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Grokking The Gimp
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4.5.3.3 Threshold and Decompose

In the previous sections, Threshold was applied directly to the image. However, this tool can often be more effective when applied to an image color component. The function Decompose, found in the Image:Image/Mode menu, can be used to separate an image into its RGB and HSV components. When the decomposition is RGB, Decompose creates three grayscale images containing the red, green, and blue channels of the image. For HSV, three grayscales are also created, but now they represent the hue, saturation, and value components of the image. (See Chapter  5 for an in-depth discussion of the relationship between an image and its RGB and HSV color components.)

Figure  4.49(a)

  
Figure 4.49: A Flower Image and the Decompose dialog
Figure 4.49

illustrates an image of a flower, and Figure  4.49(b) shows the Decompose dialog. Either an RGB or HSV decomposition of the image can be performed by clicking on the appropriate radio button. The CMY  decomposition is not useful, because it produces results that are identical to RGB when used with the Threshold tool. CMYK  may produce interesting results, but only RGB and HSV are discussed here.

Figure  4.50(a), (b), and (c)

  
Figure 4.50: The RGB and HSV Decompositions of the Flower
Figure 4.50

show the red, green, and blue components of the flower shown in Figure  4.49(a). Figure  4.50(d), (e), and (f) show the hue, saturation, and value components. Note that for each of the components, the relationship between the flower and its background is different. For example, the flower in both the red component and the saturation component seems to be better separated from the background than for the other components. Because the flower is a brightly saturated orange-red, this should not be a surprise. However, the point of using the Decompose tool is that it gives the Threshold tool an advantage that can be exploited when trying to extract a natural mask. Examples of using this technique can be found in Sections  7.3 and 7.4.




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