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<th colspan="3" align="center">Chapter 22. Using the X
Composite Extension</th>
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<th width="60%" align="center">Part I. Installation and
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<h2 class="title"><a name="xcompositeextension" id=
"xcompositeextension"></a>Chapter 22. Using the X
Composite Extension</h2>
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<p>X.Org X servers, beginning with X11R6.8.0, contain experimental
support for a new X protocol extension called Composite. This
extension allows windows to be drawn into pixmaps instead of
directly onto the screen. In conjunction with the Damage and Render
extensions, this allows a program called a composite manager to
blend windows together to draw the screen.</p>
<p>Performance will be degraded significantly if the <code class=
"computeroutput">RenderAccel</code> option is disabled in
xorg.conf. See <a href="xconfigoptions.html" title=
"Appendix B. X Config Options">Appendix B, <i>X
Config Options</i></a> for more details.</p>
<p>When the NVIDIA X driver is used with an X.Org X server
X11R6.9.0 or newer and the Composite extension is enabled, NVIDIA's
OpenGL implementation interacts properly with the Damage and
Composite X extensions. This means that OpenGL rendering is drawn
into offscreen pixmaps and the X server is notified of the Damage
event when OpenGL renders to the pixmap. This allows OpenGL
applications to behave properly in a composited X desktop.</p>
<p>If the Composite extension is enabled on an X server older than
X11R6.9.0, then GLX will be disabled. You can force GLX on while
Composite is enabled on pre-X11R6.9.0 X servers with the
<code class="computeroutput">AllowGLXWithComposite</code> X
configuration option. However, GLX will not render correctly in
this environment. Upgrading your X server to X11R6.9.0 or newer is
recommended.</p>
<p>You can enable the Composite X extension by running
<span><strong class="command">nvidia-xconfig
--composite</strong></span>. Composite can be disabled with
<span><strong class="command">nvidia-xconfig
--no-composite</strong></span>. See the nvidia-xconfig(1) man page
for details.</p>
<p>If you are using an OpenGL-based composite manager, you may also
need the <code class="computeroutput">DisableGLXRootClipping</code>
option to obtain proper output.</p>
<p>The Composite extension also causes problems with other driver
components:</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>In X servers prior to X.Org 7.1, Xv cannot draw into pixmaps
that have been redirected offscreen and will draw directly onto the
screen instead. For some programs you can work around this issue by
using an alternative video driver. For example, "mplayer -vo x11"
will work correctly, as will "xine -V xshm". If you must use Xv
with an older server, you can also disable the compositing manager
and re-enable it when you are finished.</p>
<p>On X.Org 7.1 and higher, the driver will properly redirect video
into offscreen pixmaps. Note that the Xv adaptors will ignore the
sync-to-vblank option when drawing into a redirected window.</p>
</li>
<li>
<p>Workstation overlays, stereo visuals, and the unified back
buffer (UBB) are incompatible with Composite. These features will
be automatically disabled when Composite is detected.</p>
</li>
<li>
<p>The Composite extension is incompatible with Xinerama in X.Org X
servers prior to version 1.10. Composite will be automatically
disabled when Xinerama is enabled on those servers.</p>
</li>
<li>
<p>Prior to X.Org X server version 1.15, the Damage extension does
not properly report rendering events on all physical X screens in
Xinerama configurations. This prevents most composite mangers from
rendering correctly.</p>
</li>
</ul>
</div>
<p></p>
<p>This NVIDIA Linux driver supports OpenGL rendering to 32-bit
ARGB windows on X.Org 7.2 and higher or when the <code class=
"computeroutput">AddARGBGLXVisuals</code> X config file option is
enabled. 32-bit visuals are only available on screens with depths
24 or 30. If you are an application developer, you can use these
new visuals in conjunction with a composite manager to create
translucent OpenGL applications:</p>
<pre class="screen">
int attrib[] = {
GLX_RENDER_TYPE, GLX_RGBA_BIT,
GLX_DRAWABLE_TYPE, GLX_WINDOW_BIT,
GLX_RED_SIZE, 1,
GLX_GREEN_SIZE, 1,
GLX_BLUE_SIZE, 1,
GLX_ALPHA_SIZE, 1,
GLX_DOUBLEBUFFER, True,
GLX_DEPTH_SIZE, 1,
None };
GLXFBConfig *fbconfigs, fbconfig;
int numfbconfigs, render_event_base, render_error_base;
XVisualInfo *visinfo;
XRenderPictFormat *pictFormat;
/* Make sure we have the RENDER extension */
if(!XRenderQueryExtension(dpy, &render_event_base, &render_error_base)) {
fprintf(stderr, "No RENDER extension found\n");
exit(EXIT_FAILURE);
}
/* Get the list of FBConfigs that match our criteria */
fbconfigs = glXChooseFBConfig(dpy, scrnum, attrib, &numfbconfigs);
if (!fbconfigs) {
/* None matched */
exit(EXIT_FAILURE);
}
/* Find an FBConfig with a visual that has a RENDER picture format that
* has alpha */
for (i = 0; i < numfbconfigs; i++) {
visinfo = glXGetVisualFromFBConfig(dpy, fbconfigs[i]);
if (!visinfo) continue;
pictFormat = XRenderFindVisualFormat(dpy, visinfo->visual);
if (!pictFormat) continue;
if(pictFormat->direct.alphaMask > 0) {
fbconfig = fbconfigs[i];
break;
}
XFree(visinfo);
}
if (i == numfbconfigs) {
/* None of the FBConfigs have alpha. Use a normal (opaque)
* FBConfig instead */
fbconfig = fbconfigs[0];
visinfo = glXGetVisualFromFBConfig(dpy, fbconfig);
pictFormat = XRenderFindVisualFormat(dpy, visinfo->visual);
}
XFree(fbconfigs);
</pre>
<p></p>
<p>When rendering to a 32-bit window, keep in mind that the X
RENDER extension, used by most composite managers, expects
"premultiplied alpha" colors. This means that if your color has
components (r,g,b) and alpha value a, then you must render (a*r,
a*g, a*b, a) into the target window.</p>
<p>More information about Composite can be found at <a href=
"http://freedesktop.org/Software/CompositeExt" target=
"_top">http://freedesktop.org/Software/CompositeExt</a></p>
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