Angus Dorbie (dorbie++at++multipass.asd.sgi.com)
Mon, 16 Jun 1997 18:06:10 -0700
>
> This basically draws the front frustum on top of the back frustum, with only
> a Z clear. Notice that you can get color information from the back channel
> (blending purposes) but at the seam, the translucent polygons get rendered
> twice so they are twice as dark. Basically, if you overlay the frustums,
> you will see the seam only on translucent polygons (at certain angles).
> If they are "exactly" matched you will see cracks in the polygons at the seam
> (again, at certain angles). Thus, this works really well for opaque
polygons,
> but messes up on translucent ones at the seam. I implemented this a while
ago
> in GL, but also found it works great in Performer.
>
This has been suggested before in this forum.
To avoid transparency problems & clipping crack problems, you might try
to implement an overlap where you could guarantee no object required to be
rendered to both channels and would therefore never be clipped by either.
You have to start worrying about extraneous depth complexity then and you'll
probably find that > 2 channels is worth exploring to accomodate small
objects transiting the overlap region while allowing efficient pixel fill
with 2 channel clipping on terrain. Also multisample transparency would not
suffer from the double contribution you describe.
With small or no overlap you may also get lighting/clipping &
gouraud/perspective shading problems.
Cheers,Angus.
--
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