VirtualGL web site.

The official VirtualGL binaries contain libjpeg-turbo, which is based in part on the work of the Independent JPEG Group.

The VirtualGL server components include software developed by the FLTK Project and distributed under the terms of the FLTK License.

VirtualGL is licensed under the wxWindows Library License, v3.1, a derivative of the GNU Lesser General Public License (LGPL), v2.1.

# 2 Conventions Used in This Document

This document assumes that VirtualGL will be installed in the default directory (/opt/VirtualGL). If your installation of VirtualGL resides in a different directory, then adjust the instructions accordingly.

## 2.1 Terminology

3D application
  A computer program that uses a 3D rendering API (OpenGL, for the purposes of this document) to generate 2D images/animation frames (“rendered frames”) from 3D data in real time
VirtualGL server/application server (sometimes just “server”)
  The remote machine (“remote” from the point of view of the user) on which 3D applications will be run with VirtualGL
3D graphics accelerator/graphics processing unit (GPU)
  A specialized hardware device that processes 3D rendering commands and data from a 3D application into 2D images much more quickly than a general-purpose CPU could. In order to effectively use VirtualGL, a GPU must be present in the VirtualGL server.
3D X server
  An X Window System server (“X server”) attached to one or more GPUs in the VirtualGL server. The 3D X server can be “headless” (not attached to a monitor.)
X proxy
  A virtual X server that runs on a remote machine (usually, but not always, on the VirtualGL server.) Because X proxies render X Window System (“X11”) drawing commands to a virtual framebuffer in main memory rather than to a real framebuffer in GPU memory, multiple simultaneous instances of an X proxy can co-exist on the same server. However, for the same reason, X proxies generally do not support hardware-accelerated 3D rendering (without the help of VirtualGL.) Most X proxies send only images to the client.
2D X server
  The X server to which the 3D application’s GUI will be displayed, and with which the user will interact. The 2D X server can be an actual X server running on the local machine (“local” from the point of view of the user) or a remote X proxy instance. The 2D X server does not need to support OpenGL.
VirtualGL Faker
  A shared library (“dynamic shared object” or “DSO”), loaded into a Linux/Unix OpenGL application at run time, that intercepts and modifies (“interposes”) certain GLX, EGL, OpenGL, X11, and XCB function calls in order to seamlessly redirect 3D rendering into an off-screen buffer on a GPU, read back the rendered frames, and transport the rendered frames (which normally means delivering them to the 2D X server)
back end
  The method by which the GLX interposer creates off-screen buffers on a GPU and redirects 3D rendering into them. The GLX back end uses the GLX API, so it can only access a GPU through an attached X server (the 3D X server.) The EGL back end uses the EGL API with the `EGL_EXT_platform_device` extension, so it accesses a GPU through an associated EGL device without the need for a 3D X server. The EGL back end emulates a subset of the GLX API using the EGL API. Since the EGL API does not support multi-buffering (double buffering or quad-buffered stereo) with off-screen surfaces, the EGL back end emulates multi-buffering using OpenGL renderbuffer objects (RBOs.) As of this writing, the EGL back end does not yet support all of the GLX extensions and esoteric OpenGL features that the GLX back end supports.
  The EGL/X11 interposer accesses the GPU using either the 3D X server or the EGL device, depending on whether the GLX back end or the EGL back end is enabled. The EGL/X11 function calls are modified and redirected on a 1:1 basis (without the need for API emulation), so there is no disadvantage to using an EGL device for 3D rendering with EGL/X11 applications.
frame trigger
  A function that is called by the 3D application to signal that it has finished rendering a frame. If the 3D application is using the GLX API, then VirtualGL reads back and transports a rendered frame if the application calls `glXSwapBuffers()` while rendering to the back buffer or if the application calls `glFinish()`, `glXWaitGL()`, or (optionally) `glFlush()` while rendering to the front buffer. If the 3D application is using the EGL/X11 API, then VirtualGL reads back and transports a rendered frame if the application calls `eglSwapBuffers()`.
VGL Transport
  The (optional) protocol that VirtualGL uses to transport rendered frames to a 2D X server on a machine other than the VirtualGL server
VirtualGL Client
  The (optional) application that receives rendered frames in real time from the VirtualGL Faker by way of the VGL Transport, decompresses or decodes those frames, and composites them into the 3D application’s windows
client
  The machine that is physically in front of the user. For the purposes of this document, the word “client”– when used generically– always refers to this machine, not to the VirtualGL Client application.

# 3 Overview

VirtualGL is an open source toolkit that gives any Linux or Unix remote display software the ability to run OpenGL applications with full hardware acceleration. Some remote display software cannot be used with OpenGL applications at all. Other remote display software forces OpenGL applications to use a slow, software-only renderer, to the detriment of performance as well as compatibility. The traditional method of displaying OpenGL applications to an X server on a different machine (indirect rendering) supports hardware acceleration, but this approach requires that all of the OpenGL commands and 3D data be sent over the network to be rendered. That is not a tenable proposition unless the 3D data is relatively small and static, unless the network is very fast, and unless the OpenGL application is specifically tuned for a remote X Window System (“X”) environment.

With VirtualGL, the OpenGL commands and 3D data are instead redirected to a GPU in the application server, and only the rendered frames are sent over the network. VirtualGL thus virtualizes GPU hardware, allowing it to be co-located in the “cold room” with compute and storage resources. VirtualGL also allows GPUs to be shared among multiple users, and it provides “workstation-like” levels of performance on 100-megabit and faster networks. This makes it possible for large, noisy, hot 3D workstations to be replaced with laptops or even thinner clients. More importantly, however, VirtualGL eliminates the workstation and the network as barriers to data size. Users can now visualize huge amounts of data in real time without needing to copy any of the data over the network or sit in front of the machine that is rendering the data.

Normally, a Un*x OpenGL application sends all of its graphics rendering commands and data, both 2D and 3D, to an X server, which may be located across the network from the application server. VirtualGL employs a technique called “split rendering” to redirect the 3D commands and data from the OpenGL application to a GPU in the application server. VGL accomplishes this by pre-loading a dynamic shared object (DSO), the VirtualGL Faker, into the OpenGL application at run time. The VirtualGL Faker intercepts and modifies certain GLX, EGL, OpenGL, X11, and XCB function calls in order to divert OpenGL rendering from the 3D application’s windows into corresponding off-screen buffers, which VGL creates in GPU memory on the application server. When the 3D application swaps the OpenGL drawing buffers or flushes the OpenGL command buffer to indicate that it has finished rendering a frame, VirtualGL reads back the rendered frame from the off-screen buffer and transports it (which normally involves delivering the frame to the 2D X server and compositing it into the 3D application’s window.)

The beauty of this approach is its non-intrusiveness. VirtualGL monitors a few X11 commands and events to determine when windows have been resized, etc., but it does not interfere in any way with the delivery of X11 2D drawing commands to the X server. For the most part, VGL does not interfere with the delivery of OpenGL commands to the GPU, either. VGL merely forces the OpenGL commands to be delivered to a GPU in the application server (through the 3D X server or EGL device attached to the GPU) rather than to the X server to which the 2D drawing commands are delivered (the 2D X server.) Once the OpenGL rendering has been redirected to an off-screen buffer, everything (including esoteric OpenGL extensions, fragment/vertex shaders, etc.) should “just work.” If an OpenGL application runs correctly when accessing a 3D server/workstation locally, then the same application should run correctly with VirtualGL when accessing the same machine remotely.

VirtualGL has two built-in “image transports” that can be used to deliver rendered frames to the 2D X server:

1. VGL Transport
  The VGL Transport is most often used whenever the 2D X server is located across the network from the application server– for instance, if the 2D X server is running on the client. VirtualGL uses its own protocol on a dedicated TCP socket to send the rendered frames to the client, and the VirtualGL Client decodes the frames and composites them into the appropriate X window. The VGL Transport can either deliver the frames in uncompressed form (RGB-encoded), or it can compress them in real time using a high-speed JPEG codec. It also supports the delivery of stereo image pairs, which can be reconstructed into a stereo frame by the VirtualGL Client.

Figure 3.1: The VGL Transport with a Client-Side 2D X Server

2. X11 Transport
  The X11 Transport simply draws the rendered frames into the appropriate X window using `XPutImage()` or similar X11 commands. This is most useful in conjunction with an X proxy, which can be one of any number of Un*x remote display applications, such as VNC. When using the X11 Transport, VirtualGL does not normally perform any image compression or encoding itself. It instead relies on an X proxy to encode the frames and deliver them to the client(s). Since the use of an X proxy eliminates the need to send X11 commands over the network, this is the recommended method for using VirtualGL over high-latency or low-bandwidth networks.

  The XV Transport, described in Chapter 10, is a variant of the X11 Transport.

Figure 3.2: The X11 Transport with an X Proxy

VirtualGL also provides an API that can be used to develop custom image transport plugins.

# 4 System Requirements

## 4.1 Linux and Other Un*x Operating Systems

| Server (x86) | Server (x86-64, AArch64) | Client (if using the VGL Transport)
--- | --- | --- | ---
Recommended CPU | For optimal performance, the CPU should support SSE2 extensions. Dual processors or dual cores recommended | Dual processors or dual cores recommended | For optimal performance, the CPU should support SSE2 extensions.
Graphics | AMD or nVidia GPU For optimal performance, particularly with multiple simultaneous users, a professional-grade GPU such as the AMD Radeon Pro or nVidia Quadro is recommended. Install the AMD or nVidia proprietary drivers. Open source drivers for these GPUs do not always provide full 3D acceleration, and some of those drivers are not fully compatible with VirtualGL. | Any graphics adapter with decent 2D performance If using a 3D graphics adapter (GPU), install the vendor drivers for that 3D graphics adapter.
O/S | VirtualGL should work with a variety of Linux distributions, FreeBSD, and Solaris, but currently-supported versions of Red Hat Enterprise Linux and its derivatives, Ubuntu LTS, and SuSE Linux Enterprise tend to receive the most attention from the VirtualGL community.
Other Software | X server configured to export True Color (24-bit or 32-bit) visuals

## 4.2 Mac/x86

| Client (if using the VGL Transport)
--- | ---
CPU | 64-bit Intel or Apple silicon required
O/S | OS X/macOS 10.9 “Mavericks” or later (Intel); macOS 11 “Big Sur” or later (Apple silicon)
Other Software | XQuartz 2.8.0 or later

## 4.3 Windows

| Client (if using the VGL Transport)
--- | ---
Recommended CPU | For optimal performance, the CPU should support SSE2 extensions.
Graphics | Any graphics adapter with decent 2D performance
Other Software | Cygwin/X Client display must have a 24-bit or 32-bit color depth (True Color.)

## 4.4 Additional Requirements for Stereographic Rendering

The client requirements do not apply to anaglyphic stereo. See Chapter 15 for more details.

| Server (GLX back end) | Server (EGL back end) | Client (VGL Transport required)
--- | --- | --- | ---
Linux/Unix | AMD or nVidia GPU that supports stereo (examples: AMD Radeon Pro, nVidia Quadro) The 3D X server should be configured to export stereo visuals. | No additional requirements | GPU that supports stereo The 2D X server should be configured to export stereo visuals.
Mac/x86 | N/A | N/A | GPU that supports stereo (example: nVidia Quadro)
Windows | N/A | N/A | This version of VirtualGL does not support quad-buffered stereo with Windows clients.

# 5 Obtaining and Installing VirtualGL

VirtualGL must be installed on any machine that will act as a VirtualGL server or as a client for the VGL Transport. It is not necessary to install VirtualGL on the client if using VNC or another type of X proxy.

## 5.1 Installing VirtualGL on Linux

If you are installing VirtualGL onto a fresh server, and you also intend to install the nVidia proprietary drivers, install VirtualGL prior to the nVidia drivers. Otherwise, installing VirtualGL may trigger an installation of Mesa, which can modify the libGL symlinks that the nVidia drivers created.

1. Download the appropriate VirtualGL binary package(s) for your system from the Releases area of the VirtualGL GitHub project page. RPM and Debian packages are provided for Linux distributions that contain GLIBC 2.17 or later.

   If you want to run both 32-bit and 64-bit OpenGL applications with VirtualGL on 64-bit x86 Linux systems, then you will need to install both VirtualGL-3.1.4.x86_64.rpm and VirtualGL-3.1.4.i386.rpm, or both virtualgl_3.1.4_amd64.deb and virtualgl32_3.1.4_amd64.deb. (virtualgl32_3.1.4_amd64.deb is a supplementary package that contains only the 32-bit server components.)

2. cd to the directory where you downloaded the binary package(s), and issue the following commands as root:
   RPM-based systems using YUM

         rpm -e VirtualGL --allmatches
         yum install VirtualGL*.rpm

   RPM-based systems using DNF

         rpm -e VirtualGL --allmatches
         dnf install VirtualGL*.rpm

   RPM-based systems using YaST2

         rpm -e VirtualGL --allmatches
         yast2 --install VirtualGL*.rpm

   Other RPM-based systems (dependencies will not be installed automatically)

         rpm -e VirtualGL --allmatches
         rpm -i VirtualGL*.rpm

   Debian-based systems

         dpkg -i virtualgl*.deb
         apt install -f

## 5.2 Installing the VirtualGL Client on OS X/macOS

1. Download the VirtualGL Mac disk image (VirtualGL-3.1.4.dmg) from the Releases area of the VirtualGL GitHub project page.
2. Open the disk image, then open VirtualGL-3.1.4.pkg inside the disk image. Follow the instructions to install the Mac VirtualGL Client.

## 5.3 Installing the VirtualGL Client on Windows (Cygwin)

Use Cygwin Setup to install the VirtualGL package.

## 5.4 Installing VirtualGL from Source

If you are using a platform for which there is not a pre-built VirtualGL binary package available, then download the VirtualGL source tarball (VirtualGL-3.1.4.tar.gz) from the Releases area of the VirtualGL GitHub project page, uncompress it, `cd VirtualGL-3.1.4`, and read the contents of BUILDING.md for further instructions on how to build and install VirtualGL from source.

## 5.5 Uninstalling VirtualGL

### Linux

As root, issue one of the following commands:

RPM-based systems

      rpm -e VirtualGL

  You may need to add `--all-matches` to the RPM command line if you have installed both the 32-bit and 64-bit VirtualGL RPMs.

Debian-based systems

      dpkg -r virtualgl

  If you have also installed the 32-bit supplementary package:

      dpkg -r virtualgl32

### OS X/macOS

Open the Uninstall VirtualGL application, located in the VirtualGL Applications folder. You can also open a terminal and execute:

    sudo /opt/VirtualGL/bin/uninstall

### Windows (Cygwin)

Use Cygwin Setup to uninstall the VirtualGL package.

# 6 Configuring a Linux or Unix Machine as a VirtualGL Server

## 6.1 Initial System Configuration

Before configuring VirtualGL, you should first ensure that:

1. The appropriate GPU drivers have been installed on the machine. With few exceptions, you should install the drivers supplied by your GPU vendor rather than the drivers supplied by your O/S distribution. See Section 4.1.

If you intend to configure the machine for use with the GLX back end, then you should also ensure that:

1. The 3D X server has been configured to use the GPU drivers you installed above.

2. The machine has an appropriate display manager (such as GDM, KDM, or LightDM) installed and has been configured to start the display manager and 3D X server at boot time. This is the default with most modern Linux and Unix distributions.

3. Accelerated OpenGL is working properly in the 3D X server.

- If the 3D X server is not headless, then this can be verified by logging into the X server locally and running `glxinfo`.
- If the 3D X server is headless, then proceed with the instructions below, and the Sanity Check section will provide you with an opportunity to verify that the 3D X server is using accelerated OpenGL.

