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B. OPENGL ENVIRONMENT
This structure is similar to the standard pixel format descriptor and allows each
layer to define it own device capabilities independent of the main plane or other
layer planes. This allows a double buffered, rgb main plane to coexist with single
buffered, color index overlay plane etc. Each layer defines a color which is defined
to be transparent and allows rendering in lower planes to show through. Functions
are provided for creating rendering contexts in layer planes, manipulating the pal-
ettes of color index layers and swapping layer planes independently of the main
plane if the graphics device supports this capability.
ASUS drivers support OpenGL overlays are provided in 32bpp truecolor display
modes. In standard truecolor modes each 32bit pixel is split into 8 bits of red, green,
blue and alpha. Overlays are provided by replacing the alpha component with 8 bits
of overlay. Thus a single 8 bit color index overlay is provided in addition to a 24 bit
rgb main plane.
The overlay plane is single buffered if the main plane is single buffered and double
buffered if the main plane is double buffered. Independent swaps of the main and
overlay planes are supported when blit double buffered. When full-screen double
buffered, such as when an OpenGL application takes up the entire display, indepen-
dent swaps are not supported.
To provide access to OpenGL overlays in 32bpp modes extra pixel formats are ex-
ported, in addition to the standard pixel formats. These extra pixel formats are de-
fined to have an 8 bit overlay plane but no alpha. As soon as any OpenGL applica-
tion chooses a pixel format of one type or the other all the pixel formats that are
exported are changed to only support that mode. For instance, if the first pixel
format chosen includes overlays all the remaining pixel formats exported will be
changed to export overlays but not alpha and vice versa. When all OpenGL win-
dows are closed the system reverts to exporting a choice of pixel formats. The
driver must work in this way as setting overlay mode is a global change and alpha
and overlay planes cannot be supported simultaneously.
When overlay planes are used an additional registry variable must be set, otherwise
icon corruption may be seen, DisableOffScreenBitmaps
This variable should be created as a DWORD value in the same place as the other
variables (described in the section, Registry Variables) and set to a value of one. A
reboot is necessary for this change to take affect
If the Softimage rendering package (which requires overlay plane functionality) is
used we recommend that one of the SupportSoftimage registry variables are set.
Full details on these variables are given earlier in the section describing all of the
registry variables. This will optimise the OpenGL drivers for use with the package.
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ASUS 3DP-V500TX
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