CHAPTER 8 COLOR MANIPULATION, CHANNELS, (Web servers) AND LAYER
CHAPTER 8 COLOR MANIPULATION, CHANNELS, AND LAYER MODES 317 Threshold has two sliders, though most of the time, you ll only need to adjust the left one. The sliders move along a bar showing a range of grayscales. The key to the Threshold tool: any pixel value between the two sliders marked as blue in the tool will be white in the resulting image. Anything that s darker than the left slider or brighter than the right slider will be black. (Yes, that s slightly counterintuitive. To give the image more white, you want less white more blue in the tool dialog.) To adjust the darkness of an image such as a scan, slide the left slider until you re happy with the result. So what s the right slider for, anyway? Adjusting the right slider will take anything that s very bright in the image, and make it black instead of white. This may sound like an odd thing to do, and generally it s not useful in scans. However, you can also use the Threshold tool on an image to make selection masks. Sometimes being able to select a brightness region somewhere in the middle can help a lot in making a selection. You ll see how later in this chapter. Threshold, since it creates an image that s purely black and white, will eliminate any aliasing smoothing the edges of lines, sometimes resulting in a jaggy look. When you want smoother edges and don t mind some gray in the image, Curves or Levels may sometimes be a better tool than Threshold. Try all of them to become familiar with the differences. Indexed Color An image with indexed color contains only a fixed set of colors, usually 256 or fewer. You ve already met indexed formats such as GIF and indexed PNG in Chapter 2. There, you saw an example of how the number of colors can change the quality of the image. The list of colors in the image is called the image s palette, like an artist s palette holding the pigments to be used in a painting. The file size of an indexed image depends very much on the number of colors in the palette. Indexed formats can be very efficient if they have only a small number of colors. They re ideal for small web images such as icons for buttons or forward/ backward arrows. Every image loaded into GIMP has a color mode, represented in the title bar of the image window. For instance, in Figure 8-19, the title bar shows that the image is in RGB mode. Figure 8-19. A colorful logo
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