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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.

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"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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