Render conformance

Aaron Plattner aplattner at
Wed Sep 8 13:10:14 PDT 2004


I've been looking at our RenderAccel conformance, and I have a couple of

1. On 2004-08-30 Eric Anholt changed the transformed coordinates test in
   rendercheck so that it shifts the image by 1/2 pixel in both directions.
   This seems wrong to me.  Shouldn't the default filtering be to use
   whatever pixel the transformed coordinate falls inside?

2. The protocol spec is somewhat ambiguous about what "component alpha"
   really means, and the Porter/Duff paper isn't much help either.  Our
   understanding was simply that the color channels of the mask are used to
   modulate the source image before it is composited with the destination.
   The server (and rendercheck) instead use the mask as a separate alpha
   component for each channel.  This doesn't match any of OpenGL's (or
   D3D's) blending operations and is very difficult to accelerate in
   hardware.  Other drivers don't seem to support it either, and Glitz has
   to resort to several passes with glColorMask to achieve the same thing.
   Is this really the correct way to implement component alpha?  Spec
   clarification would be appreciated.


-- Aaron Plattner

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