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 prior to 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
Auto-trace command’s various options.
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.