One of the most common techniques for separating parts of an image to reveal only the desired portion is to use a luminance key. A luminance key works with the difference in video level, or contrast, of the clip's signal. You can set your key to either drop out the lightest or brightest portion of your clip or switch it to drop out the darkest portions of the clip first. For example, let's take the black-and-white logo shown in Figure 7.3 (a white image over a black background) and see what happens when you key out different luminance values of that clip:
Figure 7.3. Use the Luminance Key filter to drop the black portion out of an image.or
or
There are three ways to view the luminance key effects in the Transparency Settings window, as shown in Figure 7.5:
Figure 7.5. There are several ways to view keyed effects in the Transparency Settings window. |