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<h2 class="title"><a name="openglenvvariables" id=
"openglenvvariables"></a>Chapter 11. Specifying OpenGL
Environment Variable Settings</h2>
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<h3>Full scene antialiasing</h3>
<p>Antialiasing is a technique used to smooth the edges of objects
in a scene to reduce the jagged "stairstep" effect that sometimes
appears. By setting the appropriate environment variable, you can
enable full-scene antialiasing in any OpenGL application on these
GPUs.</p>
<p>Several antialiasing methods are available and you can select
between them by setting the __GL_FSAA_MODE environment variable
appropriately. Note that increasing the number of samples taken
during FSAA rendering may decrease performance.</p>
<p>To see the available values for __GL_FSAA_MODE along with their
descriptions, run:</p>
<pre class="screen">
nvidia-settings --query=fsaa --verbose
</pre>
<p>The __GL_FSAA_MODE environment variable uses the same integer
values that are used to configure FSAA through nvidia-settings and
the NV-CONTROL X extension. In other words, these two commands are
equivalent:</p>
<pre class="screen">
export __GL_FSAA_MODE=5
nvidia-settings --assign FSAA=5
</pre>
<p>Note that there are three FSAA related configuration attributes
(FSAA, FSAAAppControlled and FSAAAppEnhanced) which together
determine how a GL application will behave. If FSAAAppControlled is
1, the FSAA specified through nvidia-settings will be ignored, in
favor of what the application requests through FBConfig selection.
If FSAAAppControlled is 0 but FSAAAppEnhanced is 1, then the FSAA
value specified through nvidia-settings will only be applied if the
application selected a multisample FBConfig.</p>
<p>Therefore, to be completely correct, the nvidia-settings command
line to unconditionally assign FSAA should be:</p>
<pre class="screen">
nvidia-settings --assign FSAA=5 --assign FSAAAppControlled=0 --assign FSAAAppEnhanced=0
</pre>
<p></p>
<h3>Anisotropic texture filtering</h3>
<p>Automatic anisotropic texture filtering can be enabled by
setting the environment variable __GL_LOG_MAX_ANISO. The possible
values are:</p>
<div class="informaltable">
<table summary="(no summary available)" border="0">
<colgroup>
<col>
<col></colgroup>
<thead>
<tr>
<th>__GL_LOG_MAX_ANISO</th>
<th>Filtering Type</th>
</tr>
</thead>
<tbody>
<tr>
<td>0</td>
<td>No anisotropic filtering</td>
</tr>
<tr>
<td>1</td>
<td>2x anisotropic filtering</td>
</tr>
<tr>
<td>2</td>
<td>4x anisotropic filtering</td>
</tr>
<tr>
<td>3</td>
<td>8x anisotropic filtering</td>
</tr>
<tr>
<td>4</td>
<td>16x anisotropic filtering</td>
</tr>
</tbody>
</table>
</div>
<p>4x and greater are only available on GeForce3 or newer GPUs; 16x
is only available on GeForce 6800 or newer GPUs.</p>
<h3>Vblank syncing</h3>
<p>Setting the environment variable __GL_SYNC_TO_VBLANK to a
non-zero value will force glXSwapBuffers to sync to your monitor's
vertical refresh (perform a swap only during the vertical blanking
period).</p>
<p>When using __GL_SYNC_TO_VBLANK with TwinView, OpenGL can only
sync to one of the display devices; this may cause tearing
corruption on the display device to which OpenGL is not syncing.
You can use the environment variable __GL_SYNC_DISPLAY_DEVICE to
specify to which display device OpenGL should sync. You should set
this environment variable to the name of a display device; for
example "CRT-1". Look for the line "Connected display device(s):"
in your X log file for a list of the display devices present and
their names. You may also find it useful to review <a href=
"configtwinview.html" title=
"Chapter 13. Configuring TwinView">Chapter 13,
<i>Configuring TwinView</i></a> "Configuring Twinview" and the
section on Ensuring Identical Mode Timings in <a href=
"programmingmodes.html" title=
"Chapter 19. Programming Modes">Chapter 19,
<i>Programming Modes</i></a>.</p>
<h3>Controlling the sorting of OpenGL FBConfigs</h3>
<p>The NVIDIA GLX implementation sorts FBConfigs returned by
glXChooseFBConfig() as described in the GLX specification. To
disable this behavior set __GL_SORT_FBCONFIGS to 0 (zero), then
FBConfigs will be returned in the order they were received from the
X server. To examine the order in which FBConfigs are returned by
the X server run:</p>
<pre class="screen">
nvidia-settings --glxinfo
</pre>
<p>This option may be be useful to work around problems in which
applications pick an unexpected FBConfig.</p>
<h3>OpenGL yield behavior</h3>
<p>There are several cases where the NVIDIA OpenGL driver needs to
wait for external state to change before continuing. To avoid
consuming too much CPU time in these cases, the driver will
sometimes yield so the kernel can schedule other processes to run
while the driver waits. For example, when waiting for free space in
a command buffer, if the free space has not become available after
a certain number of iterations, the driver will yield before it
continues to loop.</p>
<p>By default, the driver calls sched_yield() to do this. However,
this can cause the calling process to be scheduled out for a
relatively long period of time if there are other, same-priority
processes competing for time on the CPU. One example of this is
when an OpenGL-based composite manager is moving and repainting a
window and the X server is trying to update the window as it moves,
which are both CPU-intensive operations.</p>
<p>You can use the __GL_YIELD environment variable to work around
these scheduling problems. This variable allows the user to specify
what the driver should do when it wants to yield. The possible
values are:</p>
<div class="informaltable">
<table summary="(no summary available)" border="0">
<colgroup>
<col>
<col></colgroup>
<thead>
<tr>
<th>__GL_YIELD</th>
<th>Behavior</th>
</tr>
</thead>
<tbody>
<tr>
<td><unset></td>
<td>By default, OpenGL will call sched_yield() to yield.</td>
</tr>
<tr>
<td>"NOTHING"</td>
<td>OpenGL will never yield.</td>
</tr>
<tr>
<td>"USLEEP"</td>
<td>OpenGL will call usleep(0) to yield.</td>
</tr>
</tbody>
</table>
</div>
<p></p>
<h3>Controlling which OpenGL FBConfigs are available</h3>
<p>The NVIDIA GLX implementation will hide FBConfigs that are
associated with a 32-bit ARGB visual when the
XLIB_SKIP_ARGB_VISUALS environment variable is defined. This
matches the behavior of libX11, which will hide those visuals from
XGetVisualInfo and XMatchVisualInfo. This environment variable is
useful when applications are confused by the presence of these
FBConfigs.</p>
<p><a name="unofficialprotoenv" id="unofficialprotoenv"></a></p>
<h3>Using Unofficial GLX protocol</h3>
<p>By default, the NVIDIA GLX implementation will not expose GLX
protocol for GL commands if the protocol is not considered
complete. Protocol could be considered incomplete for a number of
reasons. The implementation could still be under development and
contain known bugs, or the protocol specification itself could be
under development or going through review. If users would like to
test the client-side portion of such protocol when using indirect
rendering, they can set the __GL_ALLOW_UNOFFICIAL_PROTOCOL
environment variable to a non-zero value before starting their GLX
application. When an NVIDIA GLX server is used, the related X
Config option <a href=
"xconfigoptions.html#AllowUnofficialGLXProtocol">AllowUnofficialGLXProtocol</a>
will need to be set as well to enable support in the server.</p>
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