Xlib Software Driver

# The Mesa 3D Graphics Library

# 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 conformant with a true GLX implementation. The differences are fairly obscure, however.

The unique features of the Xlib driver follows.

## X Visual Selection

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 back color buffer when in double-buffer mode. 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).

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.

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.)

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 tries 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 `demos/gamma.c` 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).

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 the Wikipedia article

## 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 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. This incurs a small performance penalty.

## 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)