Hi
I'm a newbe to AE, and trying to comp my maya renders.
I want to use an image sequence as a matte for a layer
insted of using the actual alpha that the render produce,
either by using BnW images,or other images alpha to mask
another image
thanks
If you don't want the embedded alpha to be used, simply check "ignore" when you first import your footage, or go to the Interpret Footage box if it's already in and check that box.
To use a different alpha there are a couple of ways. The easiest is to place your alpha source footage above the layer you want to mask and set it as the alpha source for that layer. Make sure the "Modes" column is showing and then select either Alpha Matte or Inverted Alpha Matte from the TrkMat column. Note this will automatically turn off the visibility of the alpha source layer (normally this is what you want anyway). You can also use the layer's luminance (brightness), which will read the RGB of the alpha source layer instead of the alpha itself. This can be used for some cool effects.
A more complex way to achieve this is to use the Set Matte effect (Channel > Set Matte), which allows you to use a layer other than the one above the target layer as your alpha source.