More specific instructions are unfortunately outside of the scope of this guide, since they will vary from system to system.

## 6.2 GLX Back End: Granting Access to the 3D X Server

VirtualGL requires access to a GPU in the application server so that it can create off-screen buffers and redirect the 3D rendering from X windows into these buffers. When using the GLX back end, accessing a GPU requires going through an X server attached to that GPU (the 3D X server), so the only way to share the application server’s GPU(s) among multiple users is to grant those users access to the 3D X server.

It is important to understand the security risks associated with this. Once a user has access to the 3D X server, there is nothing that would prevent the user from logging keystrokes or reading back images from that X server. A user with 3D X server and framebuffer device access can also read back the contents of an off-screen buffer created by another user, provided that the off-screen buffer’s drawable ID is known and provided that the GPU driver uses DRI2 (example: nVidia proprietary drivers) rather than DRI3 (example: Mesa-based drivers.) Using `xauth`, one can obtain “untrusted” X authentication keys that prevent such exploits, but unfortunately, those untrusted keys also disallow access to the GPU. Thus, it is necessary to grant full, trusted access to the 3D X server for any users that will need to use the GLX back end. Unless you fully trust the users to whom you are granting this access, then you should avoid logging in locally to the 3D X server (particularly as root) unless absolutely necessary. (In general, logging in locally once the machine has been configured for use with the GLX back end is discouraged, as this could disrupt VirtualGL’s connection to the 3D X server and thus cause OpenGL applications running with VirtualGL to abort or freeze.)

This section will explain how to configure a VirtualGL server such that selected users can use the GLX back end, even if the server is sitting at the login prompt.

1. Shut down the display manager. Examples:
   Red Hat/Fedora/SuSE Linux servers

         init 3

   Ubuntu Linux servers running LightDM

         service lightdm stop

   FreeBSD servers running GDM

         /usr/local/etc/rc.d/gdm stop

   Solaris 11/OpenSolaris servers running GDM

         svcadm disable gdm

2. Log in as root from the text console (or remotely using SSH.)
3. Run

       /opt/VirtualGL/bin/vglserver_config

4. Select option 1 (`Configure server for use with VirtualGL (GLX + EGL back ends)`.)

5.     Restrict 3D X server access to vglusers group (recommended)?
       [Y/n]

   Yes
     Only users in the `vglusers` group can use the GLX back end. (The configuration script will create the `vglusers` group if it doesn’t already exist.) This is the most secure option, since it prevents any users outside of the `vglusers` group from accessing (and thus exploiting) the 3D X server.
   No
     The GLX back end can be used by any user that successfully logs into the VirtualGL server. The 3D X server can also be accessed (and potentially exploited) by any user who is logged into the VirtualGL server. If you choose this option, it is recommended that you also disable the `XTEST` extension (see below.)

6.     Restrict framebuffer device access to vglusers group (recommended)?
       [Y/n]

   Yes
     Only users in the `vglusers` group can run OpenGL applications on the VirtualGL server. (The configuration script will create the `vglusers` group if it doesn’t already exist.) This limits the possibility that an unauthorized user could snoop the 3D framebuffer device(s) and thus see (or alter) the output of a 3D application that is being used with VirtualGL.

     If you are using a recent release of GDM, then the `gdm` account must be added to the `vglusers` group.

   No
     Any authenticated user can run OpenGL applications on the VirtualGL server. If it is necessary for users outside of the `vglusers` group to log in locally to this server and run OpenGL applications, then this option must be selected.

7.     Disable XTEST extension (recommended)?
       [Y/n]

   Yes
     Disabling `XTEST` will not prevent a user from logging keystrokes or reading images from the 3D X server, but if a user has access to the 3D X server, disabling `XTEST` will prevent them from inserting keystrokes or mouse events and thus hijacking local X sessions on that X server.

     If you are using GDM 2.14 through 2.20, it will be necessary to run `gdmsetup` and manually add an argument of `-tst` to the X server command line to disable `XTEST` for the first time. After this, `vglserver_config` should be able to disable and enable `XTEST` properly.

     GDM 2.22 and later no longer provide a means of editing the X server command line, so disabling `XTEST` will not work. The only known alternative as of this writing is to use a different display manager.

   No
     x11vnc and `x0vncserver` both require `XTEST`, so if you need to attach a VNC server to the 3D X server, then it is necessary to answer “No” (and thus leave `XTEST` enabled.)
8. If you chose to restrict 3D X server or framebuffer device access to the `vglusers` group, then edit /etc/group and add `root` to the `vglusers` group. If you choose, you can also add additional users to the group at this time. Note that any user you add to `vglusers` must log out and back in again before their new group permissions will take effect.
9. Restart the display manager:
   Red Hat/Fedora/SuSE Linux servers

         init 5

   Ubuntu Linux servers running LightDM

         service lightdm start

   FreeBSD servers running GDM

         /usr/local/etc/rc.d/gdm start

   Solaris 11/OpenSolaris servers running GDM

         svcadm enable gdm

### 6.2.1 Sanity Check

To verify that the application server is ready to be used with the GLX back end, log out of the server, log back into the server using SSH, and execute the following commands in the SSH session:

If you restricted 3D X server access to `vglusers`

    xauth merge /etc/opt/VirtualGL/vgl_xauth_key
    xdpyinfo -display :n
    /opt/VirtualGL/bin/glxinfo -display :n -c

If you did not restrict 3D X server access

    xdpyinfo -display :n
    /opt/VirtualGL/bin/glxinfo -display :n -c

(where `n` is the display number of the 3D X server.) Both commands should output a list of visuals and should complete with no errors. If you chose to disable the `XTEST` extension, then check the output of the `xdpyinfo` command to verify that `XTEST` does not show up in the list of extensions.

You should also examine the output of the `glxinfo` command to ensure that at least one of the visuals is 24-bit or 32-bit TrueColor and has Pbuffer support. (The latter is indicated by a `P` in the last column.) Example:

        visual  x   bf lv rg d st  colorbuffer  ax dp st accumbuffer  ms  cav  drw
      id dep cl sp  sz l  ci b ro  r  g  b  a F bf th cl  r  g  b  a ns b eat  typ
    ------------------------------------------------------------------------------
    0x151 24 tc  0  32  0 r  y  .  8  8  8  0 .  4 24  8 16 16 16 16  0 0 None PXW

If none of the visuals has Pbuffer support, then this is most likely because there is no 3D acceleration, which is most likely because the correct GPU drivers are not installed (or are misconfigured.) Lack of 3D acceleration is also typically indicated by the word `Mesa` in the client GLX vendor string and/or the OpenGL vendor string, and the words `Software Rasterizer` in the OpenGL renderer string.

## 6.3 EGL Back End: Granting Access to the GPU(s)

When using the EGL back end, the only way to share the application server’s GPU(s) among multiple users is to grant those users access to the device(s) associated with the GPU(s).

This section will explain how to configure a VirtualGL server such that selected users can use the EGL back end.

1. Log in as root from the text console (or remotely using SSH.)
2. Run

       /opt/VirtualGL/bin/vglserver_config

3. Select option 2 (`Configure server for use with VirtualGL (EGL back end only)`.)

4.     Restrict framebuffer device access to vglusers group (recommended)?
       [Y/n]

   Yes
     Only users in the `vglusers` group can run OpenGL applications on the VirtualGL server. (The configuration script will create the `vglusers` group if it doesn’t already exist.) This limits the possibility that an unauthorized user could snoop the 3D framebuffer device(s) and thus see (or alter) the output of a 3D application that is being used with VirtualGL. Note, however, that such an exploit should not normally be possible with the EGL back end, since it redirects 3D rendering into renderbuffer objects that are not accessible by other processes.
   No
     Any authenticated user can run OpenGL applications on the VirtualGL server. If it is necessary for users outside of the `vglusers` group to log in locally to this server and run OpenGL applications, then this option must be selected.
5. If you chose to restrict framebuffer device access to the `vglusers` group, then edit /etc/group and add `root` to the `vglusers` group. If you choose, you can also add additional users to the group at this time. Note that any user you add to `vglusers` must log out and back in again before their new group permissions will take effect.

### 6.3.1 Sanity Check

To verify that the application server is ready to be used with the EGL back end, log out of the server, log back into the server using SSH, and execute the following command in the SSH session:

    /opt/VirtualGL/bin/eglinfo egl0

This command should output a list of EGL configs and should complete with no errors.

## 6.4 Using VirtualGL with Multiple GPUs

VirtualGL can redirect the OpenGL commands from a 3D application to any GPU in the VirtualGL server. In order for this to work with the GLX back end, however, all of the GPUs must be attached to different screens on the same X server or to different X servers. Attaching them to different screens is the easiest and most common approach, and this allows a specific GPU to be selected by setting `VGL_DISPLAY` to (or invoking `vglrun -d` with) `:0.0`, `:0.1`, `:0.2`, etc. If the GPUs are attached to different X servers, then a specific GPU can be selected by setting `VGL_DISPLAY` to (or invoking `vglrun -d` with) `:0.0`, `:1.0`, `:2.0`, etc.

Setting `VGL_DISPLAY` to (or invoking `vglrun -d` with) a DRI device path (`/dev/dri/card0`, `/dev/dri/card1`, `/dev/dri/card2`, etc.) or an EGL device ID (`egl0`, `egl1`, `egl2`, etc.) enables the EGL back end and selects the specified GPU. See Section 18.1 for more details.

## 6.5 SSH Server Configuration

If you intend to use the VGL Transport, then the application server’s SSH daemon should have the `X11Forwarding` option enabled and the `UseLogin` option disabled. This is configured in sshd_config, which is usually located under /etc/ssh.

## 6.6 Un-Configuring the Server

You can use the `vglserver_config` script to restore the security settings that were in place before VirtualGL was installed. Option 2 (`Unconfigure server for use with VirtualGL (GLX + EGL back ends)`) will remove any shared access to the 3D X server and thus prevent VirtualGL from accessing a GPU in that manner. Additionally, this option will re-enable the `XTEST` extension on the 3D X server and will restore the framebuffer device permissions to their default. (By default, only root or the user that is currently logged into the application server locally can access the framebuffer devices.)

After selecting Option 2, you must restart the display manager before the changes will take effect.

Option 4 (`Unconfigure server for use with VirtualGL (EGL back end only)`) will restore the framebuffer device permissions to their default.

Unconfiguring the server does not remove the `vglusers` group.

# 7 Configuring a Windows Machine as a VGL Transport Client

The VirtualGL Client can take advantage of the `MIT-SHM` extension in Cygwin/X to accelerate the compositing of rendered frames into the 3D application’s windows. This can significantly improve the end-to-end performance of VirtualGL when using the VGL Transport over a local-area network.

To enable `MIT-SHM` in Cygwin/X:

1. Open a Cygwin Bash shell
2. Run `cygserver-config`
3. Answer “yes” when asked `Do you want to install cygserver as service?`
4. From a Windows (not Cygwin) administrator command prompt, run `net start cygserver`
5. Start or re-start Cygwin/X
6. Run `xdpyinfo` and verify that `MIT-SHM` appears in the list of X extensions

# 8 Using VirtualGL with the VGL Transport

### Advantages of the VGL Transport

- Seamless windows; every window from the 3D application appears as a separate window on the client
- Supports stereographic rendering (see requirements in Section 4.4)
- Consumes less memory and CPU time on the server, since the 2D (X11) rendering occurs on the client

### Disadvantages of the VGL Transport

- The VGL Transport is designed to be used with client-side 2D X servers, and thus it relies on the chatty remote X11 protocol to send the non-OpenGL elements of the 3D application’s GUI to the client. As a result, the VGL Transport is not recommended for use on high-latency or low-bandwidth networks.
- No collaboration features
- From the point of view of the 3D application, the client/server connection is not stateless. As with any remote X11 environment, if the network connection drops, then the application will exit.

## 8.1 VGL Transport with X11 Forwarding

This mode is recommended for use only on secure local-area networks. The X11 traffic is encrypted, but the VGL Transport is left unencrypted.

### 8.1.1 Procedure

1. Start the 2D X server if it isn’t started already.
   Mac clients
     Start XQuartz.
   Cygwin clients
     Start Cygwin/X.
2. Open a new terminal window.
   Mac clients
     In XQuartz, start a new xterm [Command-N] if one isn’t already started.
   Cygwin clients
     Start a new xterm if one isn’t already started. (Right-click on the Cygwin/X taskbar icon, then select Applications–>xterm.)
3. In the same terminal/xterm window, open a Secure Shell (SSH) session into the VirtualGL server:

       /opt/VirtualGL/bin/vglconnect user@server

   Replace `user` with your username on the VirtualGL server and `server` with the hostname or IP address of that server.
4. In the SSH session, start a 3D application with VirtualGL:

       /opt/VirtualGL/bin/vglrun [vglrun options] 3D-application-executable-or-script [arguments]

   Consult Chapter 18 for more information on `vglrun` command-line options.

## 8.2 VGL Transport with X11 Forwarding and SSH Tunneling

Both the VGL Transport and the X11 traffic are tunneled through SSH when using this mode, and thus it provides a completely secure solution. It is also useful when either the client or the VirtualGL server is behind a restrictive firewall and only SSH connections are allowed through.

### Procedure

The procedure for this mode is identical to the procedure for the VGL Transport with X11 forwarding, except that you should pass a `-s` argument to `vglconnect` when connecting to the server:

    /opt/VirtualGL/bin/vglconnect -s user@server

`vglconnect` will make two SSH connections into the server, the first to find an open port on the server and the second to create the SSH tunnel for the VGL Transport and open the secure shell. Because of Cygwin limitations, when connecting from a Windows client, it will be necessary to enter your SSH password twice unless you are using an SSH agent to enable password-less logins.

`vglconnect -s` can be used to create multi-layered SSH tunnels. For instance, if the VirtualGL server is not directly accessible from the Internet, then you can run `vglconnect -s` on the client to connect to an SSH gateway server, then you can run `vglconnect -s` again on the gateway server to connect to the VirtualGL server (application server.) Both the X11 traffic and the VGL Transport will be forwarded from the VirtualGL server through the gateway and to the client.

## 8.3 The VirtualGL Client Application: Nuts and Bolts

The VirtualGL Client application (`vglclient`) receives encoded and/or compressed frames on a dedicated TCP socket, decodes and/or decompresses the frames, and draws the frames into the appropriate X window. The `vglconnect` script wraps both `vglclient` and SSH to greatly simplify the process of creating VGL Transport connections.

`vglconnect` invokes `vglclient` with an argument of `-detach`, which causes the VirtualGL Client to completely detach from the console and run as a background daemon. It will remain running silently in the background, accepting VGL Transport connections for the X server on which it was started, until that X server is reset or until the VirtualGL Client process is explicitly killed. Logging out of the X server will reset the X server and thus kill all VirtualGL Client instances that are attached to it. You can also explicitly kill all instances of the VirtualGL Client running under your user account by invoking

    vglclient -kill

(`vglclient` is installed in /opt/VirtualGL/bin by default.)

`vglconnect` instructs the VirtualGL Client to redirect all of its console output to a log file named ~/.vgl/vglconnect-hostname-display.log, where hostname is the name of the computer on which `vglconnect` was invoked and display is the display name of the X server on which the VirtualGL Client was started (read from the `DISPLAY` environment or passed to `vglconnect` using the `-display` argument.) In the event that something goes wrong, this log file is the first place to check.

