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Chapter 7. Getting Creative with Keys > Using Color to Key Images

Using Color to Key Images

The chroma key effect works much like the luminance key, but it offers you more control by letting you specify a particular color value (or color range) to drop out. This effect allows you to take any image and begin dropping out specific portions of the clip based on a color you select and control. Like the luminance key effect, the greater the separation in values, the more control and accuracy you will have in creating a clean key. As you start adjusting the slider controls in the Transparency Settings window, you will notice that the key starts taking effect with the color value closest to the color you selected. The more you increase the slider settings, the more the color value starts expanding into colors that are closely related to the initial color you selected. If you continue increasing the slider values, you will eventually key out all the colors, thereby dropping out the entire image.

The more distinct the color values are in your image, the more accurate your key will appear. For instance, if you are trying to key out the blue in the sky of a shot of a tree and a cliff (made up primarily of shades of green and brown), you should be able to get a fairly clean key, as shown in Figure 7.8. If you want to key out the sky in a shot of a boat on the ocean (see Figure 7.9), the blue values in the water are probably too similar to the blue values in the sky. Therefore, you will wind up dropping out parts of the ocean as you adjust the key values for the sky.


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