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Transparency terminology in After Effects
To create effects by combining images, parts of each image must be transparent. The terminology of transparency varies by media and by software. After Effects refers to transparent areas using the following terms:
Alpha channel
An invisible channel that defines transparent areas for the footage item or layer that contains the channel. With imported items, an alpha channel provides a way to store both the footage and its transparency information in a single file without disturbing the footage item's color channels. Each After Effects layer can accommodate an alpha channel included with a footage item. (See Importing footage containing an alpha channel.)
Mask
A path, or outline, that modifies a layer's alpha channel. Use a mask when you want to draw areas of transparency in After Effects. A mask belongs to a specific layer, but each After Effects layer can contain multiple masks.
Matte
A layer (or any of its channels) that defines the transparent areas of that layer or another layer. Use a matte when you have a channel or layer that defines the desired area of transparency better than the alpha channel (or in cases where footage does not include an alpha channel).
Keying
Defining transparency by a particular color (color key) or brightness value (luminance key) in an image. Pixels matching the key color become transparent. Use keying to remove a background with a uniform color, such as a blue screen.
