# change register combiner's mux operator

Hi,

having an RGBA texture, I want to render only the parts which are in an alpha range of [a,b]. Two pass algorithm is working (using glAlphaFunc) but I want to do it in a single pass.
By using the register combiners’ mux operations this could be done, but the mux selects AB or CD by comparing spare0’s alpha to 0.5. So, my question is how to map the a and b so that they can be used in the mux operator, instead of 0.5?
Or is there another one pass algorithm to do it?

Thanks
kon

/edit adding/subtracting the difference of a,b and 0.5 to the texture’s alpha values is possible but it takes a register combiner and I’ve got only two of them and these are needed for the two muxs!

[This message has been edited by kon (edited 05-03-2002).]

well… if alpha < min, set out.alpha to 0
else set out.alpha to unsigned(max - alpha)

result: 0 in out.alpha if alpha > max, 0 in out.alpha if alpha < min…
check against 0 in the alphatest and you-re fine out…

alpha<min == alpha-min<0 == alpha-min+.5<.5
== alpha + (1-min) - .5 < .5

you have to calculate this in the general combiner 0, and store in spare0.alpha (with some nice bias and such this goes well… spare0.alpha = unsigned_identity(alpha)+unsigned_invert(min) biased by .5)

calc (in the color combiner 0 for example unsigned(max - alpha) and store this in some spare0.rgb for example…

then in the second combiner alpha you can do the mux against spare0.alpha, and return eighter 0 or spare0.rgb.

glAlphaTest(GL_NOTEQUAL,0);

everything nonzero alpha will be okay…

hope that helped, cause i’m little confused myself… long time no coding in the rc’s anymore…

I’m assuming you intend the a and b to be constants (not something pulled from a texture or anything). If so, just set the alpha test to only accept pixels with an alpha value >= a. Then all you need to do is configure the register combiners to output an alpha of 0 for pixels that have an alpha value > b, or the actual alpha value otherwise. You can do this as follows:

combiner 1: calculate b - alpha + .5 and output to spare0.alpha

combiner 2: make AB = alpha, make CD = zero, and the mux will select the correct one for you. Output the result as the spare0.alpha

final combiner: select spare0.alpha as your output alpha

Hopefully that is enough description. If you need actual code let me know and I can probably try to put it together.

Originally posted by davepermen:
hope that helped, cause i’m little confused myself

You’re not the only one confused by that

Ah, thanks I’ll give it try this weekend! I’m not that much confused anymore!

kon