magfilter strange problems

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Nicolas Gauvin (nicolas++at++cae.ca)
Fri, 5 Jul 1996 14:01:28 -0400

I ran into strange behevior of texture magnification filters lately.
I have a 128x128 texture that I use as a cloud layer on a very large
polygon covering the sky. This texture has alpha components.

Texture mag filters are:
  tex->setFilter( PFTEX_MAGFILTER_ALPHA, PFTEX_TRILINEAR );
  tex->setFilter( PFTEX_MAGFILTER_COLOR, PFTEX_TRILINEAR );

This give very good results on an Impact machine (compiled OpenGL)
and on a RE2 (compiled IrisGL) with one RM4. The texture is magnified
as expected.

However, when the same program is run on a Infinite Reality or a RE2
with 3 RM5, the same texture looks very bad. We see some very visible and
discreet gradient of colors near the edge of the clouds. Much like contour
lines.

I suspect this is related to the fact that on those two machines,
there is enough RMs to enable multisampling at the resolution I'm running
where on the other RE2 and on the Impact, there was no multisampling.

Is there something additionnal I should do to correct this sort of problem?

Nicolas Gauvin CAE Electronics Ltd., 8585 Cote De Liesse
Software Developer Saint-Laurent, Quebec, Canada, H4L-4X4
3-D Graphics Applications tel: +1 514 341 2000 extension 2275
nicolas++at++cae.ca fax: +1 514 340 5496
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