Monday, October 8, 2007

Curving RGB Color

Color correcting by numbers is a combination of art and science



Once the samplers are placed, remove the Threshold adjustment by dragging the Threshold layer to the trash icon at the lower right of the Layers palette (Figure 6). Now you can look at the Info palette and see the RGB readings for the two color samplers (Figure 7). Here, you can see the numbered samplers at the bottom showing the dark value at R=19, G=8, B=9 and the light value at R=255, G=245, B=235, although these no doubt will be different on your photograph. Ideally, we'd like the darkest and lightest point to represent black and white in the image. You can see by the numbers that all these values aren't the ideal R=10, G=10, B=10 for black and R=245, G=245, B=245 for white.

Besides being brighter and darker, the ideal values are also neutral—white and black have no color, so you should try to get R=G=B in these areas. You can see that the current values show a red bias. Armed with this information, you can start correcting by forcing these areas to be neutral.

Make a Curves adjustment layer by clicking on the New Adjustment Layer icon at the bottom of the Layers palette (Figure 8). The idea here is to get the values for RGB to hit 245, 245, 245. Since you're starting with the highest value at 255 in the Red Channel, it's probably better to move all the channels individually until everything is at 245.
Once the Curves dialog opens, start by moving the white point, in the Red composite channel, down. (Pay attention to how the gradient bars are set up; I have mine set up with the black end at the left and bottom. If yours are reversed, click on the gradient to switch it back.) The white point should be at the upper right of the curve—click on the point to select it and then you can use the arrow keys to move it (Figure 9). Look at the Info palette—the Color Sampler numbers are at the bottom, black at #1, white at #2. There are two columns of numbers for RGB; the left column is the current value, the right is the value you're changing to. Watch how the numbers change when you move the point on the curve—stop when R hits 245 (Figure 10).

Because the Green Channel is already at 245 no change is needed; however, this may not be the case for your image. Repeat the process as needed in the Green and Blue Channels—the image is starting to shape up (Figure 11).

Now we need to do the same for the black point. Look at the values for the #1 sample point: Green and Blue are both too low, but Red is way too high! Switch to the Red Channel from the Channel drop-down in the Curves dialog and select the black point on the curve (this should be at the bottom left). Move this to the right until R=10.

The moves for Green and Blue will be opposite—the black point in these channels will have to move up.

This movement may affect the value at the white point slightly, so you may have to readjust the other end of the curve a little bit to keep the white point value at 245.

So far, we've been concerning ourselves only with the black and white points, and by neutralizing the color in these areas, we've managed to accomplish quite a bit of improvement. Now we have to look at the most important color in this image—the color of the skin! Before you can do this, we need to go over a little background material.




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