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6181ab9d77
That link has been dead for 15 years... We could link to Archive.org [1] to get the last time this page existed, but I feel like Wikipedia is a better choice. [1] http://web.archive.org/web/20021211151318/http://www.inforamp.net/~poynton/notes/colour_and_gamma/GammaFAQ.html Signed-off-by: Eric Engestrom <eric.engestrom@imgtec.com> Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
283 lines
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HTML
283 lines
9.0 KiB
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html lang="en">
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<head>
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<meta http-equiv="content-type" content="text/html; charset=utf-8">
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<title>Xlib Software Driver</title>
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<link rel="stylesheet" type="text/css" href="mesa.css">
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</head>
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<body>
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<div class="header">
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<h1>The Mesa 3D Graphics Library</h1>
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</div>
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<iframe src="contents.html"></iframe>
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<div class="content">
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<h1>Xlib Software Driver</h1>
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<p>
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Mesa's Xlib driver provides an emulation of the GLX interface so that
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OpenGL programs which use the GLX API can render to any X display, even
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those that don't support the GLX extension.
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Effectively, the Xlib driver converts all OpenGL rendering into Xlib calls.
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</p>
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<p>
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The Xlib driver is the oldest Mesa driver and the most mature of Mesa's
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software-only drivers.
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</p>
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<p>
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Since the Xlib driver <em>emulates</em> the GLX extension, it's not
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totally conformant with a true GLX implementation.
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The differences are fairly obscure, however.
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</p>
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<p>
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The unique features of the Xlib driver follows.
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</p>
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<h2>X Visual Selection</h2>
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<p>
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Mesa supports RGB(A) rendering into almost any X visual type and depth.
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</p>
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<p>
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The glXChooseVisual function tries to choose the best X visual
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for the given attribute list. However, if this doesn't suit your needs
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you can force Mesa to use any X visual you want (any supported by your
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X server that is) by setting the <b>MESA_RGB_VISUAL</b> and
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<b>MESA_CI_VISUAL</b>
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environment variables.
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When an RGB visual is requested, glXChooseVisual
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will first look if the MESA_RGB_VISUAL variable is defined.
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If so, it will try to use the specified visual.
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Similarly, when a color index visual is requested, glXChooseVisual will
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look for the MESA_CI_VISUAL variable.
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</p>
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<p>
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The format of accepted values is: <code>visual-class depth</code>
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</p>
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<p>
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Here are some examples:
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</p>
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<pre>
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using csh:
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% setenv MESA_RGB_VISUAL "TrueColor 8" // 8-bit TrueColor
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% setenv MESA_CI_VISUAL "PseudoColor 12" // 12-bit PseudoColor
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% setenv MESA_RGB_VISUAL "PseudoColor 8" // 8-bit PseudoColor
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using bash:
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$ export MESA_RGB_VISUAL="TrueColor 8"
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$ export MESA_CI_VISUAL="PseudoColor 12"
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$ export MESA_RGB_VISUAL="PseudoColor 8"
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</pre>
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<h2>Double Buffering</h2>
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<p>
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Mesa can use either an X Pixmap or XImage as the back color buffer when in
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double-buffer mode.
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The default is to use an XImage.
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The <b>MESA_BACK_BUFFER</b> environment variable can override this.
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The valid values for <b>MESA_BACK_BUFFER</b> are: <b>Pixmap</b> and
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<b>XImage</b> (only the first letter is checked, case doesn't matter).
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</p>
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<p>
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Using XImage is almost always faster than a Pixmap since it resides in
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the application's address space.
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When glXSwapBuffers() is called, XPutImage() or XShmPutImage() is used
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to transfer the XImage to the on-screen window.
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</p>
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<p>
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A Pixmap may be faster when doing remote rendering of a simple scene.
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Some OpenGL features will be very slow with a Pixmap (for example, blending
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will require a round-trip message for pixel readback.)
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</p>
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<p>
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Experiment with the MESA_BACK_BUFFER variable to see which is faster
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for your application.
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</p>
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<h2>Colormaps</h2>
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<p>
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When using Mesa directly or with GLX, it's up to the application
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writer to create a window with an appropriate colormap. The GLUT
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toolkit tries to minimize colormap <em>flashing</em> by sharing
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colormaps when possible. Specifically, if the visual and depth of the
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window matches that of the root window, the root window's colormap
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will be shared by the Mesa window. Otherwise, a new, private colormap
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will be allocated.
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</p>
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<p>
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When sharing the root colormap, Mesa may be unable to allocate the colors
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it needs, resulting in poor color quality. This can happen when a
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large number of colorcells in the root colormap are already allocated.
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To prevent colormap sharing in GLUT, set the
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<b>MESA_PRIVATE_CMAP</b> environment variable. The value isn't
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significant.
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</p>
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<h2>Gamma Correction</h2>
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<p>
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To compensate for the nonlinear relationship between pixel values
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and displayed intensities, there is a gamma correction feature in
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Mesa. Some systems, such as Silicon Graphics, support gamma
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correction in hardware (man gamma) so you won't need to use Mesa's
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gamma facility. Other systems, however, may need gamma adjustment
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to produce images which look correct. If you believe that
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Mesa's images are too dim, read on.
