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The Color Difference Key
The final way we can create key out the background is with the Color Difference Key. It is similar to the Color Range Key, but with even more control. This key is useful when you are dealing with hair or semitransparent items like water or glass. At first glance, this keyer may be the most frightening of all of the key plug-ins found in After Effects, but it really is quite easy to understand if you remember that we are dealing with differences.
Without going into a lengthy explanation of what every slider does, we will just stick with the sliders we need and how they operate.
The View sets the Composition window to a variety of different outputs. The three most useful are going to be Matte Corrected, Final Output, and [A, B, Matte], Corrected, and Final Output, which will give you a quad view of all of the View Options. Since we are trying to extract a matte, set the View to Matte Corrected. This shows us our fixed, or corrected, matte.
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We are going to be working in the Preview window instead of the Comp window, so anytime you are using the eyedropper tools, make sure you are doing it in the Preview window only.
Use the Key Color eyedropper and select your background color. Like before, I suggest you make the selection as near to the face as possible.
Next we are going to select a range of colors using the difference settings. In this plug-in they are defined as Partial A and Partial B. You will also notice an A, B and alpha symbol under the second preview window. Click on the A button and using the Black eyedropper (the eyedropper in the middle), select the brightest area of the background that did not key out, but should have. In this example it is the area just above the subjects head where more light was falling on the background.
Switch over to the B preview and you will notice that the luminance ranges have been reversed. A and B are differences of one another, hence the name. Using the Black eyedropper again, select the brightest area of the background (which is actually the darkest area) that did not key out.
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Once the White In points have been set, you will probably still see areas in the Comp window that are gray, or partial alphas. Switch to the alpha preview window (the Greek letter at the end), and using the Matte in Black and Matte in White sliders adjust until all the gray areas are gone. The result should be a very clean matte. If not, repeat the choking process explained above to clean up the final bits.
This final method of keying footage also helped clean up some of the spill that resulted from light bouncing off the background.
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