* Advanced Configuration
{anchor: Advanced_Configuration}

** 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:

	#. Setting a configuration environment variable globally (for instance, in
		{file: /etc/profile})

	#. Setting a configuration environment variable on a per-user basis (for
		instance, in {file: ~/.bashrc})

	#. Setting a configuration environment variable only for the current shell
		session (for instance, {pcode: export VGL_XXX=__whatever__})

	#. 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.

{anchor: VGL_ALLOWINDIRECT}
| Environment Variable | {pcode: 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) |
#OPT: hiCol=first

	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.)
	{nl}{nl}
	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 | {pcode: VGL_CLIENT = __{c}__ } |
| ''vglrun'' argument | {pcode: -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'' |
#OPT: hiCol=first

	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.

{anchor: VGL_COMPRESS}
| Environment Variable | \
	{pcode: VGL_COMPRESS = __proxy \| jpeg \| rgb \| xv \| yuv__ } |
| ''vglrun'' argument | {pcode: -c __proxy \| jpeg \| rgb \| xv \| yuv__ } |
| Summary | Set image transport and image compression type |
| Image Transports | All |
| Default Value | (See description) |
#OPT: hiCol=first

	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 {ref prefix="Section ": X11_Proxy_Usage_Local}.)
	{nl}{nl}
	''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
	{ref prefix="Chapter ": VGL_Transport_Usage}.)
	{nl}{nl}
	''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 {ref prefix="Section ": X11_Proxy_Usage_Remote}.)
	{nl}{nl}
	''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 {ref prefix="Chapter ": X_Video_Support}.)
	{nl}{nl}
	''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
	{ref prefix="Chapter ": X_Video_Support}.)
	{nl}{nl}
	If ''VGL_COMPRESS'' is not specified, then the default is set as follows:
	{nl}{nl}
	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.
	{nl}{nl}
	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.

{anchor: VGL_DISPLAY}
| Environment Variable | {pcode: VGL_DISPLAY = __{d}__ } |
| ''vglrun'' argument | {pcode: -d __{d}__ } |
| Summary | __''{d}''__ = the X display/screen or EGL device to use for 3D \
	rendering |
| Image Transports | All |
| Default Value | '':0'' |
#OPT: hiCol=first

	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.

| Environment Variable | {pcode: VGL_EGLLIB = __{l}__ } |
| Summary | __''{l}''__ = the location of an alternate EGL library |
| Image Transports | All |
#OPT: hiCol=first

	Description :: Normally VirtualGL will call the ''eglGetProcAddress()''
	function in the EGL library against which it or the 3D application was linked
	(usually {file: 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.
	{nl}{nl}
	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 | \
	{pcode: 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 |
#OPT: hiCol=first

	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.
	{nl}{nl}
	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 | \
	{pcode: 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'' |
#OPT: hiCol=first

	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 | {pcode: VGL_FAKEOPENCL = __0 \| 1__ } |
| ''vglrun'' argument | ''-ocl'' / ''+ocl'' |
| Summary | Disable/enable OpenCL interposer |
| Image Transports | All |
| Default Value | Disabled |
#OPT: hiCol=first

	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 | {pcode: VGL_FAKEXCB = __0 \| 1__ } |
| ''vglrun'' argument | ''-xcb'' / ''+xcb'' |
| Summary | Disable/enable XCB interposer |
| Image Transports | All |
| Default Value | Enabled |
#OPT: hiCol=first

	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.

{anchor: VGL_FORCEALPHA}
| Environment Variable | {pcode: 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) |
#OPT: hiCol=first

	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.
	{nl}{nl}
	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][''VGL_READBACK'']] option for further information.

{anchor: VGL_FPS}
| Environment Variable | {pcode: VGL_FPS = __{f}__ } |
| ''vglrun'' argument | {pcode: -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) |
#OPT: hiCol=first

	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.
	{nl}{nl}
	If frame spoiling is disabled, then setting ''VGL_FPS'' effectively limits
	the server's 3D rendering frame rate as well.

{anchor: VGL_GAMMA}
| Environment Variable | {pcode: VGL_GAMMA = __{g}__ } |
| ''vglrun'' argument | {pcode: -gamma __{g}__ } |
| Summary | __''{g}''__ = gamma correction factor |
| Image Transports | All |
| Default Value | ''1.00'' (no gamma correction) |
#OPT: hiCol=first

	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 = X{^G}__, 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.
	{nl}{nl}
	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 = W{^1/G}__, where G
	is the "gamma correction factor" and should be equal to the gamma of the
	monitor.  So, the final output is __Y = X{^G} = (W{^1/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.
	{nl}{nl}
	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.

