I think the background/focus/frame approach mentioned in the manual is very convincing. But how do you create the frame (window which blocks some parts of the content underneath)? I find it quite cumbersome to achieve this with the Resolume sources or effects, but maybe there are some tricks?
I tried the Metaballs Source in the top layer, with Balls color = red and background color = black. Using an inverted auto mask, the metaballs background created a black frame, but the visible part of layer 2 (where the balls are) is now red.
I'd rather have "tranparent metaballs".. but when I turn off auto mask and reduce the alpha of the red color of the metaballs to zero, the output is just black.
I would expect the red color to be transparent with alpha=0, not to be black? Confusing.