To Create A Mask From A Channel With Auto-Trace - Adobe AFTER EFFECTS 7.0 Manual

See also
"About motion paths" on page 210

To 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 selected area while leaving the
layer intact. After Effects 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.
When you apply Auto-trace, affected layers are automatically set to Best Quality to ensure accurate results.
You can 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:
1
•
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.
2
3 Choose Layer > Auto-trace.
Select one of the following:
4
Creates mask keyframes at only the current frame.
Current Frame
Creates mask keyframes for frames within the work area.
Work Area
Set any of the following options:
5
Select to preview the mask results and the results of the Auto-trace command's various options.
Preview
Specifies the channel to convert to masks. Choices include Alpha, Red, Green, Blue, and Luminance.
Channel
Inverts the input layer prior to searching for edges. For example, if you apply Auto-trace to a square layer that
Invert
contains a circular alpha channel, Invert creates a mask for the circle only, instead of creating masks for both the
circle and the square outline.
Blurs the original image before generating the tracing result. Select this option to reduce small artifacts and to
Blur
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.
Specifies, in pixels, how far the traced shape is allowed to deviate from the shape of the channel.
Tolerance
Specifies, as a percentage, the value that a pixel's channel must have for that pixel to be considered part of
Threshold
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.
Specifies the smallest feature in the original image that will be traced. For example, a value of 4
Minimum Area
removes features smaller than 2 pixels wide by 2 pixels high from the tracing result.
Specifies the roundness of the mask curve at vertices. Enter a higher value for smoother curves.
Corner Roundness
Applies the mask to a new solid the same size as the selected layer. This control is automatically
Apply To New Layer
selected for layers that have Collapse Transformations enabled—it creates a new layer the same size as the compo-
sition that contains the layer.
ADOBE AFTER EFFECTS 7.0
248
User Guide