{anchor: VGL_GLFLUSHTRIGGER}
| Environment Variable | {pcode: VGL_GLFLUSHTRIGGER = __0 \| 1__ } |
| Summary | Disable/enable using ''glFlush()'' as a frame trigger function |
| Default Value | Enabled |
#OPT: hiCol=first

	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.
	{nl}{nl}
	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][''VGL_SPOILLAST'']] for more information on
	how VirtualGL tries to handle this, under normal circumstances.)
	{nl}{nl}
	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][Application Recipes]] for a list
	of 3D applications that are known to require this.

| Environment Variable | {pcode: VGL_GLLIB = __{l}__ } |
| Summary | __''{l}''__ = the location of an alternate OpenGL library |
| Image Transports | All |
#OPT: hiCol=first

	Description :: Normally VirtualGL will call the ''glXGetProcAddress()'' or
	''glXGetProcAddressARB()'' function in the OpenGL library against which it or
	the 3D application was linked (usually {file: 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.
	{nl}{nl}
	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 | {pcode: 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'' |
#OPT: hiCol=first

	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 {ref prefix="Chapter ": Config_Dialog}
	for more details.

{anchor: VGL_INTERFRAME}
| Environment Variable | {pcode: VGL_INTERFRAME = __0 \| 1__ } |
| Summary | Disable or enable interframe comparison |
| Image Transports | VGL (JPEG, RGB), Custom (if supported) |
| Default Value | Enabled |
#OPT: hiCol=first

	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.
	{nl}{nl}
	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][''VGL_TILESIZE'']] option

| Environment Variable | {pcode: 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 |
#OPT: hiCol=first

	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 | {pcode: 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 |
#OPT: hiCol=first

	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.

{anchor: VGL_NPROCS}
| Environment Variable | {pcode: VGL_NPROCS = __{n}__ } |
| ''vglrun'' argument | {pcode: -np __{n}__ } |
| Summary | __''{n}''__ = the number of threads to use for \
	compression/encoding |
| Image Transports | VGL (JPEG, RGB), Custom (if supported) |
| Default Value | ''1'' |
#OPT: hiCol=first

	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.
	{nl}{nl}
	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][''VGL_TILESIZE'']] option

| Environment Variable | {pcode: VGL_OCLLIB = __{l}__ } |
| Summary | __''{l}''__ = the location of an alternate OpenCL library |
| Image Transports | All |
| Default Value | {file: libOpenCL.so.1} in the system library path |
#OPT: hiCol=first

	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.)
	{nl}{nl}
	You shouldn't need to change this unless something doesn't work.

| Environment Variable | {pcode: VGL_PORT = __{p}__ } |
| ''vglrun'' argument | {pcode: -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 |
#OPT: hiCol=first

	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 | {pcode: VGL_PROFILE = __0 \| 1__ } |
| ''vglrun'' argument | ''-pr'' / ''+pr'' |
| Summary | Disable/enable profiling output |
| Image Transports | VGL, X11, XV, Custom (if supported) |
| Default Value | Disabled |
#OPT: hiCol=first

	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.
	{nl}{nl}
	See {ref prefix="Chapter ": Perf_Measurement} for more details.

{anchor: VGL_QUAL}
| Environment Variable | {pcode: VGL_QUAL = __{q}__ } |
| ''vglrun'' argument | {pcode: -q __{q}__ } |
| Summary | __''{q}''__ = the JPEG compression quality, \
	1 \<\= __''{q}''__ \<\= 100 |
| Image Transports | VGL (JPEG), Custom (if supported) |
| Default Value | ''95'' |
#OPT: hiCol=first

	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.)
	{nl}{nl}
	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
	[[http://www.mpi-inf.mpg.de/resources/hdr/vdp/][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.
	{nl}
	!!! 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.

{anchor: VGL_READBACK}
| Environment Variable | {pcode: 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'' |
#OPT: hiCol=first

	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.
	{nl}{nl}
	This option was developed initially to support running
	[[http://www.paraview.org/][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.
	{nl}{nl}
	* ''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.
	{nl}{nl}
	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][''VGL_FORCEALPHA'']] option to ''1'' could alleviate the
	issue.
	{nl}{nl}
	* ''sync'' = Synchronous readback mode.  This disables the use of PBOs
	altogether, which causes VirtualGL to always use blocking readbacks.
	{nl}{nl}
	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 | {pcode: 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'' |
#OPT: hiCol=first

	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 | {pcode: VGL_SAMPLES = __{s}__ } |
| ''vglrun'' argument | {pcode: -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 |
#OPT: hiCol=first

	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 {pcode: GLX_SAMPLES,__{s}__} to ''glXChooseVisual()'' or
	{pcode: 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.

