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 Mask tool 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, and each has a mode that says how it combines with the ones above it — the seven After Effects has: None, Add, Subtract, Intersect, Lighten, Darken and Difference.
The Mask Feather tool (press G twice — it shares the key with the Pen) places feather points along a mask, so one edge can be soft and another hard.
Layer → Mask and Shape Path holds the path commands: Closed, Set First Vertex, RotoBezier, Reverse Path Direction, Free Transform Points, Set Mask / Path Keyframe, and Convert Mask to Shape Layer. To make masks from type, see Shapes & masks from text and SVG.
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 Properties ▸ Blending and switches, or from Track Matte in a layer’s right-click menu. The matte layer 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 Properties ▸ Blending and switches. 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. A mask-scoped effect stays on the GPU.
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 |