Re: Multisample Masks

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Sharon Clay (src++at++rose.asd.sgi.com)
Wed, 26 Feb 1997 13:57:44 -0800

+>---- On Feb 26, 6:54pm, Orad Hi-Tec Systems wrote:
> Subject: Re: Multisample Masks
->Mark Baranowski wrote:

->
->Multisample transparency through fragment alpha
->(glEnable(GL_SAMPLE_ALPHA_TO_MASK_SGIS))
->has only n+1 levels of transparency (includeing totally transparent and
->totally opaque), where n is the number of multi-samples in your visual.
->glSampleMaskSGIS() produces a 2x2 transparency dithering pattern which
->boosts the number of levels to 4*n+1 - so it is advantageous in this
->respect too. You'll have to install patch1355 to fix a bug in the
->dithering
->patterns.
->
->The quality of the fade greatly depends on this n.

Very true.

Just for general reference, don't be too afraid to use blending,
particularly if your objects are simple so that you don't need to
sort amoungst them for proper results. Basic blending with
smaller RGB visual sizes (such as 8 or 10) is pretty quick. In some
cases it can even be faster because unlike the multisample mask
techniques, it doesn't create complex pixels which can slow future
rendering to those pixels.

->There is a *very* annoying SGI hardware bug which prevents the setup
->of n=8 multisamples coexisting with alpha bit-planes (no such visual
->exists). You can only do n=4 multisamples with alpha bit-planes.
->This is *independent* of your framebuffer memory size (number of RM6
->boards).

Just to clarify to avoid possible confusion here, destination alpha planes
are not needed to implement the fade techniques discussed - they only use
source (in-coming) alpha.

src.

-- 
-----{-----{---++at++   -----{----{---++at++   -----{----{---++at++   -----{----{---++at++
Sharon Rose Clay (Fischler) - Silicon Graphics, Advanced Systems Dev.
src++at++sgi.com  (415) 933 - 1002  FAX: (415) 965 - 2658  MS 8U-590
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