Simon Hayhurst (simon++at++nirvana.neu.sgi.com)
Tue, 21 May 1996 20:02:11 -0600
Impact's have 9 bit framebuffers and 36 bit framebuffers. At all resolutions
you have 1x9bit buffer on both max & high impact.
Screen Resolution Number of Available 36 bit buffers
High Impact Max Impact
1024x768 3 6
1280x1024 2 4
1600x1200 1 2
HDTV 1 2
Each 36 bit buffer can support :-
12/12/12 RGB Single Buffer (36 bits used)
8/8/8/8 RGBA Single Buffer (32 bits used)
5/5/5/1 RGBA Double Buffer (16x2 32 bits used)
4/4/4/4 RGBA Double Buffer (16x2 32 bits used)
12 Color Index Double Buffer (12x2 24 bits used)
24bit-Z & 8-bit Stencil (ZST). (24 + 8 bits used)
So you can now see how many framebuffers would be required to (eg) double
buffer.
Now there is a catch, some bits are needed from the 36 bit buffers.
On a per-buffer basis, some bits have special functions, eg for tagged clears.
A framebuffer with no free bits (12/12/12 RGB in fact) cannot do tagged clears
as an example, therefore screen clearing is a little slower.
The final catch, is that some of these control bits are essential, not on a
per-buffer basis, but on a 'graphics pipe' basis. Hence, you cannot fully
deplete all 'possible' 36 bit framebuffer bits.
Net result, at 1280x1024 on a high impact, you cannot do 2 x 12/12/12 RGB (ie
double buffered 12/12/12 RGB), because of the need for control bits, but you
could do 12/12/12 RGB double buffered on a Max Impact.
Hope this helps,
Simon
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Simon Hayhurst Phone (41)-38-433733
The Computarium Fax (41)-38-433905
Core Technology Group Voice Mail 5-7269
SGI, Cortaillod, Switzerland Internet simon ++at++ neu.sgi.com
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