Monday, October 8, 2007
Curving RGB Color
Color correcting by numbers is a combination of art and science
In reality, there's no ideal skin color—people come in a wide range of colors, dark to light, red to yellow and everything in between. In the more limited world of reproduction, we tend to accept a narrower range of colors for skin. This has to do with cultural preferences, the influence of marketing media as well as the history of print technology. Western viewers tend to prefer more saturated and brighter skin colors in all races with a surprisingly narrow hue range. It's more difficult to analyze this particular range of color using RGB numbers. That's why we set up the second color readout to CMYK in the Info palette. As it turns out, we can analyze the color of skin by observing the relationship between Cyan, Magenta and Yellow values in the Info palette. CMYK numbers work a little differently than RGB numbers. Unlike RGB, higher values in CMYK make the image darker, and in the range of colors for skin, we'll be dealing with two-digit numbers instead of three-digit numbers, so it's easier to do the mental calculations for analysis. Skin color is a shade of red that varies by how pink or yellow it is. This ratio is neatly expressed with the ink percentages of Magenta and Yellow in CMYK numbers. Ideal skin values will have Magenta and Yellow closer to each other than they are to Cyan. Yellow always will be higher than Magenta, and Cyan generally will be one-third to one-quarter of the average value of Magenta and Yellow. Darker skin will have higher Cyan values, and very dark skin will look better with Magenta closer to Yellow.
Right now, the CMY ratios are off in the image. When I place the cursor toward the elbow on the back of the arm, the values for this area of skin midtone have Yellow and Magenta reversed in strength—M=45 and Y=35! Remember, Yellow should be higher than Magenta, not lower. You can't directly manipulate Magenta and Yellow while you're in RGB, but you can adjust the ratio easily using opponent colors in RGB. What's an opponent color? This little bit of classic color theory is easily seen in the arrangement at the top of the Info palette; reading across the top row of numbers, we see R numbers, then C numbers. Red and Cyan are opponent colors. Adding Red will reduce Cyan; reducing Red will increase Cyan. The next row down shows the opponent pair, G and M. The next row shows B and Y. So the key to affecting Yellow in RGB is the Blue Channel.
To reverse the cool pink appearance of the skin, go to the Blue Channel in the Curves dialog and Command-click or Control-click near the elbow to place a point on the curve for this area. Leave the cursor where it is, and use the arrow keys to move this point down, subtracting Blue from the skin tone (Figure 12). Watch the Y values go up in the Info palette until Yellow is about 10 points higher. Now the image looks like Figure 13. Everything has warmed up again, but all the lighter colors have become more yellow. This last move has affected the white point—B is down to 240. Every time you move a point on the curve, it can affect other values, so you have to keep an eye on the white and black points whenever you change anything on the curve. It's a relatively simple matter to return to the end points and readjust. Control or Option-Tab will move from one point to the next in the Curves dialog—you can select any existing point this way without moving the cursor.
Select the white point in the Blue curve, and use the arrow keys to move the point to the left (Figure 14). Watch the #2 sampler numbers, and stop when B=245. This one move has cleaned up the white and improved the overall contrast of the image. Return to the Red and Green Channels and readjust the end points as needed to balance everything out. Don't worry if you're off by a few points, but try to keep the values within three points or so of the ideal, at least as far as the ratios go.
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