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Applying the Color Difference Key Effect

After Effects offers many keying effects. 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.

Confused yet? Most people who come across the Color Difference Key usually stop right there and run away in terror. But you don't need to fear the Color Difference Key.

  1. Select the Talent_DV layer in the Timeline panel, and choose Effect > Keying > Color Difference Key.
  2. In the Effect Controls panel, click the eyedropper for the Key Color setting. Then click the green background color in the left thumbnail preview at the top of the panel (see Figure 6).
  3. Click the A button under the right thumbnail preview to view the Matte Partial A (see Figure 7).

    Portions of the background are still visible in the Matte Partial A preview. This is actually a good thing, because it allows you to refine your key, which you'll do now.

  4. Using the Black eyedropper (the middle one between the two thumbnail previews), click the brightest portion of background that didn't key out in the Matte Partial A thumbnail preview. This will be near the top of the actor's head (see Figure 8).
  5. Click the B button under the right thumbnail preview to see the Matte Partial B, which is the inverse of Matte Partial A.
  6. Using the Black eyedropper again, click the brightest portion of the remaining background in the Matte Partial B thumbnail preview (near the bottom). This should remove any remaining traces of the background (see Figure 9).
  7. Click the Alpha button under the right thumbnail preview. If this were a clean matte, the actor would be completely white. She's not, so you need to make some adjustments to the effect (see Figure 10).
  8. In the Effect Controls panel, increase the Matte In Black amount to 50 and decrease the Matte In White amount to 109 (see Figure 11). You can think of the Matte In Black and Matte In White as being similar to the Levels effects.

    Adjusting these two controls cleans up the alpha channel of the image. The result is a clean key accomplished with just a few clicks of the mouse.

Checking the Alpha Channel for Errors

The orange (or transparent) background of the composition makes it easy to see any lingering bits of the green screen, but it may still be difficult to see any holes that may have been created with the actor. To double-check your key, switch to the alpha-channel view to see the resulting matte.

  1. At the bottom of the Composition panel, click the Show Channel button and choose Alpha from the pop-up menu (see Figure 12).

    Now you can see the black-and-white matte of your key. The black portion of the image represents the area that's transparent. The white portion represents the area that's opaque. Gray areas of the alpha channel (the feathered edge around the actor) are semi-transparent.

  2. Make sure that no portion of the actor is black or gray. If you have followed the steps so far, you should be in good shape.
  3. Click the Show Channel button again, and choose RGB to return to RGB view.
  4. Hide the Color Difference Key properties in the Effect Controls panel.
  5. Choose File > Save to save your work.
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