When the VirtualGL Client successfully starts on a given X server, it stores its listener port number in a root window property on the X server. If other VirtualGL Client instances attempt to start on the same X server, they read the X window property, determine that another VirtualGL Client instance is already running, and exit to allow the first instance to retain control. The VirtualGL Client will clean up the X property under most circumstances, even if it is explicitly killed. However, under rare circumstances (if sent a SIGKILL signal, for instance), a VirtualGL Client instance may exit uncleanly and leave the X property set. In these cases, it may be necessary to add an argument of `-force` to `vglconnect` the next time you use it. This tells `vglconnect` to start a new VirtualGL Client instance, regardless of whether the VirtualGL Client thinks that there is already an instance running on this X server. Alternately, you can simply reset the X server to clear the orphaned X window property.

### 8.3.1 The VirtualGL Client and Firewalls

To retain compatibility with previous versions of VirtualGL, the first VirtualGL Client instance on a given machine will attempt to listen on port 4242. If it fails to obtain that port, because another application or another VirtualGL Client instance is already using it, then the VirtualGL Client will try to obtain a free port in the range of 4200-4299. Failing that, it will request a free port from the operating system.

In a nutshell: if you only ever plan to run one X server at a time on the client, which means that you’ll only ever need one instance of the VirtualGL Client at a time, then it is sufficient to open inbound port 4242 in the client’s firewall. If you plan to run multiple X servers on the client, which means that you will need to run multiple VirtualGL Client instances, then you may wish to open ports 4200-4299. Similarly, if you are running the VirtualGL Client on a multi-user X proxy server that has a firewall, then you may wish to open ports 4200-4299 in the server’s firewall. Opening ports 4200-4299 will accommodate up to 100 separate VirtualGL Client instances. More instances than that cannot be accommodated on a firewalled machine, unless the firewall is able to create rules based on application executables instead of listening ports.

Note that it is not necessary to open any inbound ports in the firewall to use the VGL Transport with SSH Tunneling.

# 9 Using VirtualGL with X Proxies Such as VNC

The VGL Transport is a good solution for using VirtualGL over a fast network. However, the VGL Transport is not generally suitable for high-latency or low-bandwidth networks, due to its reliance on the X11 protocol to send the non-OpenGL elements of the 3D application’s GUI. The VGL Transport also requires an X server to be running on the client, which makes it generally more difficult to deploy and use on Windows clients. VirtualGL can be used with an X proxy to overcome these limitations. An X proxy acts as a virtual X server, receiving X11 commands from the 3D application (and from VirtualGL), rendering the X11 commands into images, compressing the resulting images, and sending the compressed images over the network to a client or clients. X proxies perform well on all types of networks, including high-latency and low-bandwidth networks. They often provide rudimentary collaboration capabilities, allowing multiple clients to simultaneously view the same X session and pass around control of the keyboard and mouse. X proxies are also stateless, meaning that the client can disconnect and reconnect at will from any machine on the network, and the 3D application will remain running on the server.

Since VirtualGL is sending rendered frames to the X proxy at a very fast rate, the proxy must be able to compress the frames very quickly in order to keep up. Unfortunately, however, most X proxies can’t. They simply aren’t designed to compress, with any degree of performance, the large and complex images generated by 3D applications. Therefore, The VirtualGL Project provides an optimized X proxy called TurboVNC, a high-speed VNC (Virtual Network Computing) variant that is designed specifically to achieve high levels of performance with VirtualGL. More information about TurboVNC, including instructions for using it with VirtualGL, can be found in the TurboVNC User’s Guide.

Many other X proxy solutions work well with VirtualGL, and some of these solutions provide compelling features (seamless windows, for instance), but none of these X proxies matches the performance of TurboVNC, as of this writing.

## 9.1 Using VirtualGL with an X Proxy on the Same Server

The most common (and optimal) way to use VirtualGL with an X proxy is to set up both on the same server. This allows VirtualGL to send rendered frames to the X proxy through shared memory rather than over a network.

With this configuration, you can usually invoke

    /opt/VirtualGL/bin/vglrun [vglrun options] 3D-application-executable-or-script [arguments]

from a terminal inside of the X proxy session, and it will “just work.” VirtualGL reads the value of the `DISPLAY` environment variable to determine whether to enable the X11 Transport by default. If `DISPLAY` begins with a colon (`:`) or with `unix:`, then VirtualGL will assume that the 2D X server is on the same machine and will enable the X11 Transport as the default. In some cases, however, the `DISPLAY` environment variable in the X proxy session may not begin with a colon or `unix:`. In these cases, it is necessary to manually enable the X11 Transport by setting the `VGL_COMPRESS` environment variable to `proxy` or by passing an argument of `-c proxy` to `vglrun`.

## 9.2 Using VirtualGL with an X Proxy on a Different Machine

If the X proxy and VirtualGL are running on different servers, then it is desirable to use the VGL Transport to send rendered frames from the VirtualGL server to the X proxy. It is also desirable to disable image compression in the VGL Transport. Otherwise, the frames would have to be compressed by the VirtualGL server, decompressed by the VirtualGL Client, then recompressed by the X proxy, which is a waste of CPU resources. However, sending images uncompressed over a network requires a fast network (generally, Gigabit Ethernet or faster), so there needs to be a fast link between the VirtualGL server and the X proxy server for this procedure to perform well.

The procedure for using the VGL Transport to display 3D applications from a VirtualGL server to an X proxy on a different machine is the same as the procedure for using the VGL Transport to display 3D applications from a VirtualGL server to a client-side 2D X server, with the following exceptions:

1. The “client” in this case is the X proxy host.
2. The “2D X server” is the X proxy.
3. It is recommended that you disable image compression in the VGL Transport by either setting the `VGL_COMPRESS` environment variable to `rgb` or passing an argument of `-c rgb` to `vglrun` when launching VirtualGL. Otherwise, VirtualGL will detect that the 2D X server is on a different machine, and it will automatically try to enable JPEG compression.

# 10 Support for the X Video Extension

The X Video extension allows applications to pre-encode or pre-compress images and send them through the X server to the graphics adapter, which presumably has on-board video decoding capabilities. This approach greatly reduces the CPU resources used by the X server, which can be beneficial if the X server is running on a different machine than the application.

In the case of VirtualGL, what this means is that the client no longer has to decode or decompress rendered frames from the 3D application. It can simply pass the frames along to the graphics adapter for decoding.

VirtualGL supports the X Video extension in two ways:

## 10.1 The VGL Transport with YUV Encoding

Setting the `VGL_COMPRESS` environment variable to `yuv` or passing an argument of `-c yuv` to `vglrun` enables the VGL Transport with YUV encoding. When this mode is enabled, VirtualGL encodes rendered frames as YUV420P (a form of YUV encoding that uses 4X chrominance subsampling and separates Y, U, and V components into separate image planes) instead of RGB or JPEG. The YUV420P-encoded frames are sent to the VirtualGL Client, which draws them to the 2D X server using the X Video extension.

On a per-frame basis, YUV encoding uses about half the server CPU time as JPEG compression and only slightly more server CPU time than RGB encoding. On a per-frame basis, YUV encoding uses about 1/3 the client CPU time as JPEG compression and about half the client CPU time as RGB encoding. YUV encoding also uses about half the network bandwidth (per frame) as RGB.

However, since YUV encoding uses 4X chrominance subsampling, the encoded frames may contain some visible artifacts. In particular, narrow, aliased lines and other sharp features may appear “soft”.

## 10.2 The XV Transport

Setting the `VGL_COMPRESS` environment variable to `xv` or passing an argument of `-c xv` to `vglrun` enables the XV Transport. The XV Transport is a special flavor of the X11 Transport that encodes rendered frames as YUV420P and draws them directly to the 2D X server using the X Video extension. This is mainly useful in conjunction with X proxies that support the X Video extension. The idea is that, if the X proxy is going to have to transcode the frame into YUV anyhow, VirtualGL may be faster at doing this, since it has a SIMD-accelerated YUV encoder.

# 11 Transport Plugins

VirtualGL 2.2 (and later) includes an API that allows you to write your own image transports. Thus, you can use VirtualGL for doing split rendering and framebuffer readback but then use your own library for delivering the rendered frames to the client.

When the `VGL_TRANSPORT` environment variable (or the `-trans` option to `vglrun`) is set to `{t}`, then VirtualGL will look for a DSO (dynamic shared object) with the name libvgltrans_{t}.so in the dynamic linker path and will attempt to access a set of API functions from this library. The functions that the plugin library must export are defined in /opt/VirtualGL/include/rrtransport.h, and an example of their usage can be found in server/testplugin.cpp and server/testplugin2.cpp in the VirtualGL source distribution. The former wraps the VGL Transport as an image transport plugin, and the latter does the same for the X11 Transport.

# 12 Using VirtualGL with setuid/setgid Executables

`vglrun` can be used to launch either binary executables or shell scripts, but there are a few things to keep in mind when launching a shell script with `vglrun`. When you launch a shell script with `vglrun`, the VirtualGL Faker (libvglfaker.so) and `dlopen()` interposer (libdlfaker.so) will be preloaded into every executable that the script launches. Normally this is innocuous, but if the script calls any executables that have the setuid and/or setgid permission bits set, then the dynamic linker will refuse to preload the faker libraries into those executables. One of the following warnings will be printed for each setuid/setgid executable that the script tries to launch:

Linux

    ERROR: ld.so: object 'libvglfaker.so' from LD_PRELOAD cannot be preloaded: ignored.

    ERROR: ld.so: object 'libdlfaker.so' from LD_PRELOAD cannot be preloaded: ignored.

Solaris

    ld.so.1: warning: libvglfaker.so: open failed: No such file in secure directories

    ld.so.1: warning: libdlfaker.so: open failed: No such file in secure directories

These are just warnings, and the setuid/setgid executables will continue to run (without VirtualGL preloaded into them.) However, if you want to get rid of the warnings, an easy way to do so is to simply edit the application script and make it store the value of the `LD_PRELOAD` environment variable until right before the 3D application executable is launched. For instance, consider the following 3D application script:

    #!/bin/sh
    setuid-executable
    3D-application-executable

You could modify the script as follows:

    #!/bin/sh
    LD_PRELOAD_SAVE=$LD_PRELOAD
    LD_PRELOAD=
    export LD_PRELOAD

    setuid-executable

    LD_PRELOAD=$LD_PRELOAD_SAVE
    export LD_PRELOAD

    3D-application-executable

This procedure may be necessary to work around certain other interaction issues between VirtualGL and the launch scripts of specific 3D applications. See Application Recipes for more details.

If the 3D application that you are intending to run with VirtualGL is itself a setuid/setgid executable, then further steps are required. Otherwise, the 3D application will launch without VirtualGL preloaded into it. Forcing VirtualGL to be preloaded into setuid/setgid executables has security ramifications, so please be aware of these before you do it. By applying one of the following workarounds, you are essentially telling the operating system that you trust the security and stability of VirtualGL as much as you trust the security and stability of the operating system. While we’re flattered, we’re not sure that we’re necessarily deserving of that accolade, so if you are in a security-critical environment, apply the appropriate level of paranoia here.

To force VirtualGL to be preloaded into setuid/setgid executables on Linux, you have to first make sure that the faker libraries are installed in the system library path (usually /usr/lib, /usr/lib64, /usr/lib32, or /usr/lib/i386-linux-gnu). Next, make the faker libraries setuid executables. To do this, run the following commands as root:

    chmod u+s /usr/lib/libvglfaker.so
    chmod u+s /usr/lib/libdlfaker.so

where `lib` is `lib`, `lib64`, `lib32`, or `lib/i386-linux-gnu`, depending on your system.

On Solaris, you can force VirtualGL to be preloaded into setuid/setgid executables by adding the VirtualGL library directories to the Solaris “secure path.” Solaris keeps a tight lid on what goes into /usr/lib and /lib, and by default, it will only allow libraries in those paths to be preloaded into an executable that is setuid and/or setgid. Generally, 3rd party packages are forbidden from installing anything into /usr/lib or /lib, but you can use the `crle` utility to add other directories to the operating system’s list of secure paths. In the case of VirtualGL, you would execute one of the following commands (as root):

32-bit VirtualGL:

      crle -u -s /opt/VirtualGL/lib32

64-bit VirtualGL:

      crle -64 -u -s /opt/VirtualGL/lib64

# 13 Using VirtualGL with VirtualBox

VirtualBox is an enterprise-class, open source virtualization solution that supports hardware-accelerated OpenGL in Windows and Linux guests running on Windows, Mac, Linux, and Solaris/x86 hosts. 3D acceleration in VirtualBox is accomplished by installing a special driver in the guest that transmits OpenGL calls through a local connection to the VirtualBox process running on the host. When used in conjunction with VirtualGL on a Linux or Solaris/x86 host, this solution provides a means of displaying Windows 3D applications remotely.

To use VirtualGL with VirtualBox, perform the following procedures:

1. Launch VirtualBox and use the VirtualBox GUI to create and test your virtual machine.
2. Follow the procedures outlined in the VirtualBox User Manual to enable 3D acceleration in the virtual machine. If possible, verify that 3D acceleration works on a local display without VirtualGL before adding VirtualGL to the mix.
3. Follow the procedure described in Chapter 12 to make libvglfaker.so and libdlfaker.so setuid executables (Linux) or to add the VirtualGL library directory to the list of secure paths (Solaris).
4. Exit VirtualBox and re-launch it with `vglrun`.

### NOTES

- If VirtualGL is working properly with VirtualBox, then the OpenGL renderer string in Windows guests should contain “SVGA3D” rather than “GDI Generic”.
- The OpenGL renderer string in Un*x guests always contains “SVGA3D”, so the only reliable way to verify that VirtualGL is working properly with VirtualBox in this case is to enable tracing (see below) and run a 3D application in the guest.
- Tracing (`VGL_TRACE` or `vglrun +tr`) will not work with VirtualBox unless the virtual machine is started from the command line, using

      vglrun VirtualBoxVM -startvm VM-name-or-UUID

- VirtualBox bypasses VirtualGL’s readback mechanism and image pipeline, which has two ramifications:
  - Features that are specific to VirtualGL’s readback mechanism or image pipeline, including the built-in profiling system and `VGL_LOGO`, do not work with VirtualBox.
  - VirtualBox sends mainly uncompressed images to the X server, so even with VirtualGL, it will not perform well over a remote X connection. Using a high-performance X proxy such as TurboVNC is highly recommended.

