Converting alpha channels to masks

You can convert the alpha channel of a layer to one or more masks by using the Auto-trace command. You can also create masks using the red, green, or blue channels of the layer. Auto-trace creates as many masks as necessary to outline the selected area while leaving the layer intact. When you apply Auto-trace, affected layers are automatically set to Best Quality to ensure accurate results.

To create a mask with Auto-trace:

1. In the Timeline window, adjust the work area to the range of frames for which you want to create mask shape keyframes.
2. Select one or more layers in the Composition window.
3. Choose Layer > Auto-trace.
4. Select the appropriate options in the Auto-trace dialog box, and then click OK. One or more masks appear in the Composition window and in the Timeline window for that layer.

Apply a keying effect to the layer to isolate your subject and reduce the number of masks created by Auto-trace.

Auto-trace includes the following options:

Current Frame

Creates masks on the selected frame only.

Work Area

Creates mask shape keyframes throughout the specified work area.

Tolerance

Specifies, in pixels, how tightly the mask stroke conforms to the shape of the alpha channel. Extremely high values create masks with sharp corners; extremely low values create masks that encompass noise in the specified channel's perimeter.

Threshold

The specified percentage value determines where to draw the mask stroke. Values over the threshold are mapped to white and are opaque; values under the threshold are mapped to black and are transparent.

Channel

Specifies the channel to convert to masks. Choices include alpha, red, green, blue, and luminance.

Invert

Inverts the input layer prior to searching for edges. For example, if you apply Auto-trace to a square layer that contains a circular alpha channel, Invert creates a mask for the circle only, instead of creating masks of both the circle and square outline of the layer.

Blur

Specifies the amount of blur in pixels to apply to the layer before the threshold is sampled. Set the value to zero to create masks that closely outline high-contrast images. Use higher values to reduce jagged mask edges.

Apply to New Layer

Applies the mask to a new solid the same size as the selected layer. This control is automatically selected for layers that have Collapse Transformations enabled--it creates a new layer the same size as the composition that contains the layer.