Graphics Administration Guide (July 2008)

Screen /dev/crt
DefaultVisual
Transparent
The display diagram would be the same as that of the “Results of Minimal Legal
X*screens File” configuration, above.
miscellaneous topics
double buffer extensions
DBE is an extension to the X Server that provides a double-buffering Application
Programming Interface (API). For more information about DBE and the API, consult
the DBE man pages:
DBE
XdbeQueryExtension
XdbeGetVisualInfo
XdbeFreeVisualInfo
XdbeAllocateBackBufferName
XdbeDeallocateBackBufferName
XdbeSwapBuffers
XdbeBeginIdiom
XdbeEndIdiom
XdbeGetBackBufferAttributes
performing buffer swaps on vertical blank
For performance reasons, the default DBE behavior is to not synchronize buffer swaps
with the monitor’s vertical retrace period. In some instances, therefore, image tearing
(seeing part of the old image and part of the new image on the display at the same
time) could be visible while swapping large DBE windows. For those instances where
tearing would occur and is undesirable, an optional X Server mode is available to allow
for synchronization of buffer swaps with vertical retrace. To activate this optional X
Server mode, set the following screen option in the X*screens File before the X Server
is started:
SwapBuffersOnVBlank
determining swap performance
The DBE API does not allow users to determine if double-buffering in a visual is through
software or hardware. However, the API does provide a way to determine relative
swapping performance on a per-visual basis. The XdbeScreenVisualInfo() function
returns information about the swapping performance levels for the double-buffering
visuals on a display. A visual with a higher performance level is likely to have better
double-buffer graphics performance than a visual with a lower performance level.
Nothing can be deduced from any of the following: the magnitude of the difference of
two performance levels, a performance level in isolation, or comparing performance
levels from different servers.
X Server configuration 43