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Angus Dorbie (dorbie++at++sgi.com)
Mon, 08 Jun 1998 09:43:13 -0700

Prakash Mahesh wrote:
>
> --
> We have been attempting to combine different texturing modes (a
> mixture of modulate and decal) in Performer to allow for the following
> behavior for 4 channel texture images:
> C = (1-At)Cf + At(Ct)
> A = At
> where At is the alpha channel for the texel,
> Cf is the color of the fragment,
> and Ct is the color of the texel.
>
> This differs from the standard approach which is:
> C = (1-At)Cf + At(Ct)
> A = Af ****
> where At is the alpha channel for the texel,
> Af is the alpha channel for the fragment,
> Cf is the color of the fragment,
> and Ct is the color of the texel.
>
> Is there a simple way to achieve such functionality when using
> Performer on top of OpenGL?

Well you are free to call down to the gl in draw callbacks, see
www.dorbie.com
for Performer based examples.

I don't see a way of implementing your request. The intention of the
OpenGL
texture environments are fairly clear and you requiring the alpha to act
as a
blender AND as fragment alpha source is very strange, for example it'd
cause
a base polygon color halo around decals if you use it this way. I can
see
that it might be usefull to disable blending and still enable alpha
writes
for subsequent multipass effects or it could be interesting to use the
base
polygon color to add a per vertex colored lining to texture modulated
clouds.
Could you give an explanation of your higher level objective and perhaps
we
can find another rendering approach.

Cheers,Angus.

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


For advanced 3D graphics Performer + OpenGL based examples and tutors:
http://www.dorbie.com/
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