Adobe AFTER EFFECTS 5.5 Help Manual page 158

Adobe After Effects Help
Using Help | Contents
Specifying which parts of a mask are inside and outside
The image area inside the mask outline is fully opaque, and the area outside the mask
outline is transparent. If you want to create the appearance of a hole in a video layer to
reveal a layer underneath, switch the inside and outside areas by inverting the mask. You
cannot change the mask state using keyframes; a mask is either inverted or not inverted
for the entire duration of a composition.
Default behavior for a drawn mask (left); same mask inverted (right)
To invert a drawn mask:
1 Select the mask you want to invert.
2 In the Timeline window, select the Inverted option next to the mask name.
If you have multiple masks in one layer, you can apply mask modes to create complex
transparent shapes or varying levels of transparency. For more information, see
"Controlling how masks in the same layer interact" on page 162.
Moving a mask and panning a layer behind a mask
You can adjust the area that displays through a mask by either moving the mask in the
Layer or Composition window or panning (moving) the layer behind the mask in the
Composition window. When you move a mask, the mask layer's Position values remain
constant, and the mask moves in relation to other objects in the Composition window.
Using Help | Contents
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Working with Masks and Transparency
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