Where did the original clips come from? Was one layer generated with After Effects? Does that clip contain alpha channel information?
The original clips were generated from Blender. When I select the clip and do View>Show in Viewer>Color Channels>Alpha, it IS all white.
This means that there are no areas that are transparent, only blacks. Right? But the clips are already composited in ‘Normal’ and show other background clips without any problem.
It is kinda like using a shape mask, but with the color picker tool from Color Board.