# 14 Other Application Recipes

3D Application | Versions Known to Require Recipe | Platform | Recipe | Notes
--- | --- | --- | --- | ---
Abaqus | v6 | Linux | It is necessary to add `import os` `os.environ['ABAQUS_EMULATE_OVERLAYS'] = "1"` to abaqus-install-dir/abaqus-version/site/abaqus_v6.env to make Abaqus v6 work properly with VirtualGL. If this is not done, then the application may fail to launch, it may fail to display any OpenGL-rendered pixels, or those pixels may become corrupted whenever other windows obscure them. | VirtualGL does not support transparent overlays, since those cannot be rendered in an off-screen buffer. Setting `ABAQUS_EMULATE_OVERLAYS` to `1` causes the application to emulate overlay rendering instead of using actual transparent overlays. This workaround is known to be necessary when running Abaqus 6.9 and 6.10.
Abaqus | v6 | Linux | `vglrun -nodl abaqus-path/abaqus` | User reports indicate that Abaqus 6.9 will not work properly if VirtualGL’s `dlopen()` interposer (libdlfaker.so) is preloaded into it. This may be true for other versions of Abaqus as well.
Cadence Allegro | v16.5 | Linux | `vglrun +sync allegro` | Allegro relies on mixed X11/OpenGL rendering, and thus certain features (specifically the `pcb_cursor_infinite` cursor style) do not work properly unless `VGL_SYNC` is enabled. If `VGL_SYNC` is not enabled, then the crosshairs may remain on the screen. Since `VGL_SYNC` automatically enables the X11 transport and disables frame spoiling, it is highly recommended that you use an X proxy when `VGL_SYNC` is enabled. See Section 18.1 for further information.
Animator | v4 | Linux | Comment out the line that reads `unsetenv LD_PRELOAD` in the `a4` script, then launch Animator 4 using `vglrun -ge a4` | When the `a4` script unsets `LD_PRELOAD`, this prevents VirtualGL from being loaded into the application. Animator 4 additionally checks the value of `LD_PRELOAD` and attempts to unset it from inside the application. Using `vglrun -ge` to launch the application fools Animator 4 into thinking that `LD_PRELOAD` is unset.
ANSA | v12.1.0 | Linux | Add `LD_PRELOAD_SAVE=$LD_PRELOAD` `export LD_PRELOAD=` to the top of the `ansa.sh` script, then add `export LD_PRELOAD=$LD_PRELOAD_SAVE` just prior to the `${ANSA_EXEC_DIR}bin/ansa_linux${ext2}` line. | The ANSA startup script directly invokes `/lib/libc.so.6` to query the glibc version. Since the VirtualGL faker libraries depend on libc, preloading VirtualGL when directly invoking `libc.so.6` creates an infinite loop. Thus, it is necessary to disable the preloading of VirtualGL in the application script and then re-enable it prior to launching the actual application.
AutoForm | v4.0x | All | `vglrun +sync xaf_version` | AutoForm relies on mixed X11/OpenGL rendering, and thus certain features (particularly the “Dynamic Section” dialog and “Export Image” feature) do not work properly unless `VGL_SYNC` is enabled. Since `VGL_SYNC` automatically enables the X11 transport and disables frame spoiling, it is highly recommended that you use an X proxy when `VGL_SYNC` is enabled. See Section 18.1 for further information.
Cedega | v6.0.x | Linux | Add `export LD_PRELOAD=libvglfaker.so` to the top of ~/.cedega/.winex_ver/winex-version/bin/winex3, then run Cedega as you would normally (without `vglrun`.) Since `vglrun` is not being used, it is necessary to use environment variables or the VirtualGL Configuration dialog to modify VirtualGL’s configuration. | The actual binary (WineX) that uses OpenGL is buried beneath several layers of Python and shell scripts. The `LD_PRELOAD` variable does not get propagated down from the initial shell that invoked `vglrun`.
Google Chrome/Chromium | v85 through v128 | Linux | `vglrun google-chrome --in-process-gpu --use-gl=egl` or `vglrun chromium --in-process-gpu --use-gl=egl` | The `--in-process-gpu` option causes Chrome/Chromium to use a thread rather than a separate process for 3D rendering, which prevents it from complaining about the X11 function calls that the VirtualGL Faker makes. The `--use-gl=egl` option forces Chrome/Chromium to use desktop OpenGL rather than ANGLE, which works around the X visual selection issue described below.
Google Chrome/Chromium | v129 and later | Linux | Set the `VGL_CHROMEHACK` environment variable to `1`, then launch the application with `vglrun google-chrome --in-process-gpu` or `vglrun chromium --in-process-gpu` | Chrome uses ANGLE, an OpenGL ES emulator that translates OpenGL ES function calls into OpenGL, OpenGL ES, or Vulkan function calls. When using ANGLE’s OpenGL back end, Chrome chooses an X visual by sending raw GLX requests to the 2D X server, bypassing VirtualGL. Thus, the X visual it chooses has rendering attributes that are only relevant for the 2D X server’s OpenGL implementation. Chrome then expects to find a GLX or EGL framebuffer configuration to which the chosen X visual is assigned. That will normally never happen when using VirtualGL. `VGL_CHROMEHACK` works around this issue by emulating Chrome’s X visual selection mechanism and assigning the chosen visual to all GLX and EGL framebuffer configurations that have an 8-bit alpha channel. When using nVidia’s proprietary Vulkan driver, which does something VirtualGL-like in an X proxy environment, or when using an X proxy that implements the DRI3 X extension, you can also enable GPU acceleration by passing `--use-angle=vulkan` to `google-chrome` or `chromium`. No additional arguments or environment variables are necessary in this case, nor is it necessary to launch the application with `vglrun`.
Compiz | All | Linux | Set the `VGL_WM` environment variable to `1` prior to launching the window manager with `vglrun`, or pass an argument of `+wm` to `vglrun`. | See Section 18.1 for further information.
Mozilla Firefox | v93 and earlier | Linux | Set the `MOZ_DISABLE_CONTENT_SANDBOX` environment variable to `1` prior to launching the application with `vglrun` | The content sandbox in Firefox v93 and earlier prevents VirtualGL from opening an X display connection to the 3D X server, which causes WebGL tabs to crash when using the GLX back end. Some users have reported that disabling the content sandbox is also necessary when using the EGL back end with certain GPUs.
ANSYS Fluent (when launched from ANSYS Workbench) | v16 and later | Linux | Set the `FLUENT_WB_OPTIONAL_ARGS` environment variable to `-driver opengl` and the `CORTEX_PRE` environment variable to `/opt/VirtualGL/bin/vglrun`. | If these environment variables are not set, Fluent will use software OpenGL when launched from ANSYS Workbench.
Heretic II | All | Linux | `vglrun heretic2 +set vid_ref glx` |
ANSYS HFSS, ANSYS ICEM CFD, Roxar RMS | All | Linux | Set the `VGL_SPOILLAST` environment variable to `0` prior to launching the application with `vglrun` | These applications draw node highlighting and/or rubber banding directly to the front buffer. In order for these front buffer operations to be displayed properly, it is necessary to use the “spoil first” frame spoiling algorithm whenever the application calls `glFlush()`. See Section 18.1 for further information.
Intel OpenCL ICD | All | Linux | `vglrun -ld path-to-Intel-OpenCL-libs application` | The Intel OpenCL installable client driver (ICD) is linked with a run-time library search path (rpath) of `$ORIGIN`, which would normally have the same effect as adding the directory in which the ICD is installed (default: /opt/intel/opencl/lib64 on 64-bit systems) to `LD_LIBRARY_PATH`. However, when VirtualGL is interposing the `dlopen()` function (which it does by default), this causes the actual `dlopen()` system calls to come from libdlfaker.so, so `$ORIGIN` will resolve to the directory in which the VirtualGL faker libraries are installed. This causes the `dlopen()` calls within the Intel ICD to fail, and because the ICD apparently does not check the return value of those calls, a segfault occurs. The workaround is simply to add the Intel ICD library path to `LD_LIBRARY_PATH`, which is most easily accomplished with `vglrun -ld`.
Mathematica | v7 | Linux | Set the `VGL_ALLOWINDIRECT` environment variable to `1` prior to launching the application with `vglrun`. Note that `VGL_ALLOWINDIRECT` requires the GLX back end. | Mathematica 7 will not draw the axis numbers on 3D charts correctly unless it is allowed to create an indirect OpenGL context. See Section 18.1 for further information.
MATLAB | R2024b and earlier | Linux | `vglrun /usr/local/MATLAB/version/bin/matlab \` `-nosoftwareopengl` | MATLAB will automatically use its built-in (unaccelerated) OpenGL implementation if it detects that it is running in a remote display environment. More specifically, it will always enable software OpenGL if the X server has an X extension called `VNC-EXTENSION`, which is the case with TurboVNC, TigerVNC, and RealVNC.
MATLAB | R2025a and later | Linux | Set the `VGL_CHROMEHACK` environment variable to `1`, then launch the application with `vglrun /usr/local/MATLAB/version/bin/matlab \` `-nosoftwareopengl` | MATLAB R2025a and later use the Chromium Embedded Framework (CEF), so `VGL_CHROMEHACK` is necessary to work around Chrome’s X visual selection mechanism. (Refer to the application recipe for Chrome v129 and later.) When using nVidia’s proprietary Vulkan driver, which does something VirtualGL-like in an X proxy environment, or when using an X proxy that implements the DRI3 X extension, you can also enable GPU acceleration by setting the `MW_CEF_STARTUP_OPTIONS` environment variable to `--use-angle=vulkan` prior to launching MATLAB. No additional arguments or environment variables are necessary in this case, nor is it necessary to launch the application with `vglrun`.
PyTorch | All | Linux | `vglrun -ld path-to-PyTorch-libs application` or `vglrun -nodl application` | The PyTorch module and its dependency libraries are linked with a run-time library search path (rpath) of `$ORIGIN`, which would normally have the same effect as adding the directory in which the module is installed (for instance, /usr/local/lib64/python3.9/site-packages/torch/lib) to `LD_LIBRARY_PATH`. However, when VirtualGL is interposing the `dlopen()` function (which it does by default), this causes the actual `dlopen()` system calls to come from libdlfaker.so, so `$ORIGIN` will resolve to the directory in which the VirtualGL faker libraries are installed. This causes the `dlopen()` calls within the PyTorch module to fail. The workaround is to add the PyTorch module path to `LD_LIBRARY_PATH`, which is most easily accomplished with `vglrun -ld`, or to disable VirtualGL’s `dlopen()` interposer. If you are using CUDA with PyTorch, then it may also be necessary to add one or more of the CUDA module paths (for instance, /usr/local/lib64/python3.9/site-packages/nvidia/cudnn/lib) to `LD_LIBRARY_PATH`.
Siemens Simcenter STAR-CCM+ | v19.xx | Linux | `vglrun path-to-STAR-CCM+-scripts/starccm+ \` `-clientldpreload libdlfaker.so:libvglfaker.so` | Failing to pass the aforementioned arguments to the STAR-CCM+ startup script prevents VirtualGL from being loaded into the application, resulting in unaccelerated OpenGL and error messages such as “ERROR: in isDisplayExcluded– XX: Unexpected NULL condition”
Tecplot 360 | 2011 and earlier | Linux | Set the `VGL_GLFLUSHTRIGGER` environment variable to `0` prior to launching the application with `vglrun` | When running Tecplot 360 with VirtualGL in a high-performance X proxy, flashing artifacts will be produced when the user zooms/pans/rotates the scene, unless VirtualGL is instructed not to use `glFlush()` as a frame trigger. This has been fixed in Tecplot 2012 and later. See Section 18.1 for further information.
Unity Hub / Unity Editor | v3.7.0 / v2021.X and later | Linux | `vglrun unityhub --use-gl=egl` | The `--use-gl=egl` option forces Unity to use desktop OpenGL rather than ANGLE, which works around an issue whereby, when using ANGLE, Unity assumes that every X visual has an EGL framebuffer configuration associated with it.

# 15 Advanced OpenGL Features

## 15.1 Stereographic Rendering

Stereographic rendering is a feature of OpenGL that creates separate rendering buffers for the left and right eyes and allows a 3D application to render a different image into each buffer. How the rendered stereo frames are subsequently displayed depends on the particulars of the GPU and the user’s environment. VirtualGL can support stereographic applications in one of two ways: (1) by sending the stereo image pairs to the VirtualGL Client to be displayed in stereo by the client’s GPU, or (2) by combining each stereo image pair into a single image that can be viewed with traditional anaglyphic 3D glasses or a passive stereo system, such as a 3D TV.

### 15.1.1 Quad-Buffered Stereo

The name “quad-buffered stereo” refers to the fact that OpenGL uses four buffers (left front, right front, left back, and right back) to support stereographic rendering with double buffering. GPUs with quad-buffered stereo capabilities generally provide some sort of synchronization signal that can be used to control various types of active stereo 3D glasses. Some also support “passive stereo”, which requires displaying the left and right eye buffers to different monitor outputs. VirtualGL supports quad-buffered stereo by rendering the stereo images on the server and sending the image pairs across the network to be displayed on the client.

In most cases, VirtualGL does not require that a GPU be present in the client. However, a GPU is required to display stereo image pairs, so one must be present in any client that will use VirtualGL’s quad-buffered stereo feature. Since the GPU is only being used to draw images, it need not necessarily be a high-end GPU. Generally, the least expensive GPU that has stereo capabilities will work fine in the client. If using the GLX back end, the VirtualGL server must also have a GPU that supports stereo, since this is the only way that VirtualGL can create a stereo off-screen buffer.

When a 3D application tries to render something in stereo, VirtualGL will default to using quad-buffered stereo rendering if the 2D X server supports OpenGL and has stereo visuals available (not currently supported in Cygwin/X.) Otherwise, VirtualGL will fall back to using anaglyphic stereo (see below.) It is usually necessary to explicitly enable stereo in the graphics driver configuration for both the client and, if using the GLX back end, the VirtualGL server. The Troubleshooting section below lists a way to verify that both the 3D X server and the 2D X server have stereo visuals available.

In quad-buffered mode, VirtualGL reads back both the left and right eye buffers on the server and sends the contents as a pair of compressed images to the VirtualGL Client. The VirtualGL Client then decompresses both images and draws them as a single stereo frame to the 2D X server using `glDrawPixels()`. It should thus be no surprise that enabling quad-buffered stereo in VirtualGL decreases performance by 50% or more and uses twice the network bandwidth to maintain the same frame rate as mono.

Quad-buffered stereo requires the VGL Transport. Attempting to enable it with any other image transport will cause VGL to fall back to anaglyphic stereo mode.

### 15.1.2 Anaglyphic Stereo

Anaglyphic stereo is the type of stereographic display used by old 3D movies. It typically relies on a set of 3D glasses consisting of red transparency film over the left eye and cyan transparency film over the right eye, although green/magenta and blue/yellow schemes can be used as well. To generate a 3D anaglyph, one color channel from the left eye buffer is combined with the other two color channels from the right eye buffer, thus allowing a monographic frame to contain stereo data. For instance, in the case of red/cyan, the red channel is taken from the left eye buffer, and the green and blue channels are taken from the right eye buffer. From the point of view of VirtualGL, an anaglyphic rendered frame is the same as a monographic rendered frame, so anaglyphic frames can be sent using any image transport to any type of client, regardless of the client’s capabilities.

VirtualGL uses anaglyphic stereo if it detects that a 3D application has rendered something in stereo but quad-buffered stereo is not available, either because the client doesn’t support it or because a transport other than the VGL Transport is being used. Anaglyphic stereo provides a cheap and easy way to view stereographic applications in X proxies and on clients that do not support quad-buffered stereo. Additionally, anaglyphic stereo performs much faster than quad-buffered stereo, since it does not require sending twice the data to the client.

As with quad-buffered stereo, anaglyphic stereo requires that the VirtualGL server have stereo rendering capabilities if using the GLX back end. However, anaglyphic stereo does not require any 3D rendering capabilities (stereo or otherwise) on the client.

### 15.1.3 Passive Stereo

As with anaglyphic stereo, passive stereo combines a stereographic image pair into a single image (a “stereogram”), and thus it can be used with any image transport. However, unlike anaglyphic stereo, passive stereo must be used with specific display hardware, such as a 3D TV or monitor, that decodes the left and right eye images from the stereogram and sends them separately to a pair of 3D glasses. (Typically, this is accomplished by way of polarization.)

VirtualGL supports three methods of encoding stereograms:

Interleaved
  The even rows of the stereogram are taken from the left eye image, and the odd rows are taken from the right eye image.
Top/Bottom
  The top half of the stereogram is taken from the left eye image, and the bottom half is taken from the right eye image. Both halves are subsampled 2X vertically.
Side-by-Side
  The left half of the stereogram is taken from the left eye image, and the right half is taken from the right eye image. Both halves are subsampled 2X horizontally.

