In After Effect CC, how can I use a layer, for example with a black shape, to use that shape as a mask for another layer? I know I can draw a transparency map on the layer I want to mask itself, but wheneven I move / resize that layer (it's a simple image, in my case) the mask gets moved / resized as well and I want the mask to stay put.
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I think you need to use a track matte - this lets you use one layer to control the transparency of another:
https://helpx.adobe.com/after-effects/using/alpha-channels-masks-mattes.html
