Masks & mattes
Three different mechanisms, often confused. The short version:
| Mechanism | Lives on | Driven by |
|---|---|---|
| Mask | The layer itself | A vector path you draw |
| Track / alpha matte | The layer below | Another layer’s luminance or alpha |
| Adjustment layer | Nothing — it affects what is beneath | Its effect stack |
Masks
A mask is a vector path on a layer that limits where the layer draws.
Draw one with the Rectangle Mask or Ellipse Mask tool while the target layer is selected, or with the pen for an arbitrary shape.
Mask paths are keyframeable. Animate the path itself to reveal, wipe, or follow something moving inside the frame. Multiple masks per layer are supported.
Track and alpha mattes
A matte uses one layer to decide the transparency of another.
| Type | Uses the matte layer’s… |
|---|---|
| Alpha matte | Alpha channel |
| Luma matte | Luminance — white keeps, black hides |
Either can be inverted.
Set it in Style → Compositing. The matte layer sits above the layer it mattes, and stops rendering on its own — it is now doing a job, not drawing.
A luma matte with an animated gradient is how you build a soft reveal that a mask path cannot express. A text layer as an alpha matte is how you fill type with video.
Adjustment layers
An adjustment layer applies its effects to everything rendered beneath it. Use it to grade a whole scene, blur everything behind a title, or apply one effect across many layers without duplicating it.
Create one with Ctrl/⌘ + Alt + Y, or from Layer → New.
Blend modes
All 38 of After Effects’ blend modes, Dissolve and Dancing Dissolve included, set per layer in Style → Compositing. They decide how a layer combines with what is already drawn beneath it. Full list: Compositing.
Scoping one effect to a mask
A mask can bound a single effect rather than the whole layer — blur just the face, grade just the sky, without pre-comping or duplicating the layer.
After Effects cannot do this; it is the one place masks here are wider than AE’s rather than narrower.
Picking the right one
| You want to… | Use |
|---|---|
| Cut a shape out of a layer | A mask |
| Apply one effect to part of a layer | A mask, scoped to that effect |
| Reveal along an arbitrary edge | A mask with an animated path |
| Fill type or a shape with footage | An alpha matte |
| Soft, gradient-driven reveal | A luma matte |
| Apply one effect to many layers | An adjustment layer |
| Group several layers so effects and opacity apply as one | A pre-comp — see Compositions |