Re: (originally no subject) Texturing modes

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Angus Dorbie (dorbie++at++sgi.com)
Mon, 08 Jun 1998 09:46:56 -0700

Moshe Nissim wrote:
>
> 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?
> >
>
> That would wonderful to have in OpenGL, wouldn't it?I guess you too are trying to overcome the OpenGL way of "mixing"
> lighting+material color with texture color by multiplication.

No, what's being requested is _very_ strange and has limited
application.
Perhaps you have something else in mind, but it's unclear exactly what.

> The standard approach that you mention is GL_DECAL in glTexEnv,
> with four-channel texture. However, not only is the
> framebuffer alpha affected (which can be worked around by subsequent
> rendering into the same area, or by disabling writes into the framebuffer
> alpha channel), but you also have to sacrifice the texture
> alpha channel for this constant value. This means you cannot combine
> this with a RGBA texture that modulates the alpha. Looking at the glTexEnv
> table, I see no other equation that blends Cf and Ct.
> The only thing that comes close is the GL_ADD with RGB texture, which does
> C = Cf + Ct Cc + Cb
> A = Af
> This is not true blending, but at least you have some control external to the
> texture alpha (Cc - the texture "environment color"), and Af is written to the
> framebuffer alpha, as you want.
> I think you can do this through Performer directly too, with pfTEnvMode
> and pfTEnvBlendColor.
>
> Bye,
> Moshe
>
> --
> Moshe Nissim, Orad Hi-Tec Systems
> Tel: (972) - 9 - 7676862
> Fax: (972) - 9 - 7676861
> Email: moshe++at++orad.co.il
>
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