Inverting a mask on a black solid top layer turns it into a cutout window onto lower layers

To reveal underlying layers only through a shaped aperture, VPT uses a topmost layer as a global cutout mask. Create a new top layer that fills the whole output, set its source to ‘solid’, draw and save a mask shape, then apply it to that layer. By default a mask makes its shape opaque and the surround transparent — the opposite of what a cutout needs — so you do two things: invert the mask, and set the layer’s colour to black. The top layer is then opaque everywhere except inside the mask shape, so all lower layers are hidden except where they show through the ‘window’. Useful for projecting through a door frame or any irregular opening.

Examples

Make a full-output top layer with a solid source, draw a circle mask, invert it, set the layer black; a circular window now reveals the video on layer 1 while everything else stays black.

Assessment

Give the exact steps and settings to make a circular cutout through which lower layers show. Why must the mask be inverted and the layer colour set to black?