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Grokking The Gimp
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4.5.3.1 Working with the Threshold Tool

The Threshold tool allows you to specify a range of values in an image. All the pixels that are in the range of the selected values are mapped to white, and the rest are mapped to black. Threshold is a powerful tool for automatically creating masks. This is illustrated in the following example.

Figure  4.43

  
Figure 4.43: Pasting the Image into Its Own Channel Mask
Figure 4.43

illustrates the first step of using Threshold to create a natural mask. In the example, we want to make a selection of the partially blooming flower in Figure  4.43(a). We begin by copying the image in Figure  4.43(a) into a channel mask. This is done by creating a new channel mask in the Channels dialog, and then copying and pasting the image layer to the mask using C-c and C-v (see Section  2.4). Figure  4.43(b) shows the resulting Channels dialog, and Figure  4.43(c) shows that yellow is chosen as the mask color. This color was chosen to contrast against the dark background of Figure  4.43(a). Since a channel mask is only 8 bits deep, pasting the color image into the channel mask immediately converts it to a grayscale. This can be seen in Figure  4.43(d), which was obtained by toggling on the channel mask's Eye icon and toggling off the image layer's Eye icon.

The Threshold dialog works by clicking and dragging out a part of the range of values in the histogram. The range of values in the histogram is in [0,255], and, as can be seen in Figure  4.44(b),

  
Figure 4.44: Applying Threshold to the Channel Mask
Figure 4.44

the range that has been selected is from 72 to 253. Sweeping out values in the Threshold dialog's histogram immediately maps to white the pixels in the active layer (here the channel mask) having these values. The pixels having values outside the swept range are mapped to black. Thus, the channel mask that was a continuously varying grayscale image is converted to a binary black-and-white one. Figure  4.44(a) shows the channel mask before the application of Threshold, and Figure  4.44(c) shows the channel mask after the application of Threshold.

Toggling the image layer's Eye icon back on allows the channel mask to be seen over the image, as illustrated in Figure  4.45.

  
Figure 4.45: The Resulting Mask Defects as Seen in the Image Window
Figure 4.45

The parts of the image layer corresponding to the white parts of the channel mask can be seen clearly in the image window. The parts of the image corresponding to black parts of the channel mask are masked by a partially transparent yellow film.

As shown in Figure  4.45, the result of using Threshold produces an almost perfect mask for the flower. However, several defect regions remain. There are certain parts of the image that should be masked but aren't, and there are parts that are masked but that shouldn't be. These regions are easily removed using the Lasso and the Paintbrush tool.

Figure  4.46(a)

  
Figure 4.46: Using the Lasso Tool to Remove Defect Regions
Figure 4.46

shows how the Lasso   has been used to select parts of the image that should be masked but aren't. Because there are several offending regions, their selections have been combined using the methods described in Section  3.2. The parts of the channel mask that are in the selected regions are repaired (that is, converted to black) in three steps. The channel mask is made active by clicking on its thumbnail in the Channels dialog, the Active Background Color is set to black, as shown in Figure  4.46(b), and the selections are cut by typing C-x in the image window. The result is shown in Figure  4.46(c).

Figure  4.47 shows how the stalk of the flower, which was not included in the mask, is restored using the Paintbrush  tool. Figure  4.47(a)

  
Figure 4.47: Using the Paintbrush Tool to Fill in Missing Regions
Figure 4.47

shows the stalk of the flower zoomed by 300%, and Figure  4.47(b) and (c) show that white is chosen as the Active Foreground Color and that a small hard brush has been chosen from the Brush Selection dialog. The Paintbrush cursor can be seen applying white paint to the mask over the region of the flower stalk in Figure  4.47(a). The semi-transparency of the mask facilitates the painting process. Figure  4.47(d) shows the result of having fully restored the flower stalk.

For the final step in this example, Figure  4.48(a)

  
Figure 4.48: Converting the Mask to a Selection
Figure 4.48

shows how the Channel to Selection function is applied by clicking on its icon in the Channel dialog's button bar. Turning off the visibility of the channel mask, the resulting selection is seen in Figure  4.48(b).

This example shows how using Threshold can produce a selection much more quickly than would have been possible with the Bezier Path tool. Making a Bezier path would have required placing and refining a large number of control points. In contrast, the procedure employed with the Threshold tool required some experimentation with values in the tool's dialog, followed by some rough selections with the Lasso and some painting with the Paintbrush.

A key element to making the Threshold tool work efficiently is finding a reasonable range of values in the tool dialog's histogram. The example used in this section shows that it is not necessary to find a perfect mask. Rather, the goal is to find a mask that separates the subject from the background enough so that tools such as the Lasso and the Paintbrush can be used to easily clean up the defects.

The range of values used to create the mask in this example is shown in Figure  4.44(b). It is important to understand that this result was obtained by using a trial-and-error, experimental approach. Several contiguous regions of the histogram were swept out by the mouse, and, each time, the parts of the image that mapped to white and black were observed. A tip for finding useful regions is to examine the ranges of values supporting the main bumps in the histogram. These are usually associated with major image features, and it is often the case that one of these bumps is the solution to our search. When a reasonable range has been discovered, the data entry boxes can be used to refine the end points of the range.

Although the Threshold tool is not a panacea and isn't guaranteed to work, it is often successful. It is worth trying to apply the Threshold tool before launching into a long selection process with the Bezier Path tool. Some good examples of using Threshold to make selection masks are illustrated in Sections  7.3 and 7.4.

Grokking The Gimp
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