From 12ad488e598499cf17a619b221e8e4afea363d02 Mon Sep 17 00:00:00 2001 From: Brian Paul Date: Wed, 19 Apr 2006 03:25:06 +0000 Subject: Replace old README.X11 with updated xlibdriver.html --- docs/xlibdriver.html | 265 +++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 265 insertions(+) create mode 100644 docs/xlibdriver.html (limited to 'docs/xlibdriver.html') diff --git a/docs/xlibdriver.html b/docs/xlibdriver.html new file mode 100644 index 0000000000..7734542dcb --- /dev/null +++ b/docs/xlibdriver.html @@ -0,0 +1,265 @@ + + +Xlib Software Driver + + + + + +

Xlib Software Driver

+ +

+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. +

+ +

+The Xlib driver is the oldest Mesa driver and the most mature of Mesa's +software-only drivers. +

+ +

+Since the Xlib driver emulates the GLX extension, it's not +totally conformance with a true GLX implementation. +The differences are fairly obscure, however. +

+ +

+The unique features of the Xlib driver follows. +

+ + +

X Display Modes

+Mesa supports RGB(A) rendering into almost any X visual type and depth. +

+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 MESA_RGB_VISUAL and +MESA_CI_VISUAL +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. +

+ +

+The format of accepted values is: visual-class depth +

+Here are some examples: +

+   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"
+

+ + +

Double buffering

+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 +MESA_BACK_BUFFER environment variable can override this. The valid +values for MESA_BACK_BUFFER are: Pixmap and XImage +(only the first letter is checked, case doesn't matter). +

+ +

+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. +

+ + +

Colormaps

+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 flashing 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. +

+ +

+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 +MESA_PRIVATE_CMAP environment variable. The value isn't +significant. +

+ + +

Gamma correction

+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. +

+ +

+Gamma correction is controlled with the MESA_GAMMA environment +variable. Its value is of the form Gr Gg Gb or just G 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: +

+	% 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
+

+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). +

+ +

+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. +

+ +

+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. +

+ +

+For more information about gamma correction see: + +

+ + +

Overlay Planes

+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: +

+	xprop -root | grep SERVER_OVERLAY_VISUALS
+

+ + +

HPCR glClear(GL_COLOR_BUFFER_BIT) dithering

+If you set the MESA_HPCR_CLEAR 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. +

+ + +

Extensions

+The following MESA-specific extensions are implemented in the Xlib driver. +

+ +

GLX_MESA_pixmap_colormap

+ +

+This extension adds the GLX function: +

+    GLXPixmap glXCreateGLXPixmapMESA( Display *dpy, XVisualInfo *visual,
+                                      Pixmap pixmap, Colormap cmap )
+

+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. +

+GLX_MESA_pixmap_colormap specification +

+ + +

GLX_MESA_release_buffers

+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. +

+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. +

+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: +

+         #ifdef GLX_MESA_release_buffers
+            glXReleaseBuffersMESA( dpy, window );
+         #endif
+         XDestroyWindow( dpy, window );
+

+GLX_MESA_release_buffers specification +

+This extension was added in Mesa 2.0. +

+ +

GLX_MESA_copy_sub_buffer

+This extension adds the glXCopySubBufferMESA() function. It works +like glXSwapBuffers() but only copies a sub-region of the window +instead of the whole window. +

+GLX_MESA_copy_sub_buffer specification +

+This extension was added in Mesa 2.6 +

+ +

Summary of X-related environment variables

+   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)
+

+ + + + -- cgit v1.2.3