Adobe After Effects CS5 : Color Correction for Image Optimization (part 2) - Individual Channels for Color Matching

Individual Channels for Color Matching

Many After Effects artists completely ignore the pop-up menu at the top of the Levels control allowing adjustment of the five basic Levels controls on an individual channel, but this is where its powers for color matching lie. Let’s take a look at these controls on the gradient image to reveal what exactly is going on.

Reset any Levels effect applied to the Ramp gradient. Pick Red, Green, or Blue in the Channel pop-up menu under Levels and adjust the Input and Output carets. The grayscale image takes on color. With the Red channel selected, move Red Output Black inward to tint the darker areas of the image red. Adjust Input White inward to make the midtones and highlights pink (light red). If, instead, you adjust Input Black or Output White inward, the tinting moves in the opposite direction—toward cyan—in the corresponding shadows and highlights.

As you probably know, each primary on the digital wheel of color (red, green, or blue) has an opposite (cyan, magenta, or yellow, respectively). As your color skills progress you will notice when your method of, say, reducing green spill has made flesh tones too magenta, but when you’re starting out it’s enough simply to be aware that adjustments to each color channel proportionally affect its opposite (Figure 9).

Figure 9. These charts were devised by John Dickinson at Motionworks (www.motionworks.com.au) ; it shows the relationship of each color to its opposite when adjusting the Levels Effect.

Gradients are one thing, but the best way to make sense of this with a real image is to develop the habit of studying footage on individual color channels as you work. This is the key to effective color matching.

Along the bottom of the Composition panel, all of the icons are monochrome by default save one: the Show Channel menu. It contains five selections: the three color channels as well as two alpha modes. Each one has a shortcut that, unfortunately, is not shown in the menu: Alt+1 through Alt+4 (Opt+1 through Opt+4) toggle each color channel. A colored outline around the edge of the composition palette reminds you which channel is displayed (Figure 10); toggling the active channel returns the image to RGB.

Figure 10. Four Views mode is generally intended for 3D use, but it can also be used to show RGB and individual red, green, and blue channels. This becomes extremely useful for color matching. Note differences in the three channels and the colored outline showing which is which.

Try adjusting a single channel of the gradient in Levels while displaying only that channel. The effect of brightness and contrast adjustment on a grayscale image is readily apparent. This is the way to work with individual channel adjustments, especially when you’re just beginning or if you have difficulty distinguishing colors. As you work with actual images instead of gradients, the histogram can offer valuable information about the image.

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