About masks
A mask in After Effects is a path that is used as a parameter to modify layer effects and properties. The most common use of a mask is the modification of a layer’s alpha channel, which determines the layer’s transparency 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.)
A mask path consists of segments and vertices. Segments are the lines or curves that connect vertices. Vertices are where each segment of a path starts and ends. For more information on paths, see About paths.
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. Some effects can use an open or closed mask path as input; these include Stroke, Path Text, Audio Waveform, Audio Spectrum, and Vegas. Others can use closed masks only; these 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 has 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.
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.
A mask’s Mask Opacity property determines the influence that a closed mask has on a layer’s alpha channel 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.
