user16994: 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.

user4426: 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

16994: <p>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.</p>
 4426: <p>I think you need to use a track matte - this lets you use one layer to control the transparency of another:</p>

<p><a href="https://helpx.adobe.com/after-effects/using/alpha-channels-masks-mattes.html" rel="nofollow">https://helpx.adobe.com/after-effects/using/alpha-channels-masks-mattes.html</a></p>
