Use the color difference key effect – Adobe After Effects CS4 User Manual
Page 512
506
USING AFTER EFFECTS CS4
Effects and animation presets
Last updated 12/21/2009
The Color Difference Key effect creates transparency from opposite starting points by dividing an image into two
mattes, Matte Partial A and Matte Partial B. Matte Partial B bases the transparency on the specified key color, and
Matte Partial A bases transparency on areas of the image that don’t contain a second, different color. By combining
the two mattes into a third matte, called the alpha
matte, the Color Difference Key creates well-defined
transparency values.
The Color Difference Key produces high-quality keying for all well-lit footage items shot against a bluescreen or
greenscreen and works especially well with images that contain transparent or semitransparent areas, such as smoke,
shadows, or glass.
This effect works with 8-bpc and 16-bpc color.
More Help topics
Keying introduction and resources
Use the Color Difference Key effect
1
Select the layer that you want to make partially transparent, and choose Effect > Keying > Color Difference Key.
Note: To use any of the eyedroppers in the Layer panel, choose Color Difference Key from the View menu in the Layer
panel.
2
In the Effect Controls panel, choose Matte Corrected from the View menu. To view and compare the source image,
both partial mattes, and the final matte at the same time, choose [A, B, Matte] Corrected, Final from the View menu.
Other views available in the View menu are described in step 10.
3
Select the appropriate key color: To key out a bluescreen, use the default blue color. To key out a nonblue screen,
select a key color in one of the following ways:
•
Thumbnail eyedropper: Select and then click in the Composition panel or the original thumbnail image on an
appropriate area.
•
Key Color eyedropper: Select and then click in the Composition or Layer panel on an appropriate area.
•
Key Color swatch: Click to select a color from the specified color space.
Note: The eyedropper tools move the sliders accordingly. Use the sliders in step 9 to fine-tune the keying results.
4
Click the
matte button to display the final combined matte in the matte thumbnail.
5
Select the Black eyedropper, and then click inside the matte thumbnail on the lightest area of black to specify
transparent regions. The transparency values in the thumbnail and Composition panel are adjusted.
6
Select the White eyedropper, and then click inside the matte thumbnail on the darkest area of white to specify
opaque regions. The opaque values in the thumbnail and the Composition panel are adjusted.
To produce the best possible key, make the black and white areas as different as you can so that the image retains as
many shades of gray as possible.
7
Select a matching accuracy from the Color Matching Accuracy menu. Choose Faster unless you use a screen that
isn’t a primary color (red, blue, or yellow). For those screens, choose More Accurate, which increases rendering
time but produces better results.
8
To further adjust transparency values, repeat steps 5 and 6 for one or both of the partial mattes. Click the Partial
Matte B button or the Partial Matte A button to select a partial matte, and then repeat the steps.