Re: Shadow bug

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Angus Dorbie (dorbie++at++sgi.com)
Wed, 13 May 1998 12:48:17 -0700

Andrew Ng wrote:
>
> > We're using the pfLightSource mechanism on an Onyx2 with iR
> > graphics to generate shadows in our application. We have found
> > that with certain camera positions, some garbage geometry appears
> > to be creating a shadow; by moving the camera slightly, the
> > effect goes away.
>
> I have also been using pfLightSource shadows on our Onyx iR (which has
> patch 1808 installed) and have noticed the anomalies that you describe. The
> only explanation I can think of is that the floor geometry is actually
> shadowing itself in these situations. I had a quick go with different
> values for the shadow displace and scale but couldn't come up with a
> satisfactory set up.

The self shadow theory seems unlikely, particularly since only parts
seem to be occulted and you tried different offsets.

Maybe there is some interaction with pfLayers (stencil or polygon
offset)
or other state information and the shadow rendering?

Or perhaps there's a problem with the framebuffer readback although this
seems unlikely since the problem is reported as view dependent.

Have you tried actually reading back z and checking the depth values in
the image array?

One thing to try is to disable antialiasing and see if the problem
persists, this might point an accusatory finger ad zbuffer compression
on iR.

Cheers,Angus.

-- 
"Only the mediocre are always at their best." -- Jean Giraudoux 


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