How to Use Masks in After Effects (Complete Guide)

Masks are one of the core tools in After Effects — they control which parts of a layer are visible.

What is a mask?
A mask is a path (shape) drawn on a layer that defines visible and invisible areas. The area inside the mask is visible; outside is hidden (by default).

Creating a mask:
1. Select the layer first (this is crucial — if no layer is selected, you'll create a shape layer instead)
2. Use the Rectangle tool (Q), Ellipse tool, or Pen tool (G)
3. Draw on the comp viewer — a mask appears on your layer

Mask properties (press M to reveal):
- Mask Path: The shape of the mask — keyframe this to animate
- Mask Feather: Softens the edges (higher = softer). Great for vignettes
- Mask Opacity: How transparent the mask area is (100% = fully visible)
- Mask Expansion: Grows or shrinks the mask without changing the path

Mask modes (important!):
- Add: Shows the area inside the mask (default)
- Subtract: Hides the area inside the mask
- Intersect: Only shows where multiple masks overlap
- Difference: Shows everything EXCEPT the overlap

You can stack multiple masks on one layer with different modes for complex cutouts.

Animating masks:
1. Press M to show mask properties
2. Click the stopwatch on Mask Path
3. Move to a new time
4. Use the Selection tool to drag mask points to new positions
5. AE interpolates the mask shape between keyframes

Mask vs Track Matte:
- Mask: A path drawn directly on the layer
- Track Matte: Uses ANOTHER layer's brightness or alpha as the mask. More flexible for complex reveals.

Common uses:
- Revealing text with an animated mask wipe
- Cutting out parts of footage (isolating a person)
- Creating vignettes (ellipse mask with high feather)
- Limiting effects to specific areas of a layer

Pro tip: Use Mask Feather and Mask Expansion together for precise control. And remember: the keyboard shortcut M, M (press M twice quickly) shows ALL mask properties at once.