Sharon Clay (src++at++rose.engr.sgi.com)
Tue, 20 Jan 1998 14:07:28 -0800
->
->It's the cone with clouds used in our data base, and when we show it on
->the Onyx2 the texture
->has this banding behaviour.
The reason the textures looked banded on Onyx2 and OK on O2 is because
of the default transparency type when multisampled is enabled - the
Onyx2 is using the multisampled screen door transparency (a percentage of
the subpixels get the full incoming color) wheras the O2 is
using blended transparency. If you run the example in perfly and just
turn off antialiasing you'll see it switch to blended transparency and
the banding disappear. When blended, all subpixels are subjected to
the full blend function. The performance trade-offs are tricky.
Blending can be slower (much) - it requires sorting for proper blending
of different colors and bigger pixels (12 bit) are slower to blend.
However, screen door can cause some performance hits as well if you
have many layers of it and the quality is not as high. However,
screen door is a great way to do something like fade LOD since sorting
is not required to get a consistent result. So, when there is
multisampling available, we use screen door. You can force blended
transparency on the pfGeoState by selecting PFTR_HIGH_QUALITY
or PFTR_BLEND_ALPHA.
src.
--
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Sharon Rose Clay - Silicon Graphics, Advanced Systems Dev.
src++at++sgi.com (650) 933 - 1002 FAX: (650) 965 - 2658 MS 8U-590
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