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Importing footage containing an alpha channel
The color information in a typical footage item is contained in three channels: red, green, and blue. In addition, a footage item can include a fourth channel, called an alpha channel, that contains transparency information for the parts of the image that are partially or completely transparent. An alpha channel is often used as a matte for creating effects. When used in After Effects, a matte defines the transparent areas for its own or another layer. White areas define what is opaque, and black areas define what is transparent. (See Using a footage item with an alpha channel.)
Many file formats can include an alpha channel, including Adobe Photoshop, ElectricImage, TGA, TIFF, EPS, PDF, QuickTime (saved at a bit depth of Millions of Colors+), and Adobe Illustrator. For Adobe Illustrator EPS and PDF files, After Effects automatically converts empty areas to an alpha channel.
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