Interpreting Alpha

When opening an image that has an Alpha Channel embedded within it, AE needs to know how to handle the data. A dialog box will pop up to clarify this point. An incorrect choice here will often result in an object with a light or dark halo around it. (AE is pretty good at guessing the correct setting, so if you are unsure... try hitting the Guess button first.)

If there is no need for the mask, then tell AE to ignore the channel.

If you want AE to use the alpha, straight is the more common option and should be your first guess.

Premultiplied includes a color swatch that can be used to isolate a background color. Files that have been rendered within a 3D program will often have the background color bleed around its edge pixels. Black is the most common background color in these cases.

You can always go back and tell AE to re-inerpret the Alpha Channel of a footage file if you find the result different than you expected or if you change your mind after working on the project. To do this, right-click on the footage in the project window and choose Interpret/Main.

HINT: If the mask cuts a hole rather than isolates the object, reinterpret the footage file and check the Invert Alpha box.

If you need more than one version of the footage file with different Alpha needs, you can re-import it and choose a different option.