This Resolume tutorial for Arena 4 & 5 and Avenue 4 shows you a technique to display a mask overlay (also known as track matte) that both displays visuals within the mask and sits above all layers below it in the layer hierarchy.
The technique involves creating an alpha channel from black & white visuals to mask out footage. The resulting masked footage is then set up as a layer router which is then routed to another layer that is overlaid on top of a background visual. In software such as Adobe After Effects this is referred to as a track matte.
Music in this tutorial by:
Astronauts by Night