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Create a mask from channel values 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.
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