About masks

A mask in After Effects is a path that is used as a parameter to modify layer attributes, effects, and properties. The most common use of a mask is the modification of an alpha channel of a layer, which determines the transparency of the layer at each pixel. Another common use of a mask is as a path along which to animate text. (See Creating and animating text on a path.)

For more information on paths in general, see About paths.

Default behavior for a drawn mask (left); same mask inverted (right)

Closed-path masks can create transparent areas for a layer. Open paths cannot create transparent areas for a layer but are useful as parameters for an effect. Effects that can use an open or closed mask path as input include Stroke, Path Text, Audio Waveform, Audio Spectrum, and Vegas. Effects that can use closed masks (but not open masks) as input include Fill, Smear, Reshape, Particle Playground, and Inner/Outer Key.

A mask belongs to a specific layer. Each layer can contain multiple masks.

You can draw masks in common geometric shapes—including polygons, ellipses, and stars—with the shape tools, or you can use the Pen tool to draw an arbitrary path.

In most ways, drawing mask paths is the same as drawing shape paths on shape layers, though the editing and interpolation of mask paths have a few additional features. You can link a mask path to a shape path using expressions, which allows you to bring the benefits of masks into shape layers, and vice versa. See Creating shapes and masks and Managing and animating shape paths and masks.

The position of a mask in the stacking order in the Timeline panel affects how it interacts with other masks. You can drag a mask to different positions within the Masks property group in the Timeline panel.

The Mask Opacity property for a mask determines the influence that a closed mask has on the alpha channel of the layer inside the mask area. A Mask Opacity value of 100% corresponds to an interior area that is completely opaque. The area outside the mask is always completely transparent. To invert what is considered inside and what is considered outside for a specific mask, select Invert next to the mask name in the Timeline panel.

Trish and Chris Meyer provide an introduction to masks in a PDF excerpt from the “Creating Transparency” chapter of their book After Effects Apprentice: Real-World Skills for the Aspiring Motion Graphics Artist.