I'm dropping a 3d object into footage of a real city and want it to be occluded by buildings as the view orbits the object. I can mask buildings fine, and use a set alpha node to combine the mask with the original image sequence no problem. But when I use the output of the set alpha as the image input for the next mask, nothing happens.

This works: Composite with single mask

This however, with the next two buildings that block it from view added does not work: 2 additional masks

Here, I highlight which mask/set alpha is applying to which building. If I reverse which mask is applied last, then the building in green is the mask that works. It's only the last mask that works. Is there another node I need to be using instead of set alpha for multiple masks. I have 4 more buildings that need to mask over the rocket as the camera orbits. I've seen examples on youtube with multiple set alpha nodes but I can't seem to get that to work. Any help would be greatly appreciated.


"Set Alpha" should be used only once, to associate the alpha channel to the RGB information on the image.

To combine alpha channel use a math node and add (or subtract) the alphas together before associating.


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