284 lines
9.0 KiB
HTML
284 lines
9.0 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
|
|
<html lang="en">
|
|
<head>
|
|
<meta http-equiv="content-type" content="text/html; charset=utf-8">
|
|
<title>Xlib Software Driver</title>
|
|
<link rel="stylesheet" type="text/css" href="mesa.css">
|
|
</head>
|
|
<body>
|
|
|
|
<div class="header">
|
|
<h1>The Mesa 3D Graphics Library</h1>
|
|
</div>
|
|
|
|
<iframe src="contents.html"></iframe>
|
|
<div class="content">
|
|
|
|
<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 conformant with a true GLX implementation.
|
|
The differences are fairly obscure, however.
|
|
</p>
|
|
|
|
<p>
|
|
The unique features of the Xlib driver follows.
|
|
</p>
|
|
|
|
|
|
<h2>X Visual Selection</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 back color buffer when in
|
|
double-buffer mode.
|
|
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>
|
|
Using XImage is almost always faster than a Pixmap since it resides in
|
|
the application's address space.
|
|
When glXSwapBuffers() is called, XPutImage() or XShmPutImage() is used
|
|
to transfer the XImage to the on-screen window.
|
|
</p>
|
|
<p>
|
|
A Pixmap may be faster when doing remote rendering of a simple scene.
|
|
Some OpenGL features will be very slow with a Pixmap (for example, blending
|
|
will require a round-trip message for pixel readback.)
|
|
</p>
|
|
<p>
|
|
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 <code>demos/gamma.c</code> program in mesa/demos repository 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 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.
|
|
This incurs a small performance penalty.
|
|
</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="specs/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="specs/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="specs/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>
|
|
|
|
</div>
|
|
</body>
|
|
</html>
|