Long story short, i've been messing with legacy OpenGL multi-texturing (OpenGL 1.2, I think) due to legacy hardware. I've been digging around some examples
here on multi-texturing and texture combiners, but none of them gives the results i'm looking for.
What i'd like is to combine two color-keyed textures, in this case a unit sprite and a health bar texture onto a single quad surface with one cleanly overlaid on the other while preserving the transparency/opacity of both. The examples in the link have a tendancy to mess up the alpha color keying, or blend it with distorted transparency, I think something like merging the opacity of the alpha layers on the respective textures then combining the RGB elements based on that might work, but i'm not sure.
Any insight on the feasibility of this would be appreciated.
Edit: Hm, well seeing as I can't seem to delete my own topic.. I figured out a solution after a bit of fiddling, which is ironic considering i've been messing with it for awhile. In case anyone else happens to find the relevant GL code useful, this subtracts the alpha from the first texture with the second textures alpha, then blends the first and second textures together with the base textures new alpha value:
glActiveTexture(GL_TEXTURE0)
glEnable(GL_TEXTURE_2D)
glBindTexture(GL_TEXTURE_2D, texture0.id)
## //Simply sample the texture
glTexEnvi(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_REPLACE)
## //------------------------
glActiveTexture(GL_TEXTURE1)
glEnable(GL_TEXTURE_2D)
glBindTexture(GL_TEXTURE_2D, texture1.id)
glTexEnvi(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_COMBINE)
glTexEnvi(GL_TEXTURE_ENV, GL_COMBINE_ALPHA, GL_SUBTRACT)# //Modulate ALPHA with ALPHA
glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE0_ALPHA, GL_PREVIOUS)
glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE1_ALPHA, GL_TEXTURE)
glTexEnvi(GL_TEXTURE_ENV, GL_OPERAND0_ALPHA, GL_SRC_ALPHA)
glTexEnvi(GL_TEXTURE_ENV, GL_OPERAND1_ALPHA, GL_SRC_ALPHA)
glTexEnvi(GL_TEXTURE_ENV, GL_COMBINE_RGB, GL_INTERPOLATE)# //Interpolate RGB with RGB
glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE0_RGB, GL_PREVIOUS)
glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE1_RGB, GL_TEXTURE)
glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE2_RGB, GL_PREVIOUS)
glTexEnvi(GL_TEXTURE_ENV, GL_OPERAND0_RGB, GL_SRC_COLOR)
glTexEnvi(GL_TEXTURE_ENV, GL_OPERAND1_RGB, GL_SRC_COLOR)
glTexEnvi(GL_TEXTURE_ENV, GL_OPERAND2_RGB, GL_SRC_ALPHA)
glActiveTexture(GL_TEXTURE2)
glEnable(GL_TEXTURE_2D)
glBindTexture(GL_TEXTURE_2D, texture1.id)
glTexEnvi(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_COMBINE)
glTexEnvi(GL_TEXTURE_ENV, GL_COMBINE_RGB, GL_INTERPOLATE)# //Interpolate RGB with RGB
glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE0_RGB, GL_PREVIOUS)
glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE1_RGB, GL_TEXTURE)
glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE2_RGB, GL_PREVIOUS)
glTexEnvi(GL_TEXTURE_ENV, GL_OPERAND0_RGB, GL_SRC_COLOR)
glTexEnvi(GL_TEXTURE_ENV, GL_OPERAND1_RGB, GL_SRC_COLOR)
glTexEnvi(GL_TEXTURE_ENV, GL_OPERAND2_RGB, GL_SRC_ALPHA)
glTexEnvi(GL_TEXTURE_ENV, GL_COMBINE_ALPHA, GL_INTERPOLATE)# //Interpolate ALPHA with ALPHA
glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE0_ALPHA, GL_PREVIOUS)
glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE1_ALPHA, GL_TEXTURE)
glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE2_ALPHA, GL_PREVIOUS)
glTexEnvi(GL_TEXTURE_ENV, GL_OPERAND0_ALPHA, GL_SRC_ALPHA)
glTexEnvi(GL_TEXTURE_ENV, GL_OPERAND1_ALPHA, GL_SRC_ALPHA)
glTexEnvi(GL_TEXTURE_ENV, GL_OPERAND2_ALPHA, GL_SRC_ALPHA)
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