Alpha in textures

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Dewey Anderson (dewey++at++evt.com)
Mon, 14 Oct 1996 12:39:28 -0700

Let's say I take a rectangle and texture map solid white onto it with a 50%
alpha value. (I think this also applies to just coloring the rectangle, but my
particular application is texturing.) When I look at the alpha signal out on
our High Impact (connecting a monitor to the alpha spigot) the alpha looks
considerably lower than 50%. It's as if the alpha value is applied not only to
the r,g,b values but to the alpha value itself.

If this is so, the reasoning escapes me. That sounds to me like buying
something for $1000 and being told the tax rate is 10%. So the salesman
multiplies 10% times $1000 and says "The tax is $100". And THEN he multiplies
10% times 10% and says "The tax rate is 1%." It makes no sense to do that 2nd
multiplication.

Similarly, if I have a color of r=1, g=1 and b=1 and the alpha value is 50%, I
take 50% and mulitply it times the color and get r=0.5, g=0.5 and b=0.5. I
don't then want to multiply 50% by 50% and say "The alpha value is 25%."

Anyway, is there some way to make this not happen? (Perhaps this is more of an
OpenGL issue. Can Performer control this aspect of OpenGL?)

(Also, I recognize that while in MY application this behavior makes no sense,
it might in others. I'd be interested in hearing an explanation of why this
might be a good thing to do.)

Dewey Anderson
dewey++at++evt.com
Evolving Video Technologies

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