Applying High Pass to individual color channels

In my continuing series of color plates devoted to adding a bit of digital color to the ages-old game of chess, Color Plate 10-5 shows the results of applying the High Pass filter set to a Radius value of 5.0 to the various color channels. This application is a pretty interesting use for this filter. When applied to all channels at once, High Pass has an irritating habit of robbing the image of color in the low-contrast areas, just where the color is needed most. But when you apply it to a single channel, there's no color to steal. In fact, the filter adds color. For example, because there is almost no contrast in the dark shadows, High Pass elevates the black to gray in each of the affected color channels. The gray in the red channel appears red, the gray in the red channel mixed with the gray in the green channel appears yellow, and so on. As a result, the filter imbues each image with a chalky glow.

Note I enhanced the High Pass effect slightly in Color Plate 10 5 by increasing the contrast of each affected color channel using the Levels command. Using the Input option boxes at the top of the Levels dialog box, I changed the first value to 65 and the third value to 190, thereby compressing the color space equally on both the black and white sides. Had I not done this, the images would appear a little more washed out. (Not a lot, but I figure that you deserve the best color I can deliver.) For detailed information on the Levels command, read Chapter 17.

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