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+<HTML>
+
+<TITLE>Xlib Software Driver</TITLE>
+
+<link rel="stylesheet" type="text/css" href="mesa.css"></head>
+
+<BODY>
+
+<H1>Xlib Software Driver</H1>
+
+<p>
+Mesa's Xlib driver provides an emulation of the GLX interface so that
+OpenGL programs which use the GLX API can render to any X display, even
+those that don't support the GLX extension.
+Effectively, the Xlib driver converts all OpenGL rendering into Xlib calls.
+</p>
+
+<p>
+The Xlib driver is the oldest Mesa driver and the most mature of Mesa's
+software-only drivers.
+</p>
+
+<p>
+Since the Xlib driver <em>emulates</em> the GLX extension, it's not
+totally conformance with a true GLX implementation.
+The differences are fairly obscure, however.
+</p>
+
+<p>
+The unique features of the Xlib driver follows.
+</p>
+
+
+<H2>X Display Modes</H2>
+<p>
+Mesa supports RGB(A) rendering into almost any X visual type and depth.
+</p>
+<p>
+The glXChooseVisual function tries to choose the best X visual
+for the given attribute list. However, if this doesn't suit your needs
+you can force Mesa to use any X visual you want (any supported by your
+X server that is) by setting the <b>MESA_RGB_VISUAL</b> and
+<b>MESA_CI_VISUAL</b>
+environment variables.
+When an RGB visual is requested, glXChooseVisual
+will first look if the MESA_RGB_VISUAL variable is defined.
+If so, it will try to use the specified visual.
+Similarly, when a color index visual is requested, glXChooseVisual will
+look for the MESA_CI_VISUAL variable.
+</p>
+
+<p>
+The format of accepted values is: <code>visual-class depth</code>
+</p>
+<p>
+Here are some examples:
+</p>
+<pre>
+ using csh:
+ % setenv MESA_RGB_VISUAL "TrueColor 8" // 8-bit TrueColor
+ % setenv MESA_CI_VISUAL "PseudoColor 12" // 12-bit PseudoColor
+ % setenv MESA_RGB_VISUAL "PseudoColor 8" // 8-bit PseudoColor
+
+ using bash:
+ $ export MESA_RGB_VISUAL="TrueColor 8"
+ $ export MESA_CI_VISUAL="PseudoColor 12"
+ $ export MESA_RGB_VISUAL="PseudoColor 8"
+</pre>
+
+
+<H2>Double buffering</H2>
+<p>
+Mesa can use either an X Pixmap or XImage as the backbuffer when in
+double buffer mode. Using GLX, the default is to use an XImage. The
+<b>MESA_BACK_BUFFER</b> environment variable can override this. The valid
+values for <b>MESA_BACK_BUFFER</b> are: <b>Pixmap</b> and <b>XImage</b>
+(only the first letter is checked, case doesn't matter).
+</p>
+
+<p>
+A pixmap is faster when drawing simple lines and polygons while an
+XImage is faster when Mesa has to do pixel-by-pixel rendering. If you
+need depth buffering the XImage will almost surely be faster.
+Experiment with the MESA_BACK_BUFFER variable to see which is faster
+for your application.
+</p>
+
+
+<H2>Colormaps</H2>
+<p>
+When using Mesa directly or with GLX, it's up to the application
+writer to create a window with an appropriate colormap. The GLUT
+toolkit tris to minimize colormap <em>flashing</em> by sharing
+colormaps when possible. Specifically, if the visual and depth of the
+window matches that of the root window, the root window's colormap
+will be shared by the Mesa window. Otherwise, a new, private colormap
+will be allocated.
+</p>
+
+<p>
+When sharing the root colormap, Mesa may be unable to allocate the colors
+it needs, resulting in poor color quality. This can happen when a
+large number of colorcells in the root colormap are already allocated.
+To prevent colormap sharing in GLUT, set the
+<b>MESA_PRIVATE_CMAP</b> environment variable. The value isn't
+significant.
+</p>
+
+
+<H2>Gamma correction</H2>
+<p>
+To compensate for the nonlinear relationship between pixel values
+and displayed intensities, there is a gamma correction feature in
+Mesa. Some systems, such as Silicon Graphics, support gamma
+correction in hardware (man gamma) so you won't need to use Mesa's
+gamma facility. Other systems, however, may need gamma adjustment
+to produce images which look correct. If you believe that
+Mesa's images are too dim, read on.
+</p>
+
+<p>
+Gamma correction is controlled with the <b>MESA_GAMMA</b> environment
+variable. Its value is of the form <b>Gr Gg Gb</b> or just <b>G</b> where
+Gr is the red gamma value, Gg is the green gamma value, Gb is the
+blue gamma value and G is one gamma value to use for all three
+channels. Each value is a positive real number typically in the
+range 1.0 to 2.5.
+The defaults are all 1.0, effectively disabling gamma correction.
