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I have a basic After Effects question, but I don't know the right words for it to find what I need.

I have a composition that consists of three side-by-side videos. But, for now, it's just three layers on top of eachother. I created three skewed rectangles (Shapes). I want each of the layers to be shown on top ONLY of one of the shape layers. So, I want footage A to be shown on top of shape A, footage B on shape B, footage C on shape C. There is also some whitespace, between the shapes, so the background video is shown as well.

How would I accomplish this?

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The easiest way would be to use the shapes as track mattes for the video layers.

Put the Footage A video layer directly beneath the Shape A layer and in the Modes column choose Alpha Matte "Shape A" from the Track Matte drop-down. Repeat for Footage B and Shape B and Footage C and Shape C

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What track mattes do is use the alpha (transparency) or luma (brightness) of a layer to set the transparency of the layer directly below it. The Alpha Inverted option reverses wht is visible or invisible.

There's always at least two ways to do anything in After Effects, so I should just mention that other ways to do the same thing include copying the shape paths and pasting as masks on the video layers, or using the silhouette alpha mode on the shape layers and pre-comping. All methods have advantages and disadvantages. Read the page here for more.

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