In After Effects there are many ways to mask. You can mask a layer with a path, or another layer or with transition effects. You also can use Lightness/Color/etc. to add transparency to other layers.
Layer Mask
- directly applied to layer
- moves, scales, rotates with layer
- layers can have several masks that can be combined in an additive or subtractive way
- defined by paths
- mask path can be animated
- edge can be feathered
- it’s fixed to the layer and will transform with it
Track Matte
- separate layer acts as matte/mask
- does not move or scale with masked layer but can be parented
- can use alpha OR luma
- both also inverted
- matte layer can be animated separately
- only one matte per layer and vice versa
Set Matte effect
- separate layer acts as matte/mask
- can therefore be applied multiple times and to multiple layers
- can use alpha, luma, color channels, etc. for matte
- matte layer can not be animated separately
- hard to understand because matte transforms with layer if applied to a Pre-Comp or Solid and collapse transformations is off and does not transform with layer if applied to a Shape Layer or if collapse transformations is on
Stencil Alpha
- masks ALL layers underneath in a comp
- layers above are not affected
- to get a background you’ll need to precomp eveything
Preserve Underlying Transparency
- layer will only be visible where anything is already in any layer underneath
- only works in Comps without background layer
Transitions
VIDEO