{anchor: VGL_SPOIL}
| Environment Variable | {pcode: VGL_SPOIL = __0 \| 1__ } |
| ''vglrun'' argument | ''-sp'' / ''+sp'' |
| Summary | Disable/enable frame spoiling |
| Image Transports | VGL, X11, XV, Custom (if supported) |
| Default Value | Enabled |
#OPT: hiCol=first

	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.
	{nl}{nl}
	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 {ref prefix="Section ": Frame_Spoiling}.)
	Disabling frame spoiling also prevents non-interactive 3D applications from
	wasting graphics resources by rendering frames that will never be seen.

{anchor: VGL_SPOILLAST}
| Environment Variable | {pcode: 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 |
#OPT: hiCol=first

	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.
	{nl}{nl}
	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][Application Recipes]] for a list of 3D applications
	that are known to require this.

{anchor: VGL_STEREO}
| Environment Variable | \
	{pcode: VGL_STEREO = __left \| right \| quad \| rc \| gm \| by \| i \| tb \| ss__ } |
| ''vglrun'' argument | \
	{pcode: -st __left \| right \| quad \| rc \| gm \| by \| i \| tb \| ss__ } |
| Summary | Specify the delivery method for stereo frames |
| Image Transports | All |
| Default Value | ''quad'' |
#OPT: hiCol=first

	Description :: {:}
	''left'' = When a 3D application renders a stereo frame, read back and
	transport only the left eye buffer
	{nl}{nl}
	''right'' = When a 3D application renders a stereo frame, read back and
	transport only the right eye buffer
	{nl}{nl}
	''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.
	{nl}{nl}
	''rc'' = Use Red/Cyan (anaglyphic) stereo, even if quad-buffered is
	available
	{nl}{nl}
	''gm'' = Use Green/Magenta (anaglyphic) stereo, even if quad-buffered is
	available
	{nl}{nl}
	''by'' = Use Blue/Yellow (anaglyphic) stereo, even if quad-buffered is
	available
	{nl}{nl}
	''i'' = Use Interleaved (passive) stereo, even if quad-buffered is
	available
	{nl}{nl}
	''tb'' = Use Top/Bottom (passive) stereo, even if quad-buffered is
	available
	{nl}{nl}
	''ss'' = Use Side-by-Side (passive) stereo, even if quad-buffered is
	available
	{nl}{nl}
	See {ref prefix="Chapter ": Advanced_OpenGL} for more details.

{anchor: VGL_SUBSAMP}
| Environment Variable | \
	{pcode: VGL_SUBSAMP = __gray \| 1x \| 2x \| 4x \| 8x \| 16x__ } |
| ''vglrun'' argument | \
	{pcode: -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'' |
#OPT: hiCol=first

	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.
	{nl}{nl}
	''1x'' = no chrominance subsampling
	{nl}{nl}
	''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.
	{nl}{nl}
	''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.
	{nl}{nl}
	''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.__
	{nl}{nl}
	''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.__
	{nl}{nl}
	''gray'' = discard all chrominance components.  This is useful when running
	3D applications (such as medical visualization applications) that are already
	generating grayscale images.
	{nl}{nl}
	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.
	{nl}{nl}
	{bold: The axis indicator from a popular visualization application displayed
	with 1x, 2x, and 4x chrominance subsampling (respectively):}
	{nl}
	{img: subsampling.gif}
	{nl}
	!!! 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.

{anchor: VGL_SYNC}
| Environment Variable | {pcode: 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 |
#OPT: hiCol=first

	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][Application
	Recipes]].)
	{nl}{nl}
	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.
	{nl}{nl}
	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.)

{anchor: VGL_TILESIZE}
| Environment Variable | {pcode: 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'' |
#OPT: hiCol=first

	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 [[#VGL_INTERFRAME][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][''VGL_NPROCS'']].)
	{nl}{nl}
	There are several tradeoffs that must be considered when choosing a tile
	size:
	{nl}{nl}
	__Parallel scalability:__
	{list:
		{item: 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.}}
	{nl}
	__Compression efficiency:__
	{list:
		{item: 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.}}
	{nl}
	__Interframe optimization:__
	{list:
		{item: 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.}}
	{nl}
	__Network efficiency:__
	{list:
		{item: 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.}}
	{nl}
	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 | {pcode: VGL_TRACE = __0 \| 1__ } |
| ''vglrun'' argument | ''-tr'' / ''+tr'' |
| Summary | Disable/enable tracing |
| Image Transports | All |
| Default Value | Disabled |
#OPT: hiCol=first

	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 | {pcode: VGL_TRANSPORT = __{t}__ } |
| ''vglrun'' argument | {pcode: -trans __{t}__ } |
| Summary | Use an image transport plugin |
| Default Value | None |
#OPT: hiCol=first