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</p>
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<p>
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Gamma correction is controlled with the <b>MESA_GAMMA</b> environment
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variable. Its value is of the form <b>Gr Gg Gb</b> or just <b>G</b> where
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Gr is the red gamma value, Gg is the green gamma value, Gb is the
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blue gamma value and G is one gamma value to use for all three
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channels. Each value is a positive real number typically in the
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range 1.0 to 2.5.
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The defaults are all 1.0, effectively disabling gamma correction.
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Examples:
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</p>
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<pre>
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% export MESA_GAMMA="2.3 2.2 2.4" // separate R,G,B values
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% export MESA_GAMMA="2.0" // same gamma for R,G,B
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</pre>
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<p>
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The <code>demos/gamma.c</code> program in mesa/demos repository may help
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you to determine reasonable gamma value for your display. With correct
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gamma values, the color intensities displayed in the top row (drawn by
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dithering) should nearly match those in the bottom row (drawn as grays).
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</p>
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<p>
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Alex De Bruyn reports that gamma values of 1.6, 1.6 and 1.9 work well
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on HP displays using the HP-ColorRecovery technology.
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</p>
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<p>
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Mesa implements gamma correction with a lookup table which translates
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a "linear" pixel value to a gamma-corrected pixel value. There is a
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small performance penalty. Gamma correction only works in RGB mode.
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Also be aware that pixel values read back from the frame buffer will
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not be "un-corrected" so glReadPixels may not return the same data
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drawn with glDrawPixels.
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</p>
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<p>
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For more information about gamma correction, see the
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<a href="https://en.wikipedia.org/wiki/Gamma_correction">Wikipedia article</a>
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</p>
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<h2>Overlay Planes</h2>
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<p>
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Hardware overlay planes are supported by the Xlib driver. To
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determine if your X server has overlay support you can test for the
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SERVER_OVERLAY_VISUALS property:
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</p>
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<pre>
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xprop -root | grep SERVER_OVERLAY_VISUALS
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</pre>
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<h2>HPCR Dithering</h2>
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<p>
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If you set the <b>MESA_HPCR_CLEAR</b> environment variable then dithering
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will be used when clearing the color buffer. This is only applicable
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to HP systems with the HPCR (Color Recovery) feature.
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This incurs a small performance penalty.
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</p>
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<h2>Extensions</h2>
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<p>
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The following Mesa-specific extensions are implemented in the Xlib driver.
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</p>
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<h3>GLX_MESA_pixmap_colormap</h3>
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<p>
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This extension adds the GLX function:
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</p>
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<pre>
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GLXPixmap glXCreateGLXPixmapMESA( Display *dpy, XVisualInfo *visual,
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Pixmap pixmap, Colormap cmap )
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</pre>
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<p>
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It is an alternative to the standard glXCreateGLXPixmap() function.
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Since Mesa supports RGB rendering into any X visual, not just True-
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Color or DirectColor, Mesa needs colormap information to convert RGB
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values into pixel values. An X window carries this information but a
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pixmap does not. This function associates a colormap to a GLX pixmap.
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See the xdemos/glxpixmap.c file for an example of how to use this
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extension.
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</p>
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<p>
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<a href="specs/MESA_pixmap_colormap.spec">GLX_MESA_pixmap_colormap specification</a>
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</p>
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<h3>GLX_MESA_release_buffers</h3>
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<p>
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Mesa associates a set of ancillary (depth, accumulation, stencil and
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alpha) buffers with each X window it draws into. These ancillary
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buffers are allocated for each X window the first time the X window
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is passed to glXMakeCurrent(). Mesa, however, can't detect when an
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X window has been destroyed in order to free the ancillary buffers.
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</p>
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<p>
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The best it can do is to check for recently destroyed windows whenever
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the client calls the glXCreateContext() or glXDestroyContext()
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functions. This may not be sufficient in all situations though.
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</p>
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<p>
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The GLX_MESA_release_buffers extension allows a client to explicitly
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deallocate the ancillary buffers by calling glxReleaseBuffersMESA()
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just before an X window is destroyed. For example:
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</p>
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<pre>
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#ifdef GLX_MESA_release_buffers
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glXReleaseBuffersMESA( dpy, window );
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#endif
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XDestroyWindow( dpy, window );
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</pre>
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<p>
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<a href="specs/MESA_release_buffers.spec">GLX_MESA_release_buffers specification</a>
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</p>
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<p>
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This extension was added in Mesa 2.0.
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</p>
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<h3>GLX_MESA_copy_sub_buffer</h3>
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<p>
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This extension adds the glXCopySubBufferMESA() function. It works
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like glXSwapBuffers() but only copies a sub-region of the window
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instead of the whole window.
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</p>
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<p>
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<a href="specs/MESA_copy_sub_buffer.spec">GLX_MESA_copy_sub_buffer specification</a>
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</p>
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<p>
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This extension was added in Mesa 2.6
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</p>
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<h2>Summary of X-related environment variables</h2>
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<pre>
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MESA_RGB_VISUAL - specifies the X visual and depth for RGB mode (X only)
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MESA_CI_VISUAL - specifies the X visual and depth for CI mode (X only)
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MESA_BACK_BUFFER - specifies how to implement the back color buffer (X only)
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MESA_PRIVATE_CMAP - force aux/tk libraries to use private colormaps (X only)
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MESA_GAMMA - gamma correction coefficients (X only)
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</pre>
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</div>
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</body>
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</html>
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