+Examples:
+</p>
+<pre>
+ % export MESA_GAMMA="2.3 2.2 2.4" // separate R,G,B values
+ % export MESA_GAMMA="2.0" // same gamma for R,G,B
+</pre>
+<p>
+The progs/demos/gamma.c program may help you to determine reasonable gamma
+value for your display. With correct gamma values, the color intensities
+displayed in the top row (drawn by dithering) should nearly match those
+in the bottom row (drawn as grays).
+</p>
+
+<p>
+Alex De Bruyn reports that gamma values of 1.6, 1.6 and 1.9 work well
+on HP displays using the HP-ColorRecovery technology.
+</p>
+
+<p>
+Mesa implements gamma correction with a lookup table which translates
+a "linear" pixel value to a gamma-corrected pixel value. There is a
+small performance penalty. Gamma correction only works in RGB mode.
+Also be aware that pixel values read back from the frame buffer will
+not be "un-corrected" so glReadPixels may not return the same data
+drawn with glDrawPixels.
+</p>
+
+<p>
+For more information about gamma correction see:
+<a href="http://www.inforamp.net/~poynton/notes/colour_and_gamma/GammaFAQ.html"
+the Gamma FAQ</a>
+</p>
+
+
+<H2>Overlay Planes</H2>
+<p>
+Hardware overlay planes are supported by the Xlib driver. To
+determine if your X server has overlay support you can test for the
+SERVER_OVERLAY_VISUALS property:
+</p>
+<pre>
+ xprop -root | grep SERVER_OVERLAY_VISUALS
+</pre>
+
+
+<H2>HPCR glClear(GL_COLOR_BUFFER_BIT) dithering</H2>
+<p>
+If you set the <b>MESA_HPCR_CLEAR</b> environment variable then dithering
+will be used when clearing the color buffer. This is only applicable
+to HP systems with the HPCR (Color Recovery) feature.
+</p>
+
+
+<H2>Extensions</H2>
+<p>
+The following MESA-specific extensions are implemented in the Xlib driver.
+</p>
+
+<h3>GLX_MESA_pixmap_colormap</h3>
+
+<p>
+This extension adds the GLX function:
+</p>
+<pre>
+ GLXPixmap glXCreateGLXPixmapMESA( Display *dpy, XVisualInfo *visual,
+ Pixmap pixmap, Colormap cmap )
+</pre>
+<p>
+It is an alternative to the standard glXCreateGLXPixmap() function.
+Since Mesa supports RGB rendering into any X visual, not just True-
+Color or DirectColor, Mesa needs colormap information to convert RGB
+values into pixel values. An X window carries this information but a
+pixmap does not. This function associates a colormap to a GLX pixmap.
+See the xdemos/glxpixmap.c file for an example of how to use this
+extension.
+</p>
+<p>
+<a href="MESA_pixmap_colormap.spec">GLX_MESA_pixmap_colormap specification</a>
+</p>
+
+
+<h3>GLX_MESA_release_buffers</h3>
+<p>
+Mesa associates a set of ancillary (depth, accumulation, stencil and
+alpha) buffers with each X window it draws into. These ancillary
+buffers are allocated for each X window the first time the X window
+is passed to glXMakeCurrent(). Mesa, however, can't detect when an
+X window has been destroyed in order to free the ancillary buffers.
+</p>
+<p>
+The best it can do is to check for recently destroyed windows whenever
+the client calls the glXCreateContext() or glXDestroyContext()
+functions. This may not be sufficient in all situations though.
+</p>
+<p>
+The GLX_MESA_release_buffers extension allows a client to explicitly
+deallocate the ancillary buffers by calling glxReleaseBuffersMESA()
+just before an X window is destroyed. For example:
+</p>
+<pre>
+ #ifdef GLX_MESA_release_buffers
+ glXReleaseBuffersMESA( dpy, window );
+ #endif
+ XDestroyWindow( dpy, window );
+</pre>
+<p>
+<a href="MESA_release_buffers.spec">GLX_MESA_release_buffers specification</a>
+</p>
+<p>
+This extension was added in Mesa 2.0.
+</p>
+
+<H3>GLX_MESA_copy_sub_buffer</H3>
+<p>
+This extension adds the glXCopySubBufferMESA() function. It works
+like glXSwapBuffers() but only copies a sub-region of the window
+instead of the whole window.
+</p>
+<p>
+<a href="MESA_copy_sub_buffer.spec">GLX_MESA_copy_sub_buffer specification</a>
+</p>
+<p>
+This extension was added in Mesa 2.6
+</p>
+
+<h2>Summary of X-related environment variables</H2>
+<pre>
+ MESA_RGB_VISUAL - specifies the X visual and depth for RGB mode (X only)
+ MESA_CI_VISUAL - specifies the X visual and depth for CI mode (X only)
+ MESA_BACK_BUFFER - specifies how to implement the back color buffer (X only)
+ MESA_PRIVATE_CMAP - force aux/tk libraries to use private colormaps (X only)
+ MESA_GAMMA - gamma correction coefficients (X only)
+</pre>
+
+
+</body>
+</html>