Adobe After Effects - Manual (Page 487)

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3. To extract more than one object, or to create a hole in an object, draw additional masks and then select them from the Additional
Foreground and Additional Background menus. For example, to key out a person’s hair blowing in the wind against a blue sky, draw the
inner mask inside the head, draw the outer mask around the outside edge of the hair, and then draw an additional mask around the gap in
the hair where you can see sky. Select the additional mask from the Additional Foreground menu to extract the gap and remove the
background image.
4. Create additional open or closed masks to clean up other areas of the image, and then select them from the Cleanup Foreground or
Cleanup Background menu. Cleanup Foreground masks increase the opacity along the mask; Cleanup Background masks decrease the
opacity along the mask. Use the Brush Radius and Brush Pressure options to control the size and density of each stroke.
You can select the Background (outer) mask as a Cleanup Background mask to clean up noise from the background portions of the
image.
5. Set Edge Thin to specify how much of the border of the matte is affected by the key. A positive value moves the edge away from the
transparent region, increasing the transparent area; a negative value moves the edge toward the transparent region and increases the size of
the foreground area.
6. Increase the Edge Feather values to soften edges of the keyed area. High Edge Feather values take longer to render.
7. Specify the Edge Threshold, which is a soft cutoff for removing low-opacity pixels that can cause unwanted noise in the image background.
8. Select Invert Extraction to reverse the foreground and background regions.
9. Set Blend With Original to specify the degree to which the resulting extracted image blends with the original image.
Linear Color Key effect
Though the color keying effects built into After Effects can be useful for some purposes, you should try keying with Keylight before
attempting to use these built-in keying effects. Some keying effects have been superseded by more modern effects like Keylight.
For information about keying in general, including links to tutorials and other resources, see Keying introduction and resources.
Linear keys create a range of transparency across an image. A linear key compares each pixel in the image to the key color you specify. If the
color of a pixel closely matches the key color, it becomes completely transparent. Pixels that don’t match as well are made less transparent, and
pixels that don’t match at all remain opaque. The range of transparency values, therefore, forms a linear progression.
The Linear Color Key effect uses RGB, hue, or chroma information to create transparency from a specified key color.
This effect works with 8-bpc, 16-bpc, and 32-bpc color.
Apply the Linear Color Key effect
In the Effect Controls panel, the Linear Color Key effect displays two thumbnail images; the left thumbnail image represents the unaltered source
image, and the right thumbnail image represents the view you’ve selected in the View menu.
You can adjust the key color, the matching tolerance, and the matching softness. The matching tolerance specifies how closely pixels must match
the key color before they start becoming transparent. The matching softness controls the softness of edges between the image and the key color.
You can also reapply this key to preserve a color that was made transparent by the first application of the key. For example, if you key out a
medium-blue screen, you may lose some or all of a light-blue piece of clothing your subject is wearing. You can bring back the light-blue color by
applying another instance of the Linear Color Key and choosing Keep This Color from the Key Operation menu.
1. Select a layer as the source layer, and then choose Effect > Keying > Linear Color Key.
2. In the Effect Controls panel, choose Key Colors from the Key Operation menu.
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