Most 3D TVs/monitors can be configured to decode at least one of these types of stereograms. In order for this to work, however, the 3D drawing area must be full-screen.

### 15.1.4 Selecting a Stereo Mode

A particular stereo mode can be selected by setting the `VGL_STEREO` environment variable or by using the `-st` argument to `vglrun`. See Section 18.1 for more details.

## 15.2 Troubleshooting

VirtualGL includes a modified version of `glxinfo` that can be used to determine whether or not the 2D and 3D X servers have stereo visuals enabled.

Run the following command sequence on the VirtualGL server to determine whether the 3D X server has a suitable visual for stereographic rendering:

    xauth merge /etc/opt/VirtualGL/vgl_xauth_key
    /opt/VirtualGL/bin/glxinfo -display :n -c -v

(where `n` is the display number of the 3D X server.) One or more of the visuals should say `stereo=1` and should list `Pbuffer` as one of the `Drawable Types`.

Run the following command sequence on the VirtualGL server to determine whether the 2D X server has a suitable visual for stereographic rendering.

    /opt/VirtualGL/bin/glxinfo -v

In order to use stereo, one or more of the visuals should say `stereo=1`.

# 16 Performance Measurement

## 16.1 VirtualGL’s Built-In Profiling System

The easiest way to uncover bottlenecks in VirtualGL’s image pipeline is to set the `VGL_PROFILE` environment variable to `1` on both server and client. (Passing an argument of `+pr` to `vglrun` on the server has the same effect.) This will cause VirtualGL to measure and report the throughput of various stages in the pipeline. For example, here are some measurements from a dual Pentium 4 server communicating with a Pentium III client on a 100-megabit LAN:

Server

      Readback   - 43.27 Mpixels/sec - 34.60 fps
      Compress 0 - 33.56 Mpixels/sec - 26.84 fps
      Total      -  8.02 Mpixels/sec -  6.41 fps - 10.19 Mbits/sec (18.9:1)

Client

      Decompress - 10.35 Mpixels/sec -  8.28 fps
      Blit       - 35.75 Mpixels/sec - 28.59 fps
      Total      -  8.00 Mpixels/sec -  6.40 fps - 10.18 Mbits/sec (18.9:1)

The total throughput of the pipeline is 8.0 Megapixels/sec, or 6.4 frames/sec, indicating that our frame is 8.0 / 6.4 = 1.25 Megapixels in size (a little less than 1280 x 1024 pixels.) The readback and compress stages, which occur in parallel on the server, are obviously not slowing things down, and we’re only using 1/10 of our available network bandwidth. Looking at the client, however, we discover that its slow decompression speed (10.35 Megapixels/second) is the primary bottleneck. Decompression and blitting on the client cannot be done in parallel, so the aggregate performance is the harmonic mean of the decompression and blitting rates: [1/ (1/10.35 + 1/35.75)] = 8.0 Mpixels/sec. In this case, we could improve the performance of the whole system by simply using a client with a faster CPU.

This example is meant to demonstrate how the client can sometimes be the primary impediment to VirtualGL’s end-to-end performance. Using “modern” hardware in both the server and client, VirtualGL can easily stream 50+ Megapixels/sec across a LAN, as of this writing.

## 16.2 Frame Spoiling

By default, VirtualGL will only transport a frame if the image transport is ready to receive it. If VirtualGL detects that the 3D application has finished rendering a new frame but there are already frames waiting in the queue to be transported, then those untransported frames are dropped (“spoiled”), and the new frame is promoted to the head of the queue. This prevents a backlog of frames on the server, which would cause a perceptible delay in the responsiveness of interactive 3D applications. However, when running non-interactive 3D applications (particularly benchmarks), frame spoiling should always be disabled. With frame spoiling disabled, the server will render frames only as quickly as VirtualGL can transport those frames, which will conserve server resources as well as allow OpenGL benchmarks to accurately measure the end-to-end performance of VirtualGL. With frame spoiling enabled, OpenGL benchmarks will report meaningless data, since the rate at which the server can render frames is decoupled from the rate at which VirtualGL can transport those frames.

In most X proxies (including VNC), there is effectively another layer of frame spoiling, since the rate at which the X proxy can send frames to the client is decoupled from the rate at which VirtualGL can draw rendered frames into the X proxy. Thus, even if frame spoiling is disabled in VirtualGL, OpenGL benchmarks will still report inaccurate data if they are run in such X proxies. TCBench, described below, provides a limited solution to this problem.

To disable frame spoiling, set the `VGL_SPOIL` environment variable to `0` on the VirtualGL server or pass an argument of `-sp` to `vglrun`. See Section 18.1 for further information.

## 16.3 VirtualGL Diagnostic Tools

VirtualGL includes several tools that can be useful for diagnosing performance problems with the system.

### NetTest

NetTest is a network benchmark that uses the same network I/O classes as VirtualGL. It can be used to test the latency and throughput of any TCP/IP connection. `nettest` is installed in /opt/VirtualGL/bin by default. For Windows users, a native Windows version of NetTest is included in the VirtualGL-Utils package, which is distributed alongside VirtualGL.

To use NetTest, first start up the NetTest server on one end of the connection:

    nettest -server

Next, start the NetTest client on the other end of the connection:

    nettest -client server

Replace `server` with the hostname or IP address of the machine on which the NetTest server is running.

The NetTest client will produce output similar to the following:

    TCP transfer performance between localhost and server:

    Transfer size  1/2 Round-Trip      Throughput      Throughput
    (bytes)                (msec)    (MBytes/sec)     (Mbits/sec)
    1                    0.093402        0.010210        0.085651
    2                    0.087308        0.021846        0.183259
    4                    0.087504        0.043594        0.365697
    8                    0.088105        0.086595        0.726409
    16                   0.090090        0.169373        1.420804
    32                   0.093893        0.325026        2.726514
    64                   0.102289        0.596693        5.005424
    128                  0.118493        1.030190        8.641863
    256                  0.146603        1.665318       13.969704
    512                  0.205092        2.380790       19.971514
    1024                 0.325896        2.996542       25.136815
    2048                 0.476611        4.097946       34.376065
    4096                 0.639502        6.108265       51.239840
    8192                 1.033596        7.558565       63.405839
    16384                1.706110        9.158259       76.825049
    32768                3.089896       10.113608       84.839091
    65536                5.909509       10.576174       88.719379
    131072              11.453894       10.913319       91.547558
    262144              22.616389       11.053931       92.727094
    524288              44.882406       11.140223       93.450962
    1048576             89.440702       11.180592       93.789603
    2097152            178.536997       11.202160       93.970529
    4194304            356.754396       11.212195       94.054712

We can see that the throughput peaks at about 94 megabits/sec, which is pretty good for a 100-megabit connection. We can also see that, for small transfer sizes, the round-trip time is dominated by latency. The “latency” is the same thing as the one-way (1/2 round-trip) transit time for a zero-byte packet, which is about 93 microseconds in this case.

### CPUstat

CPUstat is available only for Linux and is installed in the same place as NetTest (/opt/VirtualGL/bin by default.) It measures the average, minimum, and peak usage for all CPU cores combined and for each CPU core individually. On Windows, this same functionality is provided in the Windows Performance Monitor, which is part of the operating system. On Solaris, the same data can be obtained using the `vmstat` program.

CPUstat measures the CPU usage over a given sample period (a few seconds) and continuously reports how much each CPU core was utilized since the last sample period. Output for a particular sample looks something like this:

    ALL :  51.0 (Usr= 47.5 Nice=  0.0 Sys=  3.5) / Min= 47.4 Max= 52.8 Avg= 50.8
    cpu0:  20.5 (Usr= 19.5 Nice=  0.0 Sys=  1.0) / Min= 19.4 Max= 88.6 Avg= 45.7
    cpu1:  81.5 (Usr= 75.5 Nice=  0.0 Sys=  6.0) / Min= 16.6 Max= 83.5 Avg= 56.3

The first column indicates what percentage of time the CPU core was active since the last sample period. This is then broken down into what percentage of time the CPU core spent running user, nice, and system/kernel code. `ALL` indicates the average utilization across all CPU cores since the last sample period. `Min`, `Max`, and `Avg` indicate a running minimum, maximum, and average of all samples since CPUstat was started.

Generally, if a 3D application’s CPU usage is fairly steady, then you can run CPUstat for a bit and wait for the `Max` and `Avg` values in the `ALL` category to stabilize, and that will tell you the application’s peak and average CPU utilization.

### TCBench

TCBench was born out of the need to compare VirtualGL’s performance to that of other thin client software, some of which had frame spoiling features that could not be disabled. TCBench measures the frame rate of a thin client system as seen from the client’s point of view. It does this by attaching to one of the windows on the client and continuously reading back a small area at the center of the window. While this may seem to be a somewhat non-rigorous test, experiments have shown that, if care is taken to ensure that the 3D application is updating the center of the window with every frame (such as in a spin animation), TCBench can produce quite accurate results. It has been sanity checked with VirtualGL’s internal profiling mechanism and with a variety of system-specific techniques, such as monitoring redraw events on the client’s windowing system.

TCBench is installed in /opt/VirtualGL/bin by default. For Windows users, a native Windows version of TCBench is included in the VirtualGL-Utils package, which is distributed alongside VirtualGL. Run `tcbench` from the command line, and it will prompt you to click in the window you want to benchmark. That window should already have an automated animation of some sort running before you launch TCBench. Note that GLXSpheres (see below) is an ideal benchmark to use with TCBench, since GLXSpheres draws a new sphere to the center of its window every time it renders a frame.

    tcbench -?

lists the relevant command-line arguments, which can be used to adjust the benchmark time, the sampling rate, and the x and y offset of the sampling area within the window.

### GLXSpheres

GLXSpheres is a benchmark that produces very similar images to nVidia’s (long-discontinued) SphereMark benchmark. In the early days of VirtualGL’s existence, it was discovered (quite by accident) that SphereMark was a pretty good test of VirtualGL’s end-to-end performance, because that benchmark generated images with about the same proportion of solid color, and similar frequency components, to the images generated by volume visualization applications.

Thus, the goal of GLXSpheres was to create an open source Un*x version of SphereMark (SphereMark was for Windows only) completely from scratch. GLXSpheres does not use any code from the original benchmark, but it does attempt to mimic the visual output of the original as closely as possible. GLXSpheres lacks some of the advanced rendering features of the original, such as the ability to use vertex arrays, but since GLXSpheres was primarily designed as a benchmark for VirtualGL, display lists are more than fast enough for that purpose.

GLXSpheres has some additional modes that its predecessor lacked, modes that are designed specifically to test the performance of various VirtualGL features:

Stereographic rendering (`glxspheres -s`)
Immediate mode rendering (`glxspheres -m`)
  Want to really see the benefit of VirtualGL? Run `glxspheres -m` over a remote X connection, then run `vglrun -sp glxspheres -m` over the same connection and compare. Immediate mode does not use display lists, so when immediate-mode OpenGL is rendered indirectly (over a remote X connection), this causes every OpenGL command to be sent as a separate network request to the X server … with every frame. Many 3D applications do not use display lists– because the geometry they are rendering is dynamic, or for other reasons– so this test models how such applications might perform when displayed remotely without VirtualGL.
Interactive mode (`glxspheres -i`)
  In interactive mode, GLXSpheres will wait to render a frame until it receives a mouse event. Continuously dragging the mouse in the window should produce a steady frame rate, and this frame rate is a reasonable model of the frame rate that you can achieve when running interactive 3D applications with VirtualGL. Comparing this interactive frame rate (`vglrun glxspheres -i`) with the non-interactive frame rate (`vglrun -sp glxspheres`) allows you to quantify the effect of network latency on the performance of interactive applications in a VirtualGL environment.

GLXSpheres is installed in /opt/VirtualGL/bin by default. 64-bit VirtualGL builds name this program `glxspheres64` so as to allow both a 64-bit and a 32-bit version of GLXSpheres to be installed on the same system.

This version of VirtualGL also provides an EGL/X11 equivalent of GLXSpheres (EGLXSpheres), which works identically to GLXSpheres except for the absense of modes (including stereographic rendering) that the EGL API does not support.

# 17 The VirtualGL Configuration Dialog

Several of VirtualGL’s operational parameters can be changed on the fly once a 3D application has been launched. This is accomplished by using the VirtualGL Configuration dialog, which can be popped up by holding down the Ctrl and Shift keys and pressing the F9 key while any one of the 3D application’s windows is active. This displays the following dialog box:

You can use this dialog to adjust various image compression and display parameters in VirtualGL. Changes are communicated immediately to VirtualGL.

Image Compression (Transport)
  This is a drop-down menu with the following options:
  None (X11 Transport) : equivalent to setting `VGL_COMPRESS=proxy`. This option can be activated at any time, regardless of which transport was active when VirtualGL started.
  JPEG (VGL Transport) : equivalent to setting `VGL_COMPRESS=jpeg`. This option is only available if the VGL Transport was active when VirtualGL started.
  RGB (VGL Transport) : equivalent to setting `VGL_COMPRESS=rgb`. This option is only available if the VGL Transport was active when VirtualGL started.
  YUV (XV Transport) : equivalent to setting `VGL_COMPRESS=xv`. This option is only available if the 2D X server has the X Video extension and the X Video implementation supports the YUV420P (AKA “I420”) image format.
  YUV (VGL Transport) : equivalent to setting `VGL_COMPRESS=yuv`. This option is only available if the 2D X server has the X Video extension, the X Video implementation supports the YUV420P (AKA “I420”) image format, and the VGL Transport was active when VirtualGL started.
  See Section 18.1 for more information about the `VGL_COMPRESS` configuration option.

  If an image transport plugin is loaded, then this menu’s name changes to “Image Compression”, and it has options “0” through “10”.

Chrominance Subsampling
  This drop-down menu is active only when using JPEG compression or an image transport plugin. It has the following options:
  Grayscale : equivalent to setting `VGL_SUBSAMP=gray`
  1X : equivalent to setting `VGL_SUBSAMP=1x`
  2X : equivalent to setting `VGL_SUBSAMP=2x`
  4X : equivalent to setting `VGL_SUBSAMP=4x`
  See Section 18.1 for more information about the `VGL_SUBSAMP` configuration option.

  If an image transport plugin is loaded, then this menu has two additional options, “8X” and “16X”.

JPEG Image Quality
  This slider gadget is active only when using JPEG compression or an image transport plugin. It is the equivalent of setting `VGL_QUAL`. See Section 18.1 for more information about the `VGL_QUAL` configuration option.

  If an image transport plugin is loaded, then this gadget’s name changes to “Image Quality”.

