Create a mask from a channel with Auto-trace

You can convert the alpha, red, green, blue, or luminance channel of a layer to one or more masks by using the Auto-trace command. Auto-trace creates as many Bezier masks as necessary to outline the specified channel values in the layer. Auto-trace creates masks with the smallest number of vertices possible while conforming to the settings that you choose. You can modify a mask created with Auto-trace as you would any other mask, and you can link its path to other path types, such as shape paths on a shape layer, using expressions.

When you apply Auto-trace, affected layers are automatically set to Best Quality to ensure accurate results.

To reduce the number of masks created by Auto-trace, apply a keying effect to the layer to isolate your subject before applying Auto-trace.

In the Timeline panel, do one of the following:
- To create mask keyframes at a single frame, drag the current-time indicator to the desired frame.
- To create mask keyframes across a range of frames, set a work area that spans that range.
Select one or more layers.
Choose Layer > Auto-trace.
Select one of the following:
Current Frame

Creates mask keyframes at only the current frame.
Work Area

Creates mask keyframes for frames within the work area.
Set any of the following options:
Invert

Inverts the input layer before searching for edges.
Blur

Blurs the original image before generating the tracing result. Select this option to reduce small artifacts and to smooth jagged edges in the tracing result. Deselect this option to closely trace details in a high-contrast image. Specify the radius, in pixels, of the area used for the blurring operation. Larger values result in more blur.
Tolerance

How far, in pixels, the traced path is allowed to deviate from the contours of the channel.
Threshold

Specifies, as a percentage, the value that a pixel’s channel must have for that pixel to be considered part of an edge. Pixels with channel values over the threshold are mapped to white and are opaque; pixels with values under the threshold are mapped to black and are transparent.
Minimum Area

Specifies the smallest feature in the original image that will be traced. For example, a value of 4 removes features smaller than 2 pixels wide by 2 pixels high from the tracing result.
Corner Roundness

Specifies the roundness of the mask curve at vertices. Enter a higher value for smoother curves.
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.
Preview

Select to preview the mask results and the results of the various options of the Auto-trace command.