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Grokking The Gimp
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6.2.6 Getting More Detail into the Subject

Sometimes one part of an image is more important than the rest. We often refer to this part of the image as the image subject. It is typical to want the subject to have as much contrast as possible. This makes the subject stand out and look much more interesting. As was discussed in Section  6.2.1, the Curves tool can be used to improve the contrast of certain parts of the tonal range in an image. This can be applied to the subject of the image by determining its lightest highlight and deepest shadow and then steepening the part of the control curves covering this range.

However, if a lot of work has already gone into maximizing tonal range and correcting color, you might be reticent to play with the curves to get additional contrast into the subject. Clearly, manipulating a part of the curves in an attempt to improve contrast can damage the color balance obtained with much hard work.

Fortunately, there is a way to have your cake and eat it too. Up to this point, the red, green, and blue control curves have monopolized our attention. As you have already seen, modifying any of these changes the overall balance of color in the image. However, the Curves tool can also be used to modify the image's Value channel.  As discussed in Section  5.3, the Value channel has no effect on color; it only affects brightness. This, then, is the perfect channel for improving contrast while preserving the image's color balance.

To illustrate the use of the Value channel to improve contrast,  Figure  6.22(a)

Figure 6.22: Original and Color Corrected Images
Figure 6.22

shows an original image that lacks tonal range and has a terrible yellow color cast. Figure  6.22(b) shows the corrected image, which was obtained simply by applying the Auto Levels button of the Levels tool (see Section  6.1.2). Although the image is much improved, it lacks detail in the eagle's white head feathers. It would be nice to improve this part of the eagle to give it more visual depth. This can be done by modifying the Value channel of the Curves tool.

To improve the contrast of the head feathers, it is necessary to determine the value range that this part of the image is contained in. The Threshold tool is perfect for this (see Section  6.2.3). Figure  6.23

Figure 6.23: Using Threshold to Find the Correct Value Range
Figure 6.23

illustrates the use of Threshold for determining an appropriate range of values. Figure  6.23(a) shows the Threshold dialog applied to the color corrected image in Figure  6.22(b), and Figure  6.23(b) shows the result of having swept out the range [174,255] in the Threshold dialog. This range, which was determined by experimentation, pretty much covers the part of the eagle image where we want to improve the contrast.

The value range determined with the Threshold tool is noted, and the tool is then canceled. The next step is to invoke the Curves tool and to improve the contrast of the Value channel curve using the range determined using Threshold. Figure  6.24

Figure 6.24: Using Curves to Improve Contrast in the Subject Value Range
Figure 6.24

illustrates the procedure. Figure  6.24(a) shows the Curves dialog for the Value channel. A control point has been added to the curve at the input value of 174, and this point has been moved to the output value of 140. This has the effect of steepening the Value curve in the range [174,255], which is the range of values where we want to improve contrast. From previous discussion, we know that steepening the Value curve in a range has the effect of improving this range's contrast.

The result of applying the curve shown in Figure  6.24(a) is shown in Figure  6.24(b). Comparing this result to the image in Figure  6.22(b) shows that the contrast of the eagle's head feathers has been significantly improved. As a final, practical note, the amount that the Value curve is steepened is determined experimentally by moving the control points on the curve and evaluating the effect on the image.

Grokking The Gimp
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