Connection Profile
  This drop-down menu is active only if the VGL Transport was active when VirtualGL started. It has the following options:
  Low Qual (Low-Bandwidth Network) : Sets the image compression type to JPEG (VGL Transport), the chrominance subsampling to 4X, and the JPEG image quality to 30.
  Medium Qual : Sets the image compression type to JPEG (VGL Transport), the chrominance subsampling to 2X, and the JPEG image quality to 80.
  High Qual (High-Bandwidth Network) : Sets the image compression type to JPEG (VGL Transport), the chrominance subsampling to 1X, and the JPEG image quality to 95.
Gamma Correction Factor
  This floating point input gadget is the equivalent of setting `VGL_GAMMA`. This enables VirtualGL’s internal gamma correction system with the specified gamma correction factor. See Section 18.1 for more information about the `VGL_GAMMA` configuration option.
Frame Spoiling
  This toggle button is the equivalent of setting `VGL_SPOIL`. See Sections 16.2 and 18.1 for more information about the `VGL_SPOIL` configuration option.
Interframe Comparison
  This toggle button is the equivalent of setting `VGL_INTERFRAME`. See Section 18.1 for more information about the `VGL_INTERFRAME` configuration option.
Stereographic Rendering Method
  This drop-down menu has the following options:
  Send Left Eye Only : equivalent to setting `VGL_STEREO=left`.
  Send Right Eye Only : equivalent to setting `VGL_STEREO=right`
  Quad-Buffered (if available) : equivalent to setting `VGL_STEREO=quad`
  Anaglyphic (Red/Cyan) : equivalent to setting `VGL_STEREO=rc`
  Anaglyphic (Green/Magenta) : equivalent to setting `VGL_STEREO=gm`
  Anaglyphic (Blue/Yellow) : equivalent to setting `VGL_STEREO=by`
  Passive (Interleaved) : equivalent to setting `VGL_STEREO=i`
  Passive (Top/Bottom) : equivalent to setting `VGL_STEREO=tb`
  Passive (Side-by-Side) : equivalent to setting `VGL_STEREO=ss`
  See Section 18.1 for more information about the `VGL_STEREO` configuration option.
Limit Frames/second
  This floating point input gadget is the equivalent of setting `VGL_FPS`. See Section 18.1 for more information about the `VGL_FPS` configuration option.

You can set the `VGL_GUI` environment variable to change the key sequence used to pop up the VirtualGL Configuration dialog. If the default of `ctrl-shift-f9` is not suitable, then set `VGL_GUI` to any combination of `ctrl`, `shift`, `alt`, and one of `f1`, `f2`, ..., `f12`. (These are not case sensitive.) For example:

    export VGL_GUI=ctrl-f9

will cause the dialog box to pop up whenever Ctrl and F9 are pressed.

To disable the VirtualGL dialog altogether, set `VGL_GUI` to `none`.

VirtualGL monitors the 3D application’s X event loop to determine whenever a particular key sequence has been pressed. If a 3D application is not monitoring key press events in its X event loop, then the VirtualGL Configuration dialog might not pop up at all. There is unfortunately no workaround for this, but it should be a rare occurrence.

# 18 Advanced Configuration

## 18.1 Faker Settings

You can control the operation of the VirtualGL faker libraries in four different ways. Each method of configuration takes precedence over the previous method:

1. Setting a configuration environment variable globally (for instance, in /etc/profile)
2. Setting a configuration environment variable on a per-user basis (for instance, in ~/.bashrc)
3. Setting a configuration environment variable only for the current shell session (for instance, `export VGL_XXX=whatever`)
4. Passing a configuration option as an argument to `vglrun`. This effectively overrides any previous environment variable setting corresponding to that configuration option.

Image transport plugins are free to handle or ignore any configuration option as they see fit.

Environment Variable | `VGL_ALLOWINDIRECT = 0 | 1`
Summary | When using the GLX back end, allow 3D applications to request an indirect OpenGL context
Image Transports | All
Default Value | `0` (all OpenGL contexts use direct rendering)

Description
  Normally, when VirtualGL maps an off-screen buffer to a window and establishes an OpenGL rendering context with the off-screen buffer, it forces direct rendering to be used with this context. Some 3D applications erroneously try to create indirect OpenGL contexts because they detect that the X server is on a different machine and assume that the 3D rendering commands will be sent over the network. Thus, VirtualGL normally forces all contexts to be direct in order to prevent severe readback performance degradation with such applications. (Even on modern GPUs, and even when the connection to the 3D X server is local, `glReadPixels()` can perform very slowly if an indirect OpenGL context is used.)
  However, some 3D applications intentionally try to create indirect contexts so that these contexts can be shared, and those applications may not work properly when the contexts are forced to be direct. For such applications, setting `VGL_ALLOWINDIRECT` to `1` will cause VirtualGL to honor the application’s request for an indirect OpenGL context.

  EGL does not support indirect OpenGL contexts, so this option requires the GLX back end.

Environment Variable | `VGL_CLIENT = {c}`
`vglrun` argument | `-cl {c}`
Summary | `{c}` = the hostname or IP address of the client
Image Transports | VGL, Custom (if supported)
Default Value | Automatically set by `vglconnect` or `vglrun`

Description
  When using the VGL Transport, `VGL_CLIENT` should be set to the hostname or IP address of the machine on which the VirtualGL Client is running. Normally, `VGL_CLIENT` is set automatically when executing `vglconnect` or `vglrun`, so don’t override it unless you know what you’re doing.

Environment Variable | `VGL_COMPRESS = proxy | jpeg | rgb | xv | yuv`
`vglrun` argument | `-c proxy | jpeg | rgb | xv | yuv`
Summary | Set image transport and image compression type
Image Transports | All
Default Value | (See description)

Description
  `proxy` = Send rendered frames in uncompressed form using the X11 Transport. This is useful when displaying to a 2D X server or X proxy on the VirtualGL server (see Section 9.1.)
  `jpeg` = Compress rendered frames using JPEG and send them using the VGL Transport. This is useful when displaying to a 2D X server on a machine other than the VirtualGL server (see Chapter 8.)
  `rgb` = Encode rendered frames as uncompressed RGB and send them using the VGL Transport. This is useful when displaying to a 2D X server or X proxy on a machine that is connected to the VirtualGL server by a very fast network (see Section 9.2.)
  `xv` = Encode rendered frames as YUV420P (planar YUV with 4X chrominance subsampling) and display them to the 2D X server using the XV Transport. This transport is designed for use with X proxies that support the X Video extension (see Chapter 10.)
  `yuv` = Encode rendered frames as YUV420P, send them using the VGL Transport, and display them to the 2D X server using the X Video extension. This greatly reduces the CPU usage on both server and client and uses only about half the network bandwidth as RGB, but the use of 4X chrominance subsampling does produce some visible artifacts (see Chapter 10.)
  If `VGL_COMPRESS` is not specified, then the default is set as follows:
  If the `DISPLAY` environment variable begins with `:` or `unix:`, then VirtualGL assumes that the 2D X server is on the same machine and uses `proxy` compression by default.
  If VirtualGL detects that the 2D X server is on a different machine, then it uses `jpeg` compression by default.

  If an image transport plugin is being used, then you can set `VGL_COMPRESS` to any numeric value >= 0 (Default value = `0`.) The plugin can choose to respond to this value as it sees fit.

Environment Variable | `VGL_DISPLAY = {d}`
`vglrun` argument | `-d {d}`
Summary | `{d}` = the X display/screen or EGL device to use for 3D rendering
Image Transports | All
Default Value | `:0` or `:1024`

Description
  If the VirtualGL server has multiple GPUs, then you can use this option to specify which GPU should be used for 3D rendering. For instance, setting `VGL_DISPLAY` to (or invoking `vglrun -d` with) `:0.1` would cause VirtualGL to use the GLX back end and redirect all of the OpenGL rendering from the 3D application to a GPU attached to Screen 1 on X Display :0. Setting `VGL_DISPLAY` to (or invoking `vglrun -d` with) a DRI device path (such as `/dev/dri/card0`) or an EGL device ID (such as `egl0`) would cause VirtualGL to use the EGL back end and redirect all of the OpenGL rendering from the 3D application to the specified EGL device. `/opt/VirtualGL/bin/eglinfo -e` lists all valid EGL device IDs and their associated DRI device paths.

  `vglrun` automatically sets `VGL_DISPLAY` to `:1024` if `VGL_DISPLAY` is unset, X Display :1024 exists, and X Display :0 does not exist. This allows VirtualGL to use the GDM greeter’s Xwayland instance as a 3D X server.

Environment Variable | `VGL_EGLLIB = {l}`
Summary | `{l}` = the location of an alternate EGL library
Image Transports | All

Description
  Normally VirtualGL will call the `eglGetProcAddress()` function in the EGL library against which it or the 3D application was linked (usually libEGL.so.1, in the system library path), and VGL will use that function to load any other “real” EGL functions that it needs to call (“real” as opposed to the “fake”, or “interposed”, versions of those functions that VirtualGL provides, which often modify the arguments or perform other operations before calling the “real” functions.) You can use the `VGL_EGLLIB` environment variable to specify the path of a dynamic library from which VirtualGL should load “real” EGL functions.
  You shouldn’t need to change this unless something doesn’t work. However, setting this environment variable is potentially useful if one wishes to insert another EGL interposer between VirtualGL and the system’s EGL library.

Environment Variable | `VGL_EXCLUDE = {d1}[,{d2},{d3},...]`
Summary | `{d1}[,{d2},{d3},...]` = A comma-separated list of X displays/screens for which the VirtualGL Faker should be bypassed
Image Transports | All
Default Value | None

Description
  In certain parallel rendering applications, it may be desirable to directly access multiple GPUs from within worker threads and to disable VirtualGL for those threads while leaving VirtualGL enabled for the main thread, which displays the final rendered result from all of the workers.
  The `VGL_EXCLUDE` environment variable specifies a list of X display names (for instance, `:0.1`) for which VirtualGL should not interpose any X11, GLX, EGL, OpenGL, XCB, or OpenCL calls. In other words, VirtualGL treats these displays as 3D X servers instead of 2D X servers and does not attempt to redirect 3D rendering away from them. When an X display connection is opened using `XOpenDisplay()`, VirtualGL checks if the display name appears in the exclude list, and if so, all subsequent X11, GLX, EGL, OpenGL, and XCB calls intended for that display are allowed to pass through unimpeded. This variable is re-checked every time `XOpenDisplay()` is called, so it can be set dynamically from within a 3D application.

Environment Variable | `VGL_EXITFUNCTION = exit | _exit | abort`
Summary | Specify the function that the VirtualGL Faker should call when a non-recoverable error occurs
Image Transports | All
Default Value | `exit`

Description
  The VirtualGL Faker normally calls `exit()` when a non-recoverable error occurs. However, that may not be appropriate for multithreaded applications that statically instantiate objects at the global scope, because calling `exit()` can cause global objects to be cleaned up before the threads that use them are terminated. Calling `_exit()` instead of `exit()` causes the application to exit immediately without cleaning up global objects, and calling `abort()` instead of `exit()` allows a core dump to be obtained.

Environment Variable | `VGL_FAKEOPENCL = 0 | 1`
`vglrun` argument | `-ocl` / `+ocl`
Summary | Disable/enable OpenCL interposer
Image Transports | All
Default Value | Disabled

Description
  In order to support applications that use OpenCL/OpenGL interoperability functions, VirtualGL has to interpose the `clCreateContext()` function and modify its arguments before passing them to the “real” `clCreateContext()` function in libOpenCL. Since libOpenCL is not available on all platforms that VirtualGL supports, the OpenCL interposer is disabled by default.

Environment Variable | `VGL_FAKEXCB = 0 | 1`
`vglrun` argument | `-xcb` / `+xcb`
Summary | Disable/enable XCB interposer
Image Transports | All
Default Value | Enabled

Description
  Qt 5 uses XCB instead of Xlib to perform all non-OpenGL X11-related operations. Thus, in order to support Qt 5 applications, VirtualGL has to interpose enough of the XCB API to allow it to intercept window resize events and to make Qt 5 believe that a GLX extension is present, even when the 2D X server doesn’t support GLX. The XCB interposer in this release of VirtualGL should be non-intrusive, and thus it is enabled by default, and this option is provided only for troubleshooting purposes. You shouldn’t need to disable the XCB interposer unless unforeseen problems are encountered.

Environment Variable | `VGL_FORCEALPHA = 0 | 1`
Summary | Force the off-screen buffers used for 3D rendering to have an alpha channel
Image Transports | All
Default Value | `0` (honor the 3D application’s choice of visual attributes)

Description
  Normally, VirtualGL will create off-screen buffers whose attributes match those of the visuals requested by the 3D application. Setting `VGL_FORCEALPHA` to `1` causes VirtualGL to always create off-screen buffers with alpha channels. This means that a 32-bit-per-pixel (BGRA) off-screen buffer will be created if the application requests a 24-bit-per-pixel visual.
  The primary purpose of this option is to work around a limitation of certain consumer-grade GPUs whereby the pixel format requested by the framebuffer readback operation must match the pixel format of the off-screen buffer in order for pixel buffer objects (PBOs) to behave correctly. Since displaying to an X proxy typically requires VirtualGL to read back the framebuffer in BGRA format, enabling `VGL_FORCEALPHA` might be necessary in order to use PBO readback mode with the aforementioned GPUs. (As of this writing, nVidia GeForce adapters are known to require this.) See the `VGL_READBACK` option for further information.

Environment Variable | `VGL_FPS = {f}`
`vglrun` argument | `-fps {f}`
Summary | Limit the end-to-end frame rate to `{f}` frames/second, where `{f}` is a floating point number > 0.0
Image Transports | VGL, X11, XV, Custom (if supported)
Default Value | `0.0` (No limit)

Description
  This option prevents VirtualGL from transporting frames at a rate faster than the specified limit. It can be used, for instance, as a crude way to control network bandwidth or CPU usage in multi-user environments in which those resources are constrained.
  If frame spoiling is disabled, then setting `VGL_FPS` effectively limits the server’s 3D rendering frame rate as well.

Environment Variable | `VGL_GAMMA = {g}`
`vglrun` argument | `-gamma {g}`
Summary | `{g}` = gamma correction factor
Image Transports | All
Default Value | `1.00` (no gamma correction)

Description
  “Gamma” refers to the relationship between the intensity of light that your computer’s monitor is instructed to display and the intensity that it actually displays. The curve is an exponential curve of the form Y = XG, where X is between 0 and 1. G is called the “gamma” of the monitor. PC monitors and TVs usually have a gamma of around 2.2.
  Some of the math involved in 3D rendering assumes a linear gamma (G = 1.0), so technically speaking, 3D applications will not display with mathematical correctness unless the pixels are “gamma corrected” to counterbalance the non-linear response curve of the monitor. However, some systems do not have any form of built-in gamma correction, so the applications developed for such systems have usually been designed to display properly without gamma correction. Gamma correction involves passing pixels through a function of the form X = W1/G, where G is the “gamma correction factor” and should be equal to the gamma of the monitor. So, the final output is Y = XG = (W1/G)G = W, which describes a linear relationship between the intensity of the pixels drawn by the application and the intensity of the pixels displayed by the monitor.
  If `VGL_GAMMA` is set to an arbitrary floating point value, then VirtualGL will perform gamma correction on all of the rendered frames from the 3D application, using the specified value as the gamma correction factor. You can also specify a negative value to apply a “de-gamma” function. Specifying a gamma correction factor of G (where G < 0) is equivalent to specifying a gamma correction factor of -1/G.

