docs/xlibdriver.html

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  10 <div class="header">
  11   <h1>The Mesa 3D Graphics Library</h1>
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  16
  17 <h1>Xlib Software Driver</h1>
  18
  19 <p>
  20 Mesa's Xlib driver provides an emulation of the GLX interface so that
  21 OpenGL programs which use the GLX API can render to any X display, even
  22 those that don't support the GLX extension.
  23 Effectively, the Xlib driver converts all OpenGL rendering into Xlib calls.
  24 </p>
  25
  26 <p>
  27 The Xlib driver is the oldest Mesa driver and the most mature of Mesa's
  28 software-only drivers.
  29 </p>
  30
  31 <p>
  32 Since the Xlib driver <em>emulates</em> the GLX extension, it's not
  33 totally conformant with a true GLX implementation.
  34 The differences are fairly obscure, however.
  35 </p>
  36
  37 <p>
  38 The unique features of the Xlib driver follows.
  39 </p>
  40
  41
  42 <h2>X Visual Selection</h2>
  43 <p>
  44 Mesa supports RGB(A) rendering into almost any X visual type and depth.
  45 </p>
  46 <p>
  47 The glXChooseVisual function tries to choose the best X visual
  48 for the given attribute list.  However, if this doesn't suit your needs
  49 you can force Mesa to use any X visual you want (any supported by your
  50 X server that is) by setting the <b>MESA_RGB_VISUAL</b> and
  51 <b>MESA_CI_VISUAL</b>
  52 environment variables.
  53 When an RGB visual is requested, glXChooseVisual
  54 will first look if the MESA_RGB_VISUAL variable is defined.
  55 If so, it will try to use the specified visual.
  56 Similarly, when a color index visual is requested, glXChooseVisual will
  57 look for the MESA_CI_VISUAL variable.
  58 </p>
  59
  60 <p>
  61 The format of accepted values is:  <code>visual-class depth</code>
  62 </p>
  63 <p>
  64 Here are some examples:
  65 </p>
  66 <pre>
  67    using csh:
  68         % setenv MESA_RGB_VISUAL "TrueColor 8"          // 8-bit TrueColor
  69         % setenv MESA_CI_VISUAL "PseudoColor 12"        // 12-bit PseudoColor
  70         % setenv MESA_RGB_VISUAL "PseudoColor 8"        // 8-bit PseudoColor
  71
  72    using bash:
  73         $ export MESA_RGB_VISUAL="TrueColor 8"
  74         $ export MESA_CI_VISUAL="PseudoColor 12"
  75         $ export MESA_RGB_VISUAL="PseudoColor 8"
  76 </pre>
  77
  78
  79 <h2>Double Buffering</h2>
  80 <p>
  81 Mesa can use either an X Pixmap or XImage as the back color buffer when in
  82 double-buffer mode.
  83 The default is to use an XImage.
  84 The <b>MESA_BACK_BUFFER</b> environment variable can override this.
  85 The valid values for <b>MESA_BACK_BUFFER</b> are:  <b>Pixmap</b> and
  86 <b>XImage</b> (only the first letter is checked, case doesn't matter).
  87 </p>
  88
  89 <p>
  90 Using XImage is almost always faster than a Pixmap since it resides in
  91 the application's address space.
  92 When glXSwapBuffers() is called, XPutImage() or XShmPutImage() is used
  93 to transfer the XImage to the on-screen window.
  94 </p>
  95 <p>
  96 A Pixmap may be faster when doing remote rendering of a simple scene.
  97 Some OpenGL features will be very slow with a Pixmap (for example, blending
  98 will require a round-trip message for pixel readback.)
  99 </p>
 100 <p>
 101 Experiment with the MESA_BACK_BUFFER variable to see which is faster
 102 for your application.
 103 </p>
 104
 105
 106 <h2>Colormaps</h2>
 107 <p>
 108 When using Mesa directly or with GLX, it's up to the application
 109 writer to create a window with an appropriate colormap.  The GLUT
 110 toolkit tries to minimize colormap <em>flashing</em> by sharing
 111 colormaps when possible.  Specifically, if the visual and depth of the
 112 window matches that of the root window, the root window's colormap
 113 will be shared by the Mesa window.  Otherwise, a new, private colormap
 114 will be allocated.
 115 </p>
 116
 117 <p>
 118 When sharing the root colormap, Mesa may be unable to allocate the colors
 119 it needs, resulting in poor color quality.  This can happen when a
 120 large number of colorcells in the root colormap are already allocated.
 121 To prevent colormap sharing in GLUT, set the
 122 <b>MESA_PRIVATE_CMAP</b> environment variable.  The value isn't
 123 significant.
 124 </p>
 125
 126
 127 <h2>Gamma Correction</h2>
 128 <p>
 129 To compensate for the nonlinear relationship between pixel values
 130 and displayed intensities, there is a gamma correction feature in
 131 Mesa.  Some systems, such as Silicon Graphics, support gamma
 132 correction in hardware (man gamma) so you won't need to use Mesa's
 133 gamma facility.  Other systems, however, may need gamma adjustment
 134 to produce images which look correct.  If you believe that
 135 Mesa's images are too dim, read on.
 136 </p>
 137
 138 <p>
 139 Gamma correction is controlled with the <b>MESA_GAMMA</b> environment
 140 variable.  Its value is of the form <b>Gr Gg Gb</b> or just <b>G</b> where
 141 Gr is the red gamma value, Gg is the green gamma value, Gb is the
 142 blue gamma value and G is one gamma value to use for all three
 143 channels.  Each value is a positive real number typically in the
 144 range 1.0 to 2.5.