	Description :: If this option is specified, then VirtualGL will attempt to
	use an image transport plugin contained in a dynamic library named
	{file: libvgltrans\___{t}__.so} located in the dynamic linker path.  See
	{ref prefix="Chapter ": Transport_Plugins} for more information.

| Environment Variable | {pcode: VGL_TRAPX11 = __0 \| 1__ } |
| Summary | Disable/enable VirtualGL's X11 error handler |
| Image Transports | All |
| Default Value | Disabled |
#OPT: hiCol=first

	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 | {pcode: VGL_VERBOSE = __0 \| 1__ } |
| ''vglrun'' argument | ''-v'' / ''+v'' |
| Summary | Disable/enable verbose VirtualGL messages |
| Image Transports | All |
| Default Value | Disabled |
#OPT: hiCol=first

	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.

{anchor: VGL_WM}
| Environment Variable | {pcode: VGL_WM = __0 \| 1__ } |
| ''vglrun'' argument | ''-wm'' / ''+wm'' |
| Summary | Disable/enable window manager mode |
| Image Transports | All |
| Default Value | Disabled |
#OPT: hiCol=first

	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 | {pcode: VGL_X11LIB = __{l}__ } |
| Summary | __''{l}''__ = the location of an alternate X11 library |
| Image Transports | All |
#OPT: hiCol=first

	Description :: Normally VirtualGL will use the X11 library against which
	it was linked (usually {file: 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.
	{nl}{nl}
	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 | {pcode: VGL_XCBLIB = __{l}__ } |
| Summary | __''{l}''__ = the location of an alternate XCB library |
| Image Transports | All |
| Default Value | {file: libxcb.so.1} in the system library path |
#OPT: hiCol=first

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

| Environment Variable | {pcode: VGL_XCBATOMLIB = __{l}__ } |
| Summary | __''{l}''__ = the location of an alternate xcb-atom library |
| Image Transports | All |
| Default Value | {file: libxcb-atom.so.0} or {file: libxcb-atom.so.1} in \
	the system library path |
#OPT: hiCol=first

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

| Environment Variable | {pcode: VGL_XCBGLXLIB = __{l}__ } |
| Summary | __''{l}''__ = the location of an alternate xcb-glx library |
| Image Transports | All |
| Default Value | {file: libxcb-glx.so.0} in the system library path |
#OPT: hiCol=first

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

| Environment Variable | {pcode: VGL_XCBKEYSYMSLIB = __{l}__ } |
| Summary | __''{l}''__ = the location of an alternate xcb-keysyms library |
| Image Transports | All |
| Default Value | {file: libxcb-keysyms.so.0} or {file: libxcb-keysyms.so.1} \
	in the system library path |
#OPT: hiCol=first

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

| Environment Variable | {pcode: VGL_XCBX11LIB = __{l}__ } |
| Summary | __''{l}''__ = the location of an alternate X11-xcb library |
| Image Transports | All |
| Default Value | {file: libX11-xcb.so.1} in the system library path |
#OPT: hiCol=first

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

| Environment Variable | {pcode: VGL_XVENDOR = __{v}__ } |
| Summary | __''{v}''__ = a fake X11 vendor string to return when the 3D \
	application calls ''XServerVendor()'' or ''ServerVendor()'' |
| Image Transports | All |
#OPT: hiCol=first

	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.

** 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 | {pcode: 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'' |
#OPT: hiCol=first

	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 | {pcode: VGLCLIENT_IPV6 = __0 \| 1__ } |
| ''vglclient'' argument | ''-ipv6'' |
| Summary | Disable/enable IPv6 sockets |
| Default Value | Disabled |
#OPT: hiCol=first

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

| Environment Variable | {pcode: VGLCLIENT_PORT = __{p}__ } |
| ''vglclient'' argument | {pcode: -port __{p}__ } |
| Summary | __''{p}''__ = TCP port on which to listen for connections from \
	the VirtualGL Faker |
| Default Value | Automatically select a free port |
#OPT: hiCol=first

	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.
	{nl}{nl}
	Setting this option circumvents the automatic behavior described above and
	causes the VirtualGL Client to listen only on the specified TCP port.

| Environment Variable | {pcode: VGL_PROFILE = __0 \| 1__ } |
| Summary | Disable/enable profiling output |
| Default Value | Disabled |
#OPT: hiCol=first

	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.
	{nl}{nl}
	See {ref prefix="Chapter ": Perf_Measurement} for more details.

| Environment Variable | {pcode: VGL_VERBOSE = __0 \| 1__ } |
| Summary | Disable/enable verbose VirtualGL messages |
| Default Value | Disabled |
#OPT: hiCol=first

	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.
