Re: High IMPACT and Transparency

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Dewey Anderson (dewey++at++evt.com)
Mon, 11 Mar 1996 17:28:23 -0800

>>We have just upgraded one of our machines to High Impact graphics and
>>are experiencing problems with polygons textured with Alpha components
>>(i.e. IA, RGBA).
>>
>>The problem is that polygons containing alpha in their texture
>>disappear when viewed through another polygon with alpha.
>
>Sounds like classic Z buffer discard of more distant polygons. Z buffer knows
>nothing about transparency.

Sounds a lot like a problem I had until I looked into the "alpha function". As
Greg said, the Z buffer doesn't know about transparency but by setting an alpha
function you can make completely transparent pixels not affect the Z buffer. I
picked an alpha value of 10 arbitrarily as a cutoff for transparent:

  // Set alpha function to clip off alpha values less than 10.
  // This is needed to make low alpha pixels actually NOT be drawn at all.
  // THAT is necessary so that z-buffer isn't filled in by clear pixels.
  gstate->setMode(PFSTATE_ALPHAFUNC, PFAF_GREATER);
  gstate->setVal(PFSTATE_ALPHAREF, (10.0f/255.0f));

If the edges of your texture are soft you'll notice some shading at the edges
since the cutoff for not effecting the z buffer is abrupt. It WOULD be nice if
Performer sorted things so that layers of transparency could be stacked, but
then, it wouldn't really need a z-buffer at all if did that. (Painter's
algorithm.) I assume that's computationally way too expensive.

Dewey K. Anderson
Evolving Video Technologies
Arvada, Colorado

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