To Generate A Layer Matte; Previewing Grain Effects; To Change The Preview Region - Adobe AFTER EFFECTS 7.0 Manual

Select Invert Match if you want to invert the matte so that the white areas become black and the black areas
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become white. (The matching color will be black in the matte and will be processed by the grain effect regardless of
the Amount setting. The inversion does not affect any other settings.)
If you are using both a color matte and a layer mask, choose one of the following from the Combine Match and
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Mask Using menu:
Makes the matte white wherever either the mask or the color match is white.
Screen
Makes the matte white where both of the inputs are white.
Multiply
9 Choose Final Output from the Viewing Mode control.

To generate a layer matte

In some cases, you may want to use a different layer or track as a mask for the layer that is using a grain effect. This
allows unlimited control over exactly which parts of the image are modified and how much.
Apply a grain effect to your image.
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2 In the Effect Controls panel, choose the layer that you want to use as a mask from the Mask Layer control in the
Blend With Original and Masking Layer controls groups.
3 Choose a masking mode from one of the standard track matte mode options.
4 If the masking layer is a different size than the current layer, choose one of the following from the If Mask Size
Differs pop-up menu in the Masking Layer controls:
Centers the masking layer over the current layer.
Center
Resizes the masking layer to match the dimensions of the current layer.
Stretch To Fit
If you are using both a color matte and a layer mask, choose one of the following from the Combine Match And
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Mask Using menu:
Makes the matte white wherever either the mask or the color match is white.
Screen
Makes the matte white where both of the inputs are white.
Multiply
6 Choose Final Output from the Viewing Mode control.

Previewing grain effects

You'll achieve the best results by experimenting, applying small increments to each of several controls in the Effect
Controls panel, and viewing the results in the Composition panel after every adjustment. You can use the Preview
Region controls group to change the position or the size of a grain effect's preview region.
Since adding or removing grain can affect sharpness of detail, you may also want to preview an area of fine detail
such as a human face or some text. When removing grain with the Remove Grain effect, it's best to preview an area
where the grain is most clearly visible or most objectionable, such as a large expanse of solid color.

To change the preview region

1 After applying a grain effect, click the Center button
Controls panel. A cross hair appears centered in the Composition panel.
In the image, click the desired center of the preview region. The preview region redraws, centered in the new
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position.
To change the dimensions of the preview region, change the Width and Height values in the Effect Controls panel
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to the desired size in pixels. (Larger preview regions can result in slower rendering.)
in the Preview Region group of controls in the Effect
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User Guide