"Full-Stage" clips triggered over individual slices?

[IsocellatorComm]

Here is the effect I am going for...

I have four slices of video on stage, each showing independent clips; I would like to show a 5th clip that would display simultaneously on all 4 slices, but as if all 4 slices were holes in a stage-wide mask...

Let me try to explain it another way...

Imagine 4 triangles of the same size, side-by-side across the stage, each showing separate images. At some point a horizontal blue line draws across all of the triangles, one by one at the same height; but other than the blue line, all of the triangles retain their individual content. The blue line is a 5th clip, played as if on a slice that spans the whole stage, but only displays on the triangles..

Hopefully I am accurately describing this, but I'm looking to layer a stage-unifying effect (with a linear progression) over several slices.

I am trying to accomplish this by making a slice for each triangle, and then adding a 5th slice (tied to a higher layer) that overlaps all 4, with a complicated mask with 4 triangular holes that match the triangles in size and location...

Am I on the right track here? or is there a better way to build this?

I am also trying to figure out a layer structure that would still allow mixing on the triangle slices but would also allow a semi-opaque layer/slice to appear on all simultaneously...

[Zoltán]

you can do crop and masks on the advanced output. try that to mask out the areas you don't want the slice of the 5th clip to show thru.

or you can do this without slices, just position resize, keystone the clips, then on a separate layer you can play your clip 5, with some masks.
this second version would allow you to mix the contents fully with blending.