underlay(3G) underlay(3G)
underlay - allocates bitplanes for display of underlay colors
void underlay(planes)
long planes;
planes expects the number of bitplanes to be allocated for underlay
colors. Valid values are 0 (the default), 2, 4, and 8.
The IRIS physical framebuffer is divided into four separate GL
framebuffers: normal, popup, overlay, underlay. Because a single
physical framebuffer is used to implement the four GL framebuffers,
bitplanes must be allocated among the GL framebuffers. underlay
specifies the number of bitplanes to be allocated to the underlay
framebuffer. underlay does not take effect immediately. Rather, it is
considered only when gconfig is called, at which time all requests for
bitplane resources are resolved.
While only one of the four GL framebuffers can be drawn to at a time (see
drawmode), all four are displayed simultaneously. The decision of which
to display at each pixel is made based on the contents of the four
framebuffers at that pixel location, using the following hierarchical
rule:
if the popup pixel contents are non-zero
then display the popup bitplanes
else if overlay bitplanes are allocated AND
the overlay pixel contents are non-zero
then display the overlay bitplanes
else if the normal pixel contents are non-zero OR
no underlay bitplanes are allocated
then display the normal bitplanes
else display the underlay bitplanes
Thus images drawn into the overlay framebuffer appear over images in the
normal framebuffer, and images drawn into the underlay framebuffer appear
under images in the normal framebuffer. Popup images appear over
everything else.
The default configuration of the underlay framebuffer is 0 bitplanes. To
make a change to this configuration other than to change the bitplane
size, the drawing mode must be UNDERDRAW. For example, the underlay
framebuffer can be configured to be double buffered by calling
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underlay(3G) underlay(3G)
doublebuffer while draw mode is UNDERDRAW.
On models that cannot support overlay and underlay bitplanes
simultaneously, calling underlay with a non-zero argument forces overlay
to zero. When simultaneous overlay and underlay operation is supported,
calling underlay may have no effect on the number of overlay bitplanes.
doublebuffer, drawmode, gconfig, getgdesc, singlebuffer, underlay
This routine is available only in immediate mode.
IRIS-4D G, GT, and GTX models, and the Personal Iris, support only single
buffered, color map mode underlay bitplanes.
The Iris Indy supports 8 underlay single buffered bitplanes, if
configured with 24 bit normal color planes.
The Personal Iris supports 0 or 2 underlay bitplanes. There are no
overlay or underlay bitplanes in the minimum configuration of the
Personal Iris.
IRIS-4D GT and GTX models support 0, 2, or 4 underlay bitplanes. Because
4-bitplane allocation reduces the popup framebuffer to zero bitplanes,
however, its use is strongly discouraged. The window manager cannot
operate properly when no popup bitplanes are available.
IRIS-4D VGX models support 0, 2, 4, or 8 underlay bitplanes, either
single or double buffered, in color map mode only. The 4 and 8 bitplane
allocations utilize the alpha bitplanes, which must be present, and which
therefore are unavailable in draw mode NORMALDRAW.
Use getgdesc to determine the maximum number of bitplanes supported for
the underlay framebuffer.
Iris Indigo does not support underlay so getgdesc returns zero for the
number of underlay bitplanes on Iris Indigo.
The Personal Iris does not support shade model GOURAUD in the underlay
framebuffer.
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