Help using Apply Mode: Screen in merge

I have what is probably a ridiculously simple problem and I can't seem to figure it out.

I have ProRes stock footage. It's dust floating on a black background. I can add it to a scene using the merge node with Apply mode set to screen. That works great. The scene is the 2D output from 3D.

However, I'd like to use it as part of a 3D scene and so I'm trying to set it up as a standalone image that is transparent except for the dust. That way I can send it to an ImagePlane3D and add it to my 3D environment to get parralax.

So all I'm doing is creating a background node. Then I make that background node transparent by dragging RGB and Alpha to zero. Great. Now I should just need to use merge, set the transparent image as background, set the dust as foreground, use Screen as the Apply Mode and I should have a transparent background with dust in the foreground.

It doesn't work. And for the life of me I can't tell why. I've tried half a dozen things and no luck. I don't understand why I can use the merge node with 'Screen' with this exact same footage as the foreground and it makes it transparent, while using it with a standalone transparent background node and Merge in exactly the same way doesn't work.

Help!!

Thanks,

Mark.

This is my node structure:

Screen Shot 2020-01-08 at 8.22.37 PM.png (33.12 KiB) Viewed 1744 times

And this is the background node settings:

Screen Shot 2020-01-08 at 8.22.16 PM.png (116.98 KiB) Viewed 1744 times

I'm viewing the merge node in my viewers. The background remains black. I expect it to be transparent.

This is the merge node settings:

Screen Shot 2020-01-08 at 8.25.11 PM.png (182.82 KiB) Viewed 1744 times

The way I would do it is take the ProRes footage MediaIn, connect it to a LumaKeyer. Connect that to your ImagePlane3D, no Background node needed. Then connect it to your Merge3D node and connect everything else as you want.

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).

Thanks Eric, really appreciate the reply!!

Bryan, thanks so much for the explanation in terms of fundamentals. That makes a lot of sense.

~Mark.

Thanks Eric, really appreciate the reply!!

Bryan, thanks so much for the explanation in terms of fundamentals. That makes a lot of sense.

~Mark.

You're welcome.