Thu Jan 09, 2020 3:29 pm
Xunile's exactly right. LumaKey the dust; that will give you what you need. But let's tackle the matter of the Screen blending mode.
It doesn't make anything transparent. Instead, like all blend modes, it's a method of choosing what color the resulting pixel is when combining two images.
Screen does this:
1-((1-foreground) * (1-background)) = output RGB
maximum(fg alpha, bg alpha) = output A (this part is conjecture; I haven't actually tested what happens in the alpha)
In the case where the foreground is black (0), you get the background multiplied by 1: no change. This is why it appears to make black transparent. But it doesn't actually because the alpha comes through unchanged. If you Screen something over a black image, you just wind up with the original foreground because all you've done is multiply your foreground by 1.
Note that the Screen blending mode is actually a hack to deal with the limitations of integer color depths. If you Screen something that has values above 1, the result will get dimmer instead of brighter, so generally speaking you shouldn't use it if you're dealing with floating point color. The correct way of combining lights is to Add them, which you can do in a Merge node by leaving the Apply Mode in Over and reducing Alpha Gain to 0. Or you can use the Channel Booleans in Add mode (but set Alpha to Do Nothing).
Bryan Ray
http://www.bryanray.name
http://www.sidefx.com