 145 The defaults are all 1.0, effectively disabling gamma correction.
 146 Examples:
 147 </p>
 148 <pre>
 149         % export MESA_GAMMA="2.3 2.2 2.4"       // separate R,G,B values
 150         % export MESA_GAMMA="2.0"               // same gamma for R,G,B
 151 </pre>
 152 <p>
 153 The <code>demos/gamma.c</code> program in mesa/demos repository may help
 154 you to determine reasonable gamma value for your display.  With correct
 155 gamma values, the color intensities displayed in the top row (drawn by
 156 dithering) should nearly match those in the bottom row (drawn as grays).
 157 </p>
 158
 159 <p>
 160 Alex De Bruyn reports that gamma values of 1.6, 1.6 and 1.9 work well
 161 on HP displays using the HP-ColorRecovery technology.
 162 </p>
 163
 164 <p>
 165 Mesa implements gamma correction with a lookup table which translates
 166 a "linear" pixel value to a gamma-corrected pixel value.  There is a
 167 small performance penalty.  Gamma correction only works in RGB mode.
 168 Also be aware that pixel values read back from the frame buffer will
 169 not be "un-corrected" so glReadPixels may not return the same data
 170 drawn with glDrawPixels.
 171 </p>
 172
 173 <p>
 174 For more information about gamma correction, see the
 175 <a href="https://en.wikipedia.org/wiki/Gamma_correction">Wikipedia article</a>
 176 </p>
 177
 178
 179 <h2>Overlay Planes</h2>
 180 <p>
 181 Hardware overlay planes are supported by the Xlib driver.  To
 182 determine if your X server has overlay support you can test for the
 183 SERVER_OVERLAY_VISUALS property:
 184 </p>
 185 <pre>
 186         xprop -root | grep SERVER_OVERLAY_VISUALS
 187 </pre>
 188
 189
 190 <h2>HPCR Dithering</h2>
 191 <p>
 192 If you set the <b>MESA_HPCR_CLEAR</b> environment variable then dithering
 193 will be used when clearing the color buffer.  This is only applicable
 194 to HP systems with the HPCR (Color Recovery) feature.
 195 This incurs a small performance penalty.
 196 </p>
 197
 198
 199 <h2>Extensions</h2>
 200 <p>
 201 The following Mesa-specific extensions are implemented in the Xlib driver.
 202 </p>
 203
 204 <h3>GLX_MESA_pixmap_colormap</h3>
 205
 206 <p>
 207 This extension adds the GLX function:
 208 </p>
 209 <pre>
 210     GLXPixmap glXCreateGLXPixmapMESA( Display *dpy, XVisualInfo *visual,
 211                                       Pixmap pixmap, Colormap cmap )
 212 </pre>
 213 <p>
 214 It is an alternative to the standard glXCreateGLXPixmap() function.
 215 Since Mesa supports RGB rendering into any X visual, not just True-
 216 Color or DirectColor, Mesa needs colormap information to convert RGB
 217 values into pixel values.  An X window carries this information but a
 218 pixmap does not.  This function associates a colormap to a GLX pixmap.
 219 See the xdemos/glxpixmap.c file for an example of how to use this
 220 extension.
 221 </p>
 222 <p>
 223 <a href="specs/MESA_pixmap_colormap.spec">GLX_MESA_pixmap_colormap specification</a>
 224 </p>
 225
 226
 227 <h3>GLX_MESA_release_buffers</h3>
 228 <p>
 229 Mesa associates a set of ancillary (depth, accumulation, stencil and
 230 alpha) buffers with each X window it draws into.  These ancillary
 231 buffers are allocated for each X window the first time the X window
 232 is passed to glXMakeCurrent().  Mesa, however, can't detect when an
 233 X window has been destroyed in order to free the ancillary buffers.
 234 </p>
 235 <p>
 236 The best it can do is to check for recently destroyed windows whenever
 237 the client calls the glXCreateContext() or glXDestroyContext()
 238 functions.  This may not be sufficient in all situations though.
 239 </p>
 240 <p>
 241 The GLX_MESA_release_buffers extension allows a client to explicitly
 242 deallocate the ancillary buffers by calling glxReleaseBuffersMESA()
 243 just before an X window is destroyed.  For example:
 244 </p>
 245 <pre>
 246          #ifdef GLX_MESA_release_buffers
 247             glXReleaseBuffersMESA( dpy, window );
 248          #endif
 249          XDestroyWindow( dpy, window );
 250 </pre>
 251 <p>
 252 <a href="specs/MESA_release_buffers.spec">GLX_MESA_release_buffers specification</a>
 253 </p>
 254 <p>
 255 This extension was added in Mesa 2.0.
 256 </p>
 257
 258 <h3>GLX_MESA_copy_sub_buffer</h3>
 259 <p>
 260 This extension adds the glXCopySubBufferMESA() function.  It works
 261 like glXSwapBuffers() but only copies a sub-region of the window
 262 instead of the whole window.
 263 </p>
 264 <p>
 265 <a href="specs/MESA_copy_sub_buffer.spec">GLX_MESA_copy_sub_buffer specification</a>
 266 </p>
 267 <p>
 268 This extension was added in Mesa 2.6
 269 </p>
 270
 271 <h2>Summary of X-related environment variables</h2>
 272 <pre>
 273    MESA_RGB_VISUAL - specifies the X visual and depth for RGB mode (X only)
 274    MESA_CI_VISUAL - specifies the X visual and depth for CI mode (X only)
 275    MESA_BACK_BUFFER - specifies how to implement the back color buffer (X only)
 276    MESA_PRIVATE_CMAP - force aux/tk libraries to use private colormaps (X only)
 277    MESA_GAMMA - gamma correction coefficients (X only)
 278 </pre>
 279
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