User-agent: Mozilla Thunderbird 0.8 (Macintosh/20040913)
Hi Shamyl,
Wild stab: this looks bad to me:
//why doesn't this give me a constant alpha?
glTexEnvi( GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_COMBINE_ALPHA_ARB );
glTexEnvi( GL_TEXTURE_ENV, GL_SOURCE0_ALPHA_ARB, GL_CONSTANT_ARB );
glTexEnvi( GL_TEXTURE_ENV, GL_OPERAND0_ALPHA_ARB, GL_ONE );
glTexEnvi( GL_TEXTURE_ENV, GL_COMBINE_ALPHA_ARB, GL_REPLACE );
I do not think that GL_ONE is a valid operand mode for the combine
extension! (I think that GL_ONE is only used for framebuffer blending
via GL_BLEND and the imaging extensions but don't quote me on that.) At
least the spec only lists these four operands for the RGB section:
and of the four only the second two are allowed as inputs to the alpha
section.
So it looks to me like your alpha is not getting zapped like you think
it should be, and so alpha blending is zonking out your whole texture at
the fragment->framebuffer blend phase.
If this is the problem, the fix is easy: use the constant color (which
is per texture unit I _think_) to get yourself alpha:
// Set up the constant color to have alpha as 1 so we can just
// set our texes alpha to 1.
// When setting up the alpha, use replace, constant color, src alpha.
glTexEnvi(GL_TEXTURE_ENV, GL_COMBINE_ALPHA_EXT, GL_REPLACE);
glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE0_ALPHA_EXT, GL_CONSTANT_EXT);
glTexEnvi(GL_TEXTURE_ENV, GL_OPERAND0_ALPHA_EXT, GL_SRC_ALPHA);
I'm amazed you saw anything in your test actually...in my experience the
combine extension is very picky, and most of the time when I screw up I
just get black, so I'm surprised that passing a strange token to the
extension gave you any output.
I have to assume I'm doing something boneheaded. But please, don't just
point me to the COMBINE documentation. I've read it a dozen times. I
just don't *get* it, I guess. Could somebody clear it up for me? I'm so
baffled, and heartbroken.
The combine output is pretty opaque...I've been thinking that when I
finish this release I should write some kind of intro to combine, since
there seems to be nothing out there that shows how to use combine for
anything more complex than emulating GL_MODULATE (and what good is that
:-). It only took a stinkin' year to figure out how it worked via trial
and error...
Anyway, hope this helps! Combine is a very cool extension in that you
can fake a lot of shader-like effects on really ancient hardware. Too
bad the Gf-MX chipset has only 2 TUs...
*Cheers*
Ben
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