Environment Variable | `VGL_GLFLUSHTRIGGER = 0 | 1`
Summary | Disable/enable using `glFlush()` as a frame trigger function
Default Value | Enabled

Description
  `glFlush()` is a sort of “asynchronous synchronization” command. It flushes the OpenGL command buffers, which generally has the effect of ensuring that the commands have been delivered to the GPU. However, unlike `glFinish()`, `glFlush()` does not wait until the commands have been rendered before it returns.
  The use of `glFlush()` can vary widely from application to application. When doing front buffer rendering, some 3D applications call `glFlush()` after each object is rendered. Some call it only at the end of the frame. Others call `glFlush()` much more often, even as frequently as every time a few primitives are rendered. This creates problems for VirtualGL, since it has to guess the application’s intent. Not all 3D applications that use front buffer rendering call `glFinish()` to signal the end of a frame, so VirtualGL cannot usually get away with ignoring `glFlush()`. However, some 3D applications call `glFlush()` so often that VirtualGL cannot get away with reading back/transporting a frame every time `glFlush()` is called, either. (See `VGL_SPOILLAST` for more information on how VirtualGL tries to handle this, under normal circumstances.)
  Some 3D applications use `glFlush()` very liberally and intend for it to be an intermediate rather than a final synchronization command. Such applications will call `glFinish()` after a sequence of `glFlush()` calls, so for those applications, using `glFlush()` as a frame trigger is a waste of resources and can sometimes create visual artifacts. (For instance, this can happen if the application clears the front buffer with a particular color, calls `glFlush()`, then clears it again with another color. We wouldn’t mention it if it hadn’t happened before.) For such applications, setting `VGL_GLFLUSHTRIGGER` to `0` should make them display properly with VirtualGL. See Application Recipes for a list of 3D applications that are known to require this.

Environment Variable | `VGL_GLLIB = {l}`
Summary | `{l}` = the location of an alternate OpenGL library
Image Transports | All

Description
  Normally VirtualGL will call the `glXGetProcAddress()` or `glXGetProcAddressARB()` function in the OpenGL library against which it or the 3D application was linked (usually libGL.so.1, in the system library path), and VGL will use that function to load any other “real” OpenGL or GLX functions that it needs to call (“real” as opposed to the “fake”, or “interposed”, versions of those functions that VirtualGL provides, which often modify the arguments or perform other operations before calling the “real” functions.) You can use the `VGL_GLLIB` environment variable to specify the path of a dynamic library from which VirtualGL should load “real” GLX and OpenGL functions.
  You shouldn’t need to change this unless something doesn’t work. However, setting this environment variable is potentially useful if one wishes to insert another OpenGL interposer between VirtualGL and the system’s OpenGL library.

Environment Variable | `VGL_GUI = {k}`
Summary | `{k}` = the key sequence used to pop up the VirtualGL Configuration dialog, or `none` to disable the dialog
Image Transports | All
Default Value | `ctrl-shift-f9`

Description
  VirtualGL normally monitors a 3D application’s X event queue and pops up the VirtualGL Configuration dialog whenever Ctrl-Shift-F9 is pressed. In the event that this interferes with a key sequence that the application is already using, then you can redefine the key sequence used to pop up the VirtualGL Configuration dialog by setting `VGL_GUI` to some combination of `shift`, `ctrl`, `alt`, and one of `f1`, `f2`, ..., `f12`. You can also set `VGL_GUI` to `none` to disable the configuration dialog altogether. See Chapter 17 for more details.

Environment Variable | `VGL_INTERFRAME = 0 | 1`
Summary | Disable or enable interframe comparison
Image Transports | VGL (JPEG, RGB), Custom (if supported)
Default Value | Enabled

Description
  The VGL Transport normally compares each rendered frame with the previous frame and sends only the portions of the frame that have changed. Setting `VGL_INTERFRAME` to `0` disables this behavior.
  This setting was introduced in order to work around a specific application interaction issue, but since a proper fix for that issue was introduced in VirtualGL 2.1.1, this option isn’t really useful anymore.

  When using the VGL Transport, interframe comparison is affected by the `VGL_TILESIZE` option

Environment Variable | `VGL_LOG = {l}`
Summary | Redirect all messages from VirtualGL to a log file specified by `{l}`
Image Transports | All
Default Value | Print all messages to stderr

Description
  Setting this environment variable to the pathname of a log file (or “stdout”) on the VirtualGL server will cause VirtualGL to redirect all of its messages (including profiling and trace output) to the specified log file rather than to stderr.

Environment Variable | `VGL_LOGO = 0 | 1`
Summary | Disable or enable the display of a VGL logo in the 3D window(s)
Image Transports | All
Default Value | Disabled

Description
  Setting `VGL_LOGO` to `1` will cause VirtualGL to add a small logo to the bottom right-hand corner of all of the rendered frames from the 3D application. This is meant as a debugging tool to allow users to determine whether or not VirtualGL is active.

Environment Variable | `VGL_NPROCS = {n}`
`vglrun` argument | `-np {n}`
Summary | `{n}` = the number of threads to use for compression/encoding
Image Transports | VGL (JPEG, RGB), Custom (if supported)
Default Value | `1`

Description
  The VGL Transport can use multiple threads to divide the task of compressing/encoding each rendered frame among multiple server CPU cores. This might speed up the overall throughput in rare circumstances in which the server CPU is significantly slower than the client CPU.
  VirtualGL will not allow more than 4 threads total to be used for compression, nor will it allow you to set this parameter to a value greater than the number of CPU cores in the system.

  When using the VGL Transport, multithreaded compression is affected by the `VGL_TILESIZE` option

Environment Variable | `VGL_OCLLIB = {l}`
Summary | `{l}` = the location of an alternate OpenCL library
Image Transports | All
Default Value | libOpenCL.so.1 in the system library path

Description
  Specifies the path of a dynamic library from which VirtualGL should load any “real” OpenCL functions that it needs to call (“real” as opposed to the “fake”, or “interposed”, versions of those functions that VirtualGL provides, which modify the arguments before calling the “real” functions.)
  You shouldn’t need to change this unless something doesn’t work.

Environment Variable | `VGL_PORT = {p}`
`vglrun` argument | `-p {p}`
Summary | `{p}` = the TCP port to use when connecting to the VirtualGL Client
Image Transports | VGL, Custom (if supported)
Default Value | Read from X property stored by VirtualGL Client

Description
  The connection port for the VGL Transport is normally determined by reading an X property that the VirtualGL Client stores on the 2D X server, so don’t override this unless you know what you’re doing.

Environment Variable | `VGL_PROFILE = 0 | 1`
`vglrun` argument | `-pr` / `+pr`
Summary | Disable/enable profiling output
Image Transports | VGL, X11, XV, Custom (if supported)
Default Value | Disabled

Description
  If profiling output is enabled, then VirtualGL will continuously benchmark itself and periodically print out the throughput of various stages in its image pipeline.
  See Chapter 16 for more details.

Environment Variable | `VGL_QUAL = {q}`
`vglrun` argument | `-q {q}`
Summary | `{q}` = the JPEG compression quality, 1 <= `{q}` <= 100
Image Transports | VGL (JPEG), Custom (if supported)
Default Value | `95`

Description
  In digital imaging, “frequency” refers to how quickly the color changes between light and dark as you move either horizontally or vertically in the image. Images with very sharp, bright features on a dark background, for instance, consist of both low-frequency and high-frequency components, whereas images with smooth transitions between neighboring pixels contain only low-frequency components. JPEG compression works by breaking down the image into its constituent frequencies and then throwing out the highest of these frequencies. The JPEG image “quality” determines which frequencies are thrown out. A JPEG quality of 1 throws out all but the lowest frequencies and thus produces a very impressionistic, but generally not very useful, compressed image. A JPEG quality of 100 retains all frequencies in the original image (but, due to roundoff errors, the compressed image is still not completely lossless.)
  Because the human eye usually cannot detect the highest frequencies in the image, and often because the image lacks those high-frequency elements to begin with, a sufficiently high JPEG quality setting can produce a “perceptually lossless” image. A “perceptually lossless” image contains a small amount of mathematical error when compared to the original image, but this error is so small that, under normal circumstances, human vision cannot detect it. The threshold quality level at which JPEG compression becomes perceptually lossless is different for each image, but experiments with various visual difference benchmarks (such as HDR-VDP) suggest that a JPEG quality of 95 is sufficient to guarantee perceptual losslessness for the types of 3D applications (volume visualization applications, in particular) in which image quality is critical. As with any benchmarks, Your Mileage May Vary. Those who are particularly paranoid about image quality can set the JPEG quality to 100 or use RGB encoding, but a fast network is required for both.

  If using an image transport plugin, then this setting need not necessarily correspond to JPEG image quality. The plugin can choose to respond to the `VGL_QUAL` option as it sees fit.

Environment Variable | `VGL_READBACK = none | pbo | sync`
Summary | Specify the method used by VirtualGL to read back the rendered frames from the GPU
Image Transports | All
Default Value | `pbo`

Description

- `none` = Do not read back the rendered frames at all. On rare occasions, it might be desirable to have VirtualGL redirect OpenGL rendering from an application’s window into an off-screen buffer but not automatically read back and transport the rendered frames. Some 3D applications have their own mechanisms for reading back the rendered frames, so setting `VGL_READBACK=none` disables VirtualGL’s readback mechanism and prevents duplication of effort.
  This option was developed initially to support running ParaView in parallel using MPI. ParaView/MPI normally uses MPI Processes 1 through N as rendering servers, each rendering a portion of the geometry into a separate window on a separate X display. ParaView reads back these server windows and composites the pixels into the main application window, which is controlled by MPI Process 0. By creating a script that passes a different value of `VGL_DISPLAY` and `VGL_READBACK` to each MPI process, it is possible to make all of the ParaView server processes render to off-screen buffers on different GPUs while preventing VirtualGL from displaying any pixels except those generated by Process 0.
- `pbo` = PBO readback mode. Attempt to use pixel buffer objects (PBOs) to read back the rendered frames from the GPU. A PBO is an opaque memory buffer managed by OpenGL, so it can be locked down for direct DMA transfers. This improves readback performance as well as makes the readback operation non-blocking. Because PBOs are managed buffers, VirtualGL has to perform an additional memory copy to transfer a rendered frame out of the PBO and into the image transport’s buffer. However, on high-end GPUs, PBO readback mode will still generally perform better than synchronous readback mode, even with this additional memory copy. Furthermore, since the non-blocking nature of PBO readback reduces the load on the GPU, PBOs can improve performance dramatically when multiple simultaneous users are sharing a professional-grade GPU.
  As of this writing, some nVidia GeForce adapters will fall back to using blocking readbacks if the pixel format requested by the readback operation does not match the pixel format of the off-screen buffer. If VirtualGL detects that this is occurring– that is, if PBOs are no longer behaving asynchronously– then VGL will fall back to synchronous readback mode until the next time the compression type is changed. If you are using an X proxy, then this situation could occur because the X proxy’s virtual framebuffer is BGRA but, unless the 3D application has requested an alpha channel, its off-screen buffer is likely BGR. In this specific case, setting the `VGL_FORCEALPHA` option to `1` could alleviate the issue.
- `sync` = Synchronous readback mode. This disables the use of PBOs altogether, which causes VirtualGL to always use blocking readbacks.
  Setting `VGL_VERBOSE=1` will cause VirtualGL to print the current readback mode being used, as well as the pixel format requested by the readback operation and the pixel format of the off-screen buffer. Additionally, a notification will be printed if VirtualGL falls back from PBO readback mode to synchronous readback mode.

Environment Variable | `VGL_REFRESHRATE = {r}`
Summary | `{r}` = the “virtual” refresh rate, in Hz, for the `GLX_EXT_swap_control` and `GLX_SGI_swap_control` extensions and the `eglSwapInterval()` function
Image Transports | All
Default Value | `60.0`

Description
  The `GLX_EXT_swap_control` and `GLX_SGI_swap_control` extensions and the `eglSwapInterval()` function allow applications to specify that buffer swaps should be synchronized with the refresh rate of the monitor. When one of the aforementioned extensions or the aforementioned function is used, `glXSwapBuffers()` or `eglSwapBuffers()` will not return until a specified number of refreshes (the “swap interval”) has occurred. Although refresh rate has no meaning when rendering into an off-screen buffer, VirtualGL uses an internal timer to emulate the refresh rate so that 3D applications can control their own frame rate. (This is often used by games, for instance, in which maintaining a constant frame rate is important.) Setting `VGL_REFRESHRATE` changes the interval of VirtualGL’s internal timer.

Environment Variable | `VGL_SAMPLES = {s}`
`vglrun` argument | `-ms {s}`
Summary | Force OpenGL multisampling to be enabled with `{s}` samples (`{s}` = 0 to force OpenGL multisampling to be disabled)
Image Transports | All
Default Value | Allow the 3D application to determine the level of multisampling

Description
  This option was added primarily because certain vendor-specific methods of enabling full-scene antialiasing at a global level (such as nVidia’s `__GL_FSAA_MODE` environment variable) do not work with off-screen buffers and, consequently, do not work with VirtualGL. If `VGL_SAMPLES` is > 0, then VirtualGL will attempt to create off-screen buffers with the specified number (or a greater number) of samples. This effectively forces the 3D application to render with the specified multisampling level, as if the application had explicitly passed attributes of `GLX_SAMPLES,{s}` to `glXChooseVisual()` or `EGL_SAMPLES,{s}` to `eglChooseConfig()`. If `VGL_SAMPLES` is `0`, then VirtualGL forces multisampling to be disabled, even if the 3D application explicitly tries to enable it.

  Multisampling cannot be used with Pixmap rendering. Any application that uses Pixmap rendering will fail if `VGL_SAMPLES` is set to a value other than 0.

Environment Variable | `VGL_SPOIL = 0 | 1`
`vglrun` argument | `-sp` / `+sp`
Summary | Disable/enable frame spoiling
Image Transports | VGL, X11, XV, Custom (if supported)
Default Value | Enabled

Description
  In remote display environments, the mouse movement is generally sampled 40-60 times per second. 3D applications generally render a new frame for every mouse motion event, so if the image transport and (if applicable) the client cannot process the frames quickly enough, the server’s TCP buffers will fill up, causing delays in the delivery of each frame. Effectively, unless the remote display software is able to transport at least 40-60 frames per second, the movement of a 3D scene will appear to lag behind the mouse motion. VirtualGL’s default behavior is to compensate for this by dropping (spoiling) each frame that the transport isn’t ready to receive. This ensures that the movement of the 3D scene will appear to “keep up” with the mouse, even though not all rendered frames are actually being transported.
  Frame spoiling is usually necessary with interactive 3D applications, but it should be turned off when running benchmarks or other non-interactive 3D applications. Turning off frame spoiling will force every frame rendered on the server to be transported, and thus the frame rate reported by OpenGL benchmarks will accurately reflect the end-to-end performance of VirtualGL. (However, in X proxy environments, this may still not accurately reflect the frame rate seen by the user. See Section 16.2.) Disabling frame spoiling also prevents non-interactive 3D applications from wasting graphics resources by rendering frames that will never be seen.

Environment Variable | `VGL_SPOILLAST = 0 | 1`
Summary | Disable/enable “spoil last” frame spoiling algorithm for frames triggered by `glFlush()`
Image Transports | VGL, X11, XV, Custom (if supported)
Default Value | Enabled

Description
  VirtualGL normally reads back a rendered frame when the 3D application calls one of the frame trigger functions, such as `glXSwapBuffers()`. When frame spoiling is enabled and the image transport is busy transporting a frame, the newly-rendered frame is normally promoted to the head of the queue, and the rest of the frames in the queue are “spoiled” (discarded.) This algorithm, called “spoil first”, ensures that when a frame is actually transported (rather than spoiled), the transported frame will be the most recently rendered frame. However, this algorithm requires that VirtualGL read back every frame that the application renders, even if the frame is ultimately discarded.
  Some 3D applications call `glFlush()` many thousands of times per frame while rendering to the front buffer. Thus, VirtualGL’s default behavior is to use a different spoiling algorithm, “spoil last”, to process frames triggered by `glFlush()` calls. “Spoil last” discards the most recently rendered frame if the image transport is busy. Thus, the only frames that are read back are the frames that are actually transported. However, there is no guarantee in this case that the transported frame will be the most recently rendered frame, so applications that perform front buffer rendering and call `glFlush()` in response to an interactive operation may not display properly. For such applications, setting the `VGL_SPOILLAST` environment variable to `0` prior to launching the application with `vglrun` will cause the “spoil first” algorithm to be used for all frame triggers, including `glFlush()`. This should fix the display problem, at the expense of increased load on the GPU (because VirtualGL is now reading back the rendered frame every time `glFlush()` is called.) See Application Recipes for a list of 3D applications that are known to require this.

