[Cin] How do I crop a picture-in-picture?

[David Given]

I have two video tracks; one is overlayed on the other. I'm using projector
automation to put the overlay video in the corner of the underlying video.
I would like to crop the overlay video so it only shows the section I'm
interested in. The overlayed video is a 1:1 image in the middle of a 16:9
video, and I just want to show the 1:1 bit. I can't figure out how to do
this.

- I thought I would be able to use the camera automation to scale the image
after I had placed the projector, but while this works, it happens *before* any
track plugins are rendered, and as one of these is rotation I always end up
with a rotated rectangle on the screen. (With black borders.)
- using the crop plugin adds more black borders.
- I tried a mask, but I haven't found out how to invert the mask (i.e. I
draw the area I want displayed so that everything else is cropped); plus
cropped areas show up as black.
- I simply can't get my head around the crop tool.

The best I've come up with is to add a scale plugin to the track to resize
it up a bit, so that the black wedges caused by the rotate plugin aren't
visible; then adjusting the camera to zoom in on the are I'm interested in;
but I haven't found a way to change the *shape* of the projector yet. It's
always a 16:9 rectangle, but the area I want to show isn't that shape.

What's the right way to do this? Ideally what I would like is the ability
to draw a path on an object on the screen and say 'clip this object to this
path'. Does anything like that exist?

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