Adobe 65008009 - After Effects CS4 Using Manual page 319

Last updated 12/21/2009
To create a mask that you can move independently of the primary layer that it's masking, create the mask on a
separate white solid layer, and use that solid layer as a track matte for the primary layer. Then use parenting to make
the solid layer a child of the primary layer, so that the mask moves with the primary layer as if it were applied directly.
Because the solid layer is a child layer, it can also be animated independently of its parent. You can use motion tracking
to make the solid layer (and therefore the mask) follow moving objects in the primary layer. (See
track matte" on page 381 and
Create a rectangular or elliptical mask numerically
1 Select a layer in the Composition panel, or display a layer in the Layer panel.
Choose Layer > Mask > New Mask. A new mask appears in the Composition or Layer panel with its handles at the
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outer edges of the frame.
3 Choose Layer > Mask > Mask Shape.
Select Reset To, choose Rectangle or Ellipse from the Shape menu, and specify the size and location of the bounding
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box for the mask.
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.
1 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.
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Choose Layer > Auto-trace.
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Select one of the following:
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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:
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Inverts the input layer before searching for edges.
Invert
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.
How far, in pixels, the traced path is allowed to deviate from the contours 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.
"Parent and child layers" on page 166.)
USING AFTER EFFECTS CS4
Drawing, painting, and paths
"Convert a layer into a
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