I've used images with an alpha channel as textures, with varying degrees of success. (For instance, in my never-ending quest to find a functional 'frosted glass' material, I came up with this:
That uses a black-to-grey diffusion cloud image with a different black-to-white diffusion cloud as the alpha channel. While it's an interesting effect, I can't say it's an entirely successful one: For one thing, in the scene there's a nude woman[1] standing immediately beyond where you see that square grey shadow; for another, that square grey shadow is somehow being cast by the cube in the left foreground -- through the wall, with light from a nonexistent source.) So, as long as you don't mind the resulting image thoroughly violates the laws of physics of the virtual universe where the scene resides, I guess you could say alpha channels work.
Actually, I probably could have lived with a chaotic upheaval of the basic rules of existence had I only been able to get them to fail dependably. Instead, the outcome of the render was incredibly variable, based on a range of factors I could never identify. For instance, when I took the texture used in the previous example and applied it to individual bricks in this 8 x 4 glass brick bathroom wall[2], this was the result:[3]
Ultimately, I threw in the towel and tried to find another way to fake it.
Maz
__________
1. After all, this was a test for a frosted glass shower stall.
2. Similar to the one in the house where I grew up.
3. The other very obvious bug -- namely, the 'melting' marble splash panel -- was the result of my having erroneously both softened and smoothed edges in the original SketchUp model.
