Re: Coplanar Planes

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Allan Schaffer (aschaffe++at++shark.paris.sgi.com)
Thu, 16 Feb 1995 18:39:10 +0100

On Feb 16, 8:43am, Benito Graniela wrote:
>
> What can you do to solve this problem on a Indigo Extreme that can
> not support stencil or displacepolygon commands?
>
> We have roads over terrain that flash constantly, even when separated
> from the terrain by 0.1 meters. We believe this is a Z-buffer
> priority problem, can anybody elaborate!

The precision of the Z-buffer is what causes this effect. The
Z-buffer has sufficient resolution to distinguish objects close to
the eyepoint but as the objects get further away, in as much as the
zbuffer can tell, they fall within the same set of Z values.

> The best solution that we
> can come out with is to "stitch" the roads in, in other word
> replacing the terrain polygons with road polygons. This is a very
> time consuming task and we are looking for other solutions to this
> problem.

I think that the Indigo2 Extreme does support the stencil command, so
you should still be able to use pfLayer nodes. The 'pflayer.c'
sample from /usr/src/Performer/src/pguide/libpf/progs works on the
Indigo2 EX here my desk. Just be sure to specify PFDECAL_BASE_STENCIL
or PFDECAL_BASE_HIGH_QUALITY.

However, "stitching" the road is probably the best solution, since
stencil operations on EXtreme are fairly slow.

Allan

-- 
Allan Schaffer
Silicon Graphics
aschaffe++at++sgi.com
http://reality.sgi.com/employees/aschaffe

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