Environment Variable | `VGL_STEREO = left | right | quad | rc | gm | by | i | tb | ss`
`vglrun` argument | `-st left | right | quad | rc | gm | by | i | tb | ss`
Summary | Specify the delivery method for stereo frames
Image Transports | All
Default Value | `quad`

Description
  `left` = When a 3D application renders a stereo frame, read back and transport only the left eye buffer
  `right` = When a 3D application renders a stereo frame, read back and transport only the right eye buffer
  `quad` = Attempt to use quad-buffered stereo, which will result in a pair of images being transported for every rendered frame. Using quad-buffered stereo requires the VGL Transport (or a transport plugin that can handle stereo image pairs.) Using quad-buffered stereo with the VGL Transport also requires that the 2D X server support OpenGL and be connected to a GPU that supports stereo rendering. The 2D X server should additionally be configured to export stereo visuals. Quad-buffered stereo is not supported when using the VGL Transport with YUV encoding. If quad-buffered stereo is requested but the transport or the client does not support it, then VirtualGL will fall back to using Red/Cyan (anaglyphic) stereo.
  `rc` = Use Red/Cyan (anaglyphic) stereo, even if quad-buffered is available
  `gm` = Use Green/Magenta (anaglyphic) stereo, even if quad-buffered is available
  `by` = Use Blue/Yellow (anaglyphic) stereo, even if quad-buffered is available
  `i` = Use Interleaved (passive) stereo, even if quad-buffered is available
  `tb` = Use Top/Bottom (passive) stereo, even if quad-buffered is available
  `ss` = Use Side-by-Side (passive) stereo, even if quad-buffered is available
  See Chapter 15 for more details.

Environment Variable | `VGL_SUBSAMP = gray | 1x | 2x | 4x | 8x | 16x`
`vglrun` argument | `-samp gray | 1x | 2x | 4x | 8x | 16x`
Summary | Specify the level of chrominance subsampling in the JPEG compressor
Image Transports | VGL (JPEG), Custom (if supported)
Default Value | `1x`

Description
  When an image is compressed using JPEG, each pixel in the image is first converted from RGB (Red/Green/Blue) to YCbCr. An RGB pixel has three values that specify the amounts of red, green, and blue that make up the pixel’s color. A YCbCr pixel has three values that specify the overall brightness of the pixel (Y, or “luminance”) and the overall color of the pixel (Cb and Cr, or “chrominance”.)

  In the digital world, the terms “YCbCr” and “YUV” are often used interchangeably. Per the convention of the image processing and digital video communities, we use “YCbCr” when discussing JPEG compression and “YUV” when discussing video formats, but they are really the same thing.

  Since the human eye is less sensitive to changes in color than it is to changes in brightness, the chrominance components for some of the pixels can be discarded without much noticeable loss in image quality. This technique, called “chrominance subsampling”, significantly reduces the size of the compressed image.
  `1x` = no chrominance subsampling
  `2x` = discard the chrominance components for every other pixel along the image’s X direction (this is also known as “4:2:2” or “2:1” subsampling.) All else being equal, 2x subsampling generally reduces the image size by about 20-25% when compared to no subsampling.
  `4x` = discard the chrominance components for every other pixel along both the X and Y directions of the image (this is also known as “4:2:0” or “2:2” subsampling.) All else being equal, 4x subsampling generally reduces the image size by about 35-40% when compared to no subsampling.
  `8x` = discard the chrominance components for 3 out of every 4 pixels along the image’s X direction and half the pixels along the image’s Y direction (this is also known as “4:1:0” or “4:2” subsampling.) This option is available only when using an image transport plugin that supports it.
  `16x` = discard the chrominance components for 3 out of every 4 pixels along both the X and Y directions of the image (this is also known as “4:4” subsampling.) This option is available only when using an image transport plugin that supports it.
  `gray` = discard all chrominance components. This is useful when running 3D applications (such as medical visualization applications) that are already generating grayscale images.
  Subsampling artifacts are less noticeable with volume data, since it usually only contains 256 colors to begin with, but narrow, aliased lines and other sharp features on a black background will tend to produce very noticeable artifacts when subsampling is enabled.
  The axis indicator from a popular visualization application displayed with 1x, 2x, and 4x chrominance subsampling (respectively):

  If using an image transport plugin, then this setting need not necessarily correspond to JPEG chrominance subsampling. How the plugin responds to the `VGL_SUBSAMP` option is implementation-specific.

Environment Variable | `VGL_SYNC = 0 | 1`
`vglrun` argument | `-sync` / `+sync`
Summary | Disable/enable strict 2D/3D synchronization
Image Transports | VGL, X11, XV, Custom (if supported)
Default Value | Disabled

Description
  Normally, VirtualGL’s operation is asynchronous from the point of view of the 3D application. The application calls a frame trigger function, and VirtualGL reads back the rendered frame and transports it to the 2D X server … eventually. This is fine for the vast majority of 3D applications, but it does not strictly conform to the GLX specification. Technically speaking, when an application calls a frame trigger function, it is well within its rights to expect the rendered frame to be immediately available in the X window. Fortunately, very few applications actually do expect this, but on rare occasions, a 3D application may try to use `XGetImage()` or other X11 functions to obtain a bitmap of the pixels that were rendered by OpenGL. Enabling `VGL_SYNC` is a somewhat extreme measure that may be needed to make such applications display properly with VirtualGL. It was developed initially as a way to pass the GLX conformance suite (`conformx`, specifically), but at least one commercial application is known to require it as well (see Application Recipes.)
  When `VGL_SYNC` is enabled, every call to a frame trigger function will cause VirtualGL to read back the rendered frame and synchronously draw it into the 3D application’s window using the X11 Transport with no frame spoiling. The frame trigger function will not return control to the 3D application until VirtualGL has verified that the rendered frame has been composited into the application’s window. Therefore, this mode can have potentially dire effects on performance when used with a 2D X server on a machine other than the VirtualGL server. It is strongly recommended that `VGL_SYNC` be used only in conjunction with an X proxy running on the VirtualGL server.
  In order to implement frame spoiling, and also to allow the last frame to be transported while the current frame is being rendered and read back, VirtualGL’s built-in image transports use a separate “transport thread”. To ensure thread safety, the X11 Transport normally creates multiple X display connections for each OpenGL window. However, when `VGL_SYNC` is enabled, the X11 Transport instead reuses the 3D application’s X display connection(s). Thus, enabling `VGL_SYNC` may prevent a 3D application running with VirtualGL from exceeding the X server’s connection limit if the application opens an excessive number of OpenGL windows.

  If an image transport plugin is being used, then VirtualGL does not automatically enable the X11 Transport or disable frame spoiling when `VGL_SYNC` is set. This allows the plugin to handle synchronous image delivery as it sees fit (or to simply ignore this option.)

Environment Variable | `VGL_TILESIZE = {t}`
Summary | `{t}` = the image tile size (`{t}` x `{t}` pixels) to use for multithreaded compression and interframe comparison (8 <= `{t}` <= 1024)
Image Transports | VGL (JPEG, RGB), Custom (if supported)
Default Value | `256`

Description
  Normally, the VGL Transport divides a rendered frame into equal-sized square tiles, compares each tile against the same tile in the previous frame, and compresses/sends only the tiles that have changed (assuming interframe comparison is enabled.) The VGL Transport also divides the task of compressing or encoding these tiles among the available CPUs in a round robin fashion, if multithreaded compression is enabled (see `VGL_NPROCS`.)
  There are several tradeoffs that must be considered when choosing a tile size:
  Parallel scalability:
  - Smaller tiles can more easily be distributed among multiple CPUs. If the tile size is too large, then there may not be enough tiles to go around.
  Compression efficiency:
  - Compressing images requires a certain amount of fixed CPU overhead, so compressing larger tiles makes more efficient use of the CPUs and increases the aggregate throughput of the compressor.
  Interframe optimization:
  - When using smaller tiles, there is more of a chance that a given tile will remain unchanged from frame to frame and thus not need to be re-transported.
  Network efficiency:
  - Each tile requires a certain amount of fixed overhead to represent it on the network. Thus, using smaller tiles increases the total number of tiles per frame, which increases the total network usage.
  256x256 was chosen as the default because, in experiments, it provided the best balance between scalability and efficiency on the platforms that VirtualGL supports.

Environment Variable | `VGL_TRACE = 0 | 1`
`vglrun` argument | `-tr` / `+tr`
Summary | Disable/enable tracing
Image Transports | All
Default Value | Disabled

Description
  When tracing is enabled, VirtualGL will log all calls to the functions it is interposing, as well as the arguments, return values, and execution times for those functions. This is useful when diagnosing interaction problems between VirtualGL and a particular OpenGL application.

Environment Variable | `VGL_TRANSPORT = {t}`
`vglrun` argument | `-trans {t}`
Summary | Use an image transport plugin
Default Value | None

Description
  If this option is specified, then VirtualGL will attempt to use an image transport plugin contained in a dynamic library named libvgltrans_{t}.so located in the dynamic linker path. See Chapter 11 for more information.

Environment Variable | `VGL_TRAPX11 = 0 | 1`
Summary | Disable/enable VirtualGL’s X11 error handler
Image Transports | All
Default Value | Disabled

Description
  If a 3D application does not install its own X11 error handler, then the default X11 error handler is used, thus causing the application to exit if an X11 error occurs. Enabling the `VGL_TRAPX11` option causes VirtualGL to install its own X11 error handler, which prints a warning message but allows the application to continue running.

Environment Variable | `VGL_VERBOSE = 0 | 1`
`vglrun` argument | `-v` / `+v`
Summary | Disable/enable verbose VirtualGL messages
Image Transports | All
Default Value | Disabled

Description
  When this option is enabled, VirtualGL will reveal some of the decisions it is making behind the scenes, such as which type of X11 drawing it is using in the X11 Transport, etc. This can be helpful when diagnosing performance problems.

Environment Variable | `VGL_WM = 0 | 1`
`vglrun` argument | `-wm` / `+wm`
Summary | Disable/enable window manager mode
Image Transports | All
Default Value | Disabled

Description
  When window manager mode is enabled, VirtualGL will disable some of its internal features that interfere with the correct operation of compositing window managers such as Compiz.

Environment Variable | `VGL_X11LIB = {l}`
Summary | `{l}` = the location of an alternate X11 library
Image Transports | All

Description
  Normally VirtualGL will use the X11 library against which it was linked (usually libX11.so.6, in the system library path) to load any “real” X11 functions that it needs to call (“real” as opposed to the “fake”, or “interposed”, versions of those functions that VirtualGL provides, which often modify the arguments or perform other operations before calling the “real” functions.) You can use the `VGL_X11LIB` environment variable to specify the path of a dynamic library from which VirtualGL should load “real” X11 functions.
  You shouldn’t need to change this unless something doesn’t work. However, setting this environment variable is potentially useful if one wishes to insert another X11 interposer between VirtualGL and the system’s X11 library.

Environment Variable | `VGL_XCBLIB = {l}`
Summary | `{l}` = the location of an alternate XCB library
Image Transports | All
Default Value | libxcb.so.1 in the system library path

Description
  Specifies the path of a dynamic library from which VirtualGL should load “real” XCB functions.

Environment Variable | `VGL_XCBATOMLIB = {l}`
Summary | `{l}` = the location of an alternate xcb-atom library
Image Transports | All
Default Value | libxcb-atom.so.0 or libxcb-atom.so.1 in the system library path

Description
  Specifies the path of a dynamic library from which VirtualGL should load “real” XCB atom functions.

Environment Variable | `VGL_XCBGLXLIB = {l}`
Summary | `{l}` = the location of an alternate xcb-glx library
Image Transports | All
Default Value | libxcb-glx.so.0 in the system library path

Description
  Specifies the path of a dynamic library from which VirtualGL should load “real” XCB GLX functions.

Environment Variable | `VGL_XCBKEYSYMSLIB = {l}`
Summary | `{l}` = the location of an alternate xcb-keysyms library
Image Transports | All
Default Value | libxcb-keysyms.so.0 or libxcb-keysyms.so.1 in the system library path

Description
  Specifies the path of a dynamic library from which VirtualGL should load “real” XCB keysyms functions.

Environment Variable | `VGL_XCBX11LIB = {l}`
Summary | `{l}` = the location of an alternate X11-xcb library
Image Transports | All
Default Value | libX11-xcb.so.1 in the system library path

Description
  Specifies the path of a dynamic library from which VirtualGL should load “real” X11 XCB functions.

Environment Variable | `VGL_XVENDOR = {v}`
Summary | `{v}` = a fake X11 vendor string to return when the 3D application calls `XServerVendor()` or `ServerVendor()`
Image Transports | All

Description
  Some 3D applications expect the X11 vendor string to contain a particular value, which the application (sometimes erroneously) uses as an indicator of whether it is being displayed to an X server on the same machine or a different machine. This setting allows you to fool such applications into thinking that they are being displayed to an X server on the same machine.

## 18.2 Client Settings

These settings control the VirtualGL Client, which is used only with the VGL Transport. `vglclient` is normally launched automatically from `vglconnect` and should not require any further configuration except in exotic circumstances. These settings are meant only for advanced users or those wishing to build additional infrastructure around VirtualGL.

Environment Variable | `VGLCLIENT_DRAWMODE = ogl | x11`
`vglclient` argument | `-gl` / `-x`
Summary | Specify the API used to composite the rendered frames into the 3D application’s windows
Default Value | `x11`

Description
  If the client has a GPU, then it may be faster in some rare instances to draw the rendered frames using OpenGL rather than 2D (X11) drawing commands.

Environment Variable | `VGLCLIENT_IPV6 = 0 | 1`
`vglclient` argument | `-ipv6`
Summary | Disable/enable IPv6 sockets
Default Value | Disabled

Description
  Enabling this option will cause the VirtualGL Client to listen on IPv6 sockets and to support both IPv4 and IPv6 connections.

Environment Variable | `VGLCLIENT_PORT = {p}`
`vglclient` argument | `-port {p}`
Summary | `{p}` = TCP port on which to listen for connections from the VirtualGL Faker
Default Value | Automatically select a free port

Description
  The default behavior of the VirtualGL Client is to first try listening for connections on port 4242, to maintain backward compatibility with VirtualGL v2.0.x. If port 4242 is not available, then the VirtualGL Client will try to find a free port in the range of 4200-4299. If none of those ports is available, then the VirtualGL Client will request a free port from the operating system.
  Setting this option circumvents the automatic behavior described above and causes the VirtualGL Client to listen only on the specified TCP port.

Environment Variable | `VGL_PROFILE = 0 | 1`
Summary | Disable/enable profiling output
Default Value | Disabled

Description
  If profiling output is enabled, then VirtualGL will continuously benchmark itself and periodically print out the throughput of various stages in its image pipelines.
  See Chapter 16 for more details.

Environment Variable | `VGL_VERBOSE = 0 | 1`
Summary | Disable/enable verbose VirtualGL messages
Default Value | Disabled

Description
  When this option is enabled, the VirtualGL Client will reveal some of the decisions it is making behind the scenes, such as which type of X11 drawing it is using, etc. This can be helpful when diagnosing performance problems.