User Guide

CHAPTER 2
72
Preparing and Importing Footage
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” on page 238.
Many file formats can include an alpha channel, including Adobe Photoshop,
ElectricImage IMAGE, 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.
Interpreting alpha channels as straight or premultiplied
When you import a footage file containing an alpha channel, After Effects determines if
it is straight or premultiplied. If the alpha channel is unlabeled, After Effects displays the
Interpret Footage dialog box and prompts you to select Ignore Alpha, Straight (Unmatted),
Premultiplied (Matted with Black), Premultiplied (Matted with White), or Guess. When
you select Guess, After Effects determines the interpretation method. For information on
straight and premultiplied alpha channels, see online Help.
Correctly identifying the type of alpha channel when you import a file can prevent prob-
lems, such as undesirable colors at the edge of an image or a loss of image quality at the
edges of the alpha channel. If a color inaccuracy, such as a halo, appears along the edges of
an alpha channel in a composition, try changing the interpretation method.
If a footage item doesn’t have an alpha channel or transparency information, you can
create an alpha channel in After Effects using a layer mask. See “Working with masks” on
page 238.
Choosing an interpretation method for alpha channels
You can change the interpretation method for a file after you import the file. You can also
change the default alpha channel interpretation in the Import Preferences dialog box, in
the Interpret Unlabeled Alpha As pop-up menu. This is especially useful for importing
footage from an application that consistently uses one type of unlabeled alpha channel.
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