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I am going to cover a couple possible ways you can key your subject, each has its strengths and weaknesses. Our ultimate goal is to key out the blue background to create an alpha matte for the rest of the project. Keying is what we do, a matte is the result.
Color Key
The first inclination might be to use the Color Key effect found in Effect>Key>Color Key panel. This will work great if you have an evenly lit background with no variation in color. If there is variation you can use the effects Color Tolerance to help key those subtle shades, but if taken to the extreme it can cause your foreground subject to key out as well.
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| Color Key may not always key all of the tonal ranges in your background. |
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Color Range
Another solution would be to use the Color Range Keyer. This will let you select an area of the image to key and then use the eyedropper tools to refine the shape of the matte. Since the footage was captured with DV, your first inclination might be to switch the Color space setting to YUV, but you would then be keying out a luminance range, which might cause some of your subject to disappear as well. Lab is for print work (CMYK) but might work in some cases. Let’s try RGB and begin selecting a blue area near and along the same line as the face. You can see that even the blue screen has areas that vary in tonal range because of an uneven paint job, and not just because of poor lighting.
By using the eyedropper add tool, you can "paint" over the other color ranges to get a much better key. There are however, small areas that can still go unchecked. To tweak this, use the Fuzziness slider. Zoom in on the frame to make sure everything is taken care of. If you need to do more fine-tuning, the minimum and maximum RGB sliders can be used.
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| While the Color Range Key is better than Color Key, some of the fine areas of your subject may not key out well (such as hair or whiskers). |
Once you are satisfied with your matte, scroll through your timeline and make sure that the key doesn’t change dramatically. Yes, you should spot check the entire timeline, because shadows and luminance changes can alter the key. Hair is often the culprit and you will need to do some additional refinements. Using the Color Range Keyer will probably not be enough and you will probably get to the point where you want to choke your keying system. So let’s do that.
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