CHAPTER 2 IMPROVING DIGITAL PHOTOS This process
CHAPTER 2 IMPROVING DIGITAL PHOTOS This process of approximation is called dithering: it involves combining pixels of several different colors. You can choose from several different types of dithering that create different effects, or you can specify no dithering, which may create a smaller file size and a cleaner appearance for images with sharp lines and only a few colors. If the image contains translucent areas, the GIMP can also use dithering to simulate this in formats such as GIF, which don t support partial transparency (Figure 2-9). Figure 2-9. Different levels of indexing, showing file sizes when saved as GIF. Note that all the GIF versions of this photo, even the smallest, result in a bigger file than the best JPEG. The Convert Image to Indexed Colors dialog does not include a Preview button. So you ll probably need to run it several times: choose some settings, click OK, and then use Undo to go back to RGB mode and try again with different settings. It s worth spending some time on this if you re trying to squeeze an indexed GIF or PNG image as small as possible. Note The GIMP offers a third image mode, grayscale, for black-and-white images. Grayscale mode is useful for scans of black-and-white documents or photographs, or simply for converting color photos to black and white. When saving an RGB image as GIF, if you skip the step of converting it to indexed mode, the GIMP will offer to do the indexing for you. In this case, the GIMP will choose a palette intended to represent as many of the image s colors as possible. This helps preserve image quality, but it doesn t do much to reduce the file size. You may even end up with a larger, yet poorer quality file than if you d used JPEG. You re much better off doing your own conversion. The Save as GIF dialog offers a few additional options.
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