Use a garbage matte, Use a hold-out matte – Adobe After Effects User Manual
Page 416
or adding the Linear Color Key.
To key dark areas or shadows, use the Extract Key on the Luminance channel.
To make a static background scene transparent, use the Difference Matte Key. Add the Simple Choker and other effects as needed to refine
the matte.
After you have used a key to create transparency, use Matte effects to remove traces of key color and create clean edges.
Blurring the alpha channel after keying can soften the edges of the matte, which can improve compositing results.
Use a garbage matte
A garbage matte (or junk matte) removes unneeded portions of the scene, resulting in a rough area that contains only the subject that you want to
keep. When you are working with a poorly lit or uneven color screen (for example, a bluescreen or greenscreen), sketching a garbage matte
around the subject can greatly reduce the amount of work that you have to do in keying out the background. However, if you spend a lot of time
making a perfect garbage matte that exactly outlines the subject—essentially rotoscoping—you lose the time-saving advantage of keying.
1. Create a mask to roughly outline a subject.
2. Apply one or more keying effects to mask out the remainder of the background.
3. Apply Matte effects as necessary to fine-tune the matte.
Aharon Rabinowitz provides a video tutorial on the
that shows how to create a super-tight garbage matte using Auto-trace.
Use a hold-out matte
Use a hold-out matte (also known as a hold-back matte) to patch a scene to which a keying effect has been applied.
A hold-out matte is a masked-out portion of a duplicate of a layer that you have keyed. The duplicate is masked to include only the area of the
image that contains the key color that you want to preserve as opaque. The hold-out matte is then placed directly on top of the keyed layer.
Example of using a hold-out matte
A. Original bluescreen image. The background for the number is also blue. B. After keying, the background for the number is also transparent. C.
Hold-out matte containing the part of the image you want to remain opaque D. When the hold-out matte is placed on top of the keyed image, the
background for the number is now opaque.
1. Duplicate the layer containing the color screen.
2. Apply keying effects and Matte effects to the original layer to create transparency.
412