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Grokking The Gimp
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6.1.2 Using the Levels Tool

The Levels tool is found in the Image:Image/Colors menu, and it was used in the previous section to examine the tonal range of a grayscale image. The features of this tool and how it can be applied to color images is discussed in detail here.

Figure  6.3

  
Figure 6.3: The Levels Tool
Figure 6.3

illustrates the Levels dialog. Note that the Channels menu is open in the figure, showing five different channels that can be displayed and modified. Because we are interested in color correction, only the Red, Green, and Blue channels apply in this section.

The histogram is a very important feature of the Levels tool because it immediately shows whether a channel occupies its entire tonal range or not. Just below the histogram is a grayscale called the input value domain.  In this grayscale, black represents pixel values of 0 and white values of 255. Thus, for the Green channel shown in Figure  6.3, the black of the input value domain represents dark values of green and the white represents bright values of green. Having no histogram over a part of the input value range means that there are no pixels in the image having these values. Thus, in Figure  6.3, you can see that there is a lack of tonal range because there are significant parts of the upper and lower input value range that have no histogram values over them.

The remaining features of the Levels dialog are for adjusting the distribution of the histogram. The leftmost and rightmost arrows of the input control slider  are used for stretching the tonal range of the image; the middle arrow can be used to warp the range. The leftmost arrow is called the shadow control arrow, the rightmost is the highlight control arrow, and the middle is the midtone control arrow. The arrows of the output control slider  are used for shrinking the tonal range. Adjustments to the control arrows can be made interactively by clicking and dragging them. The arrows can also be controlled numerically by entering values for the min, gamma, and max input levels or for the min and max output levels.

The following example shows how the adjustment features of the Levels tool function. Figure  6.4

  
Figure 6.4: Image of Deep Sea Turtle Having Compressed Tonal Range
Figure 6.4

shows an image that has a severely compressed tonal range. This can be seen by looking at the distribution of pixel values in the Red, Green, and Blue channels shown in Figures  6.5(a), (b), and (c), respectively.
  
Figure 6.5: Levels for R, G, and B Channels
Figure 6.5

The figure shows that the Red channel has the poorest tonal range. The Green and Blue channels, which are similar to each other, have tonal ranges that are hardly much better.

Figure  6.6

  
Figure 6.6: Levels for R, G, and B Channels with Adjusted Input Control Arrows
Figure 6.6

shows how the input shadows and highlights have been set for each of the channels, and the resulting image is shown in Figure  6.7
  
Figure 6.7: Deep Sea Turtle with Maximized Tonal Range
Figure 6.7

which shows that this simple readjustment of levels, producing a maximum tonal range, has greatly improved the image. The turtle now seems to pop right off the page, and the depth of field seems almost three dimensional. In comparison, the turtle in Figure  6.4 seems flat, the colors are muddy, and the image lacks life.

The maximization of an image's tonal range using the Levels tool can introduce color casts (see Section  6.2 for a definition of color casts). In fact, the turtle in Figure  6.7 appears to have a slight magenta cast. But don't worry! Color casts introduced using the Levels tool can be corrected using the Curves tool, which is treated in the next section.

A final note before leaving this section is appropriate. The Levels dialog shown in Figure  6.3 shows a button labeled Auto Levels. This button pretty much automates what has been described in this section. That is, it maximizes the tonal range in the Red, Green, and Blue channels. In fact, it moves the shadow and highlight input control sliders for each channel to about the 5% and 95% points of the histogram. After applying Auto Levels, any of the Red, Green, or Blue channels can be reviewed and modified.




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