FCPX Custom Video Shapes

Easiest way: Make your shape white on black, put it on top of your footage, and in the Inspector change the "Blend mode" from "Normal" to "Stencil Luma".

This will, however, affect all video clips below your shape. If you want to have something else playing "in the back", you will have to create a "Compound clip" from the shape and the video you want to cut out.

Tell me if this was going too fast... but that's a quick and easy way I can think of.

 

what you're trying to do is called an "alpha matte"

for your star example do the following:

-create a black star on white bg (or white star on black bg) in photoshop.
-place the image of the star above the video you want in star shape
-select the clip with the star image, go to the inspector, scroll down to compositing and select "blend mode - silhouette luma" (if you made a black star on white bg - for the other way round select "stencil luma")
-create a compound clip containing the star image and the video that's in the star
-place it over the video you want to have outside the star

it's all about blend modes (and compound clips). instead of a b&w matte (btw. greys are semi-transparent) you could also create a transparent image in photoshop, save it with an alpha channel (e.g. as .png ) and then use stencil/silhouette alpha as blend modes). this also works with videos as mattes.

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damn, you beat me by 1 minute, floh. gute nacht!

 

Well, that's because your answer was more elaborate and useful, so I was done with mine sooner. At least we had the same idea and hopefully didn't confuse the OP...