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Grokking The Gimp
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9.5.4 What Is the Best Choice?

Given that the designer chooses to limit the color palette to one that is web-safe, converting to Indexed format still requires making choices about the type of color distortion that is acceptable when it can't be avoided. Usually the choice depends on the type of image to be displayed.

If the image is a photograph, or one that has smooth color variations, chances are good that there are more than 256 colors in the image, and many will not be from the web-safe palette. Normally the color distortion to photos on an 8-bit display is not noticeable, and it could be argued that it is fruitless to convert the image to an Indexed format under these conditions. However, when photographs are mixed with other graphic materials the argument is no longer valid. Under these circumstances, the best choice is to dither. This is illustrated in Figure  9.28.

  
Figure 9.28: Choosing the Type of Color Distortion for Images with Smooth Color Variations
Figure 9.28

Figure  9.28(a) shows a splash screen used with version 1.0.4 of the GIMP. A dithered version of the splash screen, obtained using the web-safe color palette, is shown in Figure  9.28(b). A color clipped version of the splash screen using the same palette is shown in Figure  9.28(c). Due to the heavy banding seen in Figure  9.28(c), the color distortion due to dithering, seen in Figure  9.28(b), seems much more acceptable. This example shows that for images with smoothly varying color, the dithered solution is better because clipping   gives rise to a strongly objectionable color distortion.

If the image is not a photograph and does not have smoothly varying variations in color but rather large regions of constant color, the strategy is completely different. This is illustrated in Figure  9.29.

  
Figure 9.29: Choosing for Images with Large Uniform Color Regions
Figure 9.29

Figure  9.29(a) shows a graphic design celebrating the $150^{\mbox{th}}$ anniversary of the creation of the U.S. Department of the Interior. This design is constructed of four main colors: black, white, a blue consisting of 46R 110G 207B, and a yellow consisting of 241R 214G 47B. There are other colors in the image, but they are there for antialiasing  (see Section  3.1.2) and sharpness (see Section  6.4.1). From the previous discussion, you can see that the blue and the yellow are not from the web-safe palette, which only take RGB components from the set of values [0, 51, 102, 153, 204, 255]. This means that some color distortion will be introduced if the image is saved to Indexed format with the web-safe palette. Indeed, Figure  9.29(b) shows the result of applying Floyd-Steinberg dithering, and Figure  9.29(c) shows the result of color clipping.

The dithering in Figure  9.29(b) produces a very undesirable effect. The blue and yellow regions both suffer from unsightly color speckle. On the other hand, the color clipped version in Figure  9.29(c) produces a blue and a yellow that are slightly different than those of the original. Nevertheless, it is likely that this result is preferable because it preserves the homogeneity of the large, uniform color regions.

In conclusion, images that have a lot of detail and subtle color variations are better served by dithering than color clipping. Alternatively, images that have broad uniform color regions should avoid dithering and use color clipping. For images that consist of a mix of the two conditions, some experimentation is necessary. Unfortunately there are sometimes no easy choices.




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