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Grokking The Gimp
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6.1 Improving Tonal Range

Improving the tonal range of an image is the first step that should be taken in almost every effort to touch up and enhance a photo. To do this, it is necessary to understand the basic elements of tonal range and the tools the GIMP provides to measure and affect it.

6.1.1 Highlights and Shadows

What is tonal range? To answer this question, let's look at the grayscale photo shown in Figure  6.1(a).

Figure 6.1: Highlights and Shadows in a Grayscale Image
Figure 6.1

The figure shows a tiger image consisting of a complete range of grayscale values from 0, or completely black, to 255, fully white. Furthermore, from the histogram  shown in Figure  6.1(b), you can see that the distribution of pixel values in the image smoothly covers the entire available range. The histogram shown in Figure  6.1(b) is part of the Levels tool, which is discussed in detail in the next section.

The lightest part of an image is called the highlight and the darkest is called the shadow. It is important to note that not all images will have the maximum highlight of 255 and/or the minimum shadow of 0. Thus, the tonal range of an image is just the numeric difference between the image's maximum highlight and its minimum shadow. You will see shortly that measuring highlight and shadow values is useful for performing image enhancement, but before developing this idea, let's examine why tonal range is so important.

Having a full tonal range is generally a good thing. A full tonal range means that the image has, in a general sense, the fullest possible contrast. To illustrate this idea, the tonal range of the image in Figure  6.1(a) can be synthetically diminished by setting the output sliders of the Levels tool to values well inside the range of 0 to 255. This adjustment is shown Figure  6.2(b),

Figure 6.2: Limited Tonal Range
Figure 6.2

and the result on the image is shown in Figure  6.2(c). Here, you can see the effect of compressing the tonal range. Figure  6.2(b) shows the settings of the Levels tool used to limit the tonal range. Notice that the resulting contrast of the image in Figure  6.2(c) is much poorer than the contrast in Figure  6.2(a). The image with the smaller tonal range looks muddy and washed out in comparison to the original.

This example is based on a grayscale image whose tonal range has been synthetically impoverished. Nevertheless, the conclusions that can be drawn from it are general. That is, maximizing tonal range is usually a great way to enhance an image. However, sometimes it is better not to maximize the tonal range. An image of white lace gloves on a white linen tablecloth background is such an example. Under these circumstances, there is a subtle interplay of whites and off-whites in the image, and a deep black shadow is most likely undesirable. In most cases, however, getting the most tonal range out of an image improves contrast, which in turn significantly enhances the image.

Grokking The Gimp
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  Published under the terms of the Open Publication License Design by Interspire