Re: info-performer Mar 10 1998

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Rob Jenkins (robj++at++quid.engr.sgi.com)
Wed, 11 Mar 1998 10:35:51 -0800

>
> >Adjusting far/near clip did not seem to improve the result at all ??
> >Adding a little offset (0.01 meters) to the subfaces seems
> >to do the trick though. Why? I'm a little bit confused about that.
>
> I thought I might add that after some experimenting with objects I have
> discovered
> that the further away from origo an object is the more inaccurate the
> polygon calculations
> are. If you model a square with a subface and put it far away from origo it
> will start to flicker even though the depth-buffer should be able to handle
> it.
>
> I hope this might help!!!
>

This still sounds like zbuffer precision problems in that the problem gets
worse as you move further from the eye. Bear in mind when adjusting the
near/far clipping planes that it's the ratio of near:far that determines the
precision spread so adjusting near to be a bit larger can make a huge
difference to that ratio ( whereas reducing far doens't change it much ). What
ratio of near:far do you have ?

When you say 'Adding a little offset' do you mean in the model itself ? Are you
using PFDECAL_BASE_DISPLACE or PFDECAL_BASE_DISPLACE | PFDECAL_LAYER_OFFSET if
you added the offest in your model then what you see is exaplained by the
zbuffer precision stuff. Coudl you try adding the PFDECAL_LAYER_OFFSET ?
Performer then calculates an offset based on the amount of zbuffer bits you
have. The limitations of displace are described in the pfLayer man page, the
glPolygonOffsetEXT has maybe more insight which might help make this clearer.

Cheers
Rob

-- 
________________________________________________________________
Rob Jenkins mailto:robj++at++sgi.com
Silicon Graphics, Mtn View, California, USA
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