Import a still-image file or a motion-footage file that contains an alpha channel. Auto-trace creates separate masks for each area of transparency, so try to avoid using footage with several separate areas of transparency, such as a line of text, as it could require several minutes to process.

Add the footage to a composition.

Next, create masks using the alpha channel of your layer by selecting the layer in the Composition window or Timeline window and choosing Layer > Auto-trace.

In the Auto-trace dialog box, select Work Area to create masks for the duration of the layer, and choose Alpha from the Channel menu. (While you can create masks using the luminance channel, it’s best to use Luminance with high-contrast, black-and-white images only.)

If you want the resulting masks to tightly conform to the contours of the opaque areas of the layer, keep the Tolerance value low—high values result in angular masks. Changing the Threshold value has a subtler effect and shifts the mask vertexes only slightly. Blur helps to combine masks for a discontiguous alpha channel. If you want to create masks that outline transparent rather than opaque areas, select Invert. Or, to create masks on a different layer than your original image, select Apply to New Layer. Leave the other options at their default settings and click OK.

Auto-trace creates one or more masks on your footage or on a new layer. If the masks don’t outline your image as closely as you’d like, choose Edit > Undo and reapply Auto-trace, adjusting the Tolerance, Threshold, or Blur settings.