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324 CHAPTER 8 COLOR MANIPULATION, CHANNELS, AND LAYER MODES Figure 8-28. The color channels How to Use Color Decomposition Remember Decompose, discussed earlier under Working with Grayscale or Black and White ? There, you saw how to use it to convert images to grayscale in several different ways. But it s far more useful than that. What interesting things can you do with color decomposition? Well, for example, you could select that flower in Figure 8-28. If you right-click on the blue channel (in the Channel Dialog) and choose Channel to Selection, the marching ants make it look like you have a pretty good selection of the flower. But that s deceptive. There s a problem: nothing is completely selected. The flower was mostly blue, but not pure blue, so it gets mostly selected. The grass around the flower has some blue, so a little bit of that gets into the selection too. Instead of getting all of the flower and none of the grass, you get a mostly opaque flower and mostly transparent grass. If you paste the selection onto a white background, you ll see something like Figure 8-29. It might make a nice background for stationery, but it s not what you d want most of the time. Decomposing to RGB can give a better result. Figure 8-30 shows the blue layer, which is basically the same as the blue channel. You can see lots of pieces of the background, not just the flower. But since it s in a layer now, you can modify it to be more useful.
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CHAPTER 8 COLOR MANIPULATION, CHANNELS, AND LAYER MODES 323 Figure 8-27. Grabbing a color The Color Channels The GIMP maintains three special color masks that show the red, green, and blue components of the current image. You can see them in the Channels dialog, usually docked as a tab with Layers and Paths (Figure 8-28). Light means a lot of the indicated color; dark means little. Notice the white flower in the blue channel, the general lightness of the green channel (there s a lot of green in the photo), and darkness of the red (there s not much red anywhere). However, the previews in the dialog are too small to see much detail. (You can make them a bit larger using the Preview Size item in the tab configuration menu to the right of the Layers label in the dialog but they ll still be small compared to the full-sized image.) Also, not many operations work directly on the color channels. You can turn a color channel into a selection (via the context menu in the Channels dialog), but for more detailed work with color channels, you re usually better off using Decompose to create an image with red, green, and blue layers.
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322 CHAPTER 8 COLOR MANIPULATION, CHANNELS, AND LAYER MODES Figure 8-26. Editing a colormap Color index lets you move to the next or previous colors, though clicking on the appropriate square is usually just as easy. The + button in the lower right is Add color from FG. It adds a new entry in the colormap, using GIMP s current foreground color. Note that this will slightly increase the size of your colormap, and therefore your image. Between the colormap editor and making corrections with the Pencil tool, you can usually fix any dithering problems in your indexed image. In the end, you get a clean image to put on the web. Pick Colors from the Image While cleaning up that indexed image, the Pick Colors from the Image or Color Picker tool was awfully helpful. It was introduced back in Chapter 3, but it comes in especially handy here. One problem you may have with indexed images is difficulty in using GIMP s normal color chooser. For instance, in the indexed Wilber image, if I set GIMP s foreground color to bright red and try to draw on the image, what actually appears is a muddy brown. What gives? An indexed image can display only the colors in the palette. If you try to draw with a color that s not in the palette, GIMP will try to pick the closest color almost certainly not what you want. The solution is to use GIMP s Color Picker tool to grab a specific color from the image, as in Figure 8-27. That way, you ll be sure of getting a color from the right palette.
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CHAPTER 8 COLOR MANIPULATION, CHANNELS, AND LAYER MODES 321 If you can change the design to eliminate shading, you can make a much smaller image. In Figure8-25, all the shading (except in Wilber himself) has been removed and the image converted to indexed with no dithering. This resulted in a GIF file less than 5K, as compared to 6.1K for the dithered version and 15.8K for the image from Figure 8-24. Figure 8-25. Shading removed, converted as eight colors with no dithering Doubtless you re wondering, Why all this fuss over a mere 10 kilobytes? But some web pages have hundreds of images on them. If you design your icons to have small file sizes, that can make the difference between a web page that loads instantly and one that loads so slowly that users give up on it and go someplace else. Tip Do you ever find that the GIMP function you want to use is grayed out when you re working with an image you loaded from a GIF file? Most often, this is because your image is in indexed mode, perhaps because it was originally a GIF image. Many of GIMP s color functions will only work on an RGB image (or sometimes a grayscale image), and will be disabled for indexed images. (This is because an indexed image does not allow a full range of colors.) If you see color functions disabled and wonder why, check the title bar of the image window and see if it says Indexed. If so, a quick conversion (using the Mode menu) back to RGB mode will solve the problem. (If that doesn t enable the function, check whether the image has an alpha channel; some filters require one.) Of course, if you ultimately want to save to GIF or indexed PNG, you ll have to convert back to indexed mode before you save. Editing the Palette What if you want to change one of the colors in an indexed image? Dialogs . Colormap brings up the colormap editor (Figure 8-26). This window shows every color currently in the image. To change a color, simply double-click on the color square to bring up a color chooser. Alternatively, click on a square and then edit the HTML notation box and hit Return. Or click on a color, and then click the lower-left button in the dialog, Edit color.
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320 CHAPTER 8 COLOR MANIPULATION, CHANNELS, AND LAYER MODES Figure 8-23. Figure 8-22 is zoomed in to show how dithering helps the background look smooth. Flatten for Better Dithering What s the problem? Well, part of the problem is that converting an image with multiple layers to indexed mode often doesn t work very well. This image has five layers: the background, the text, the text s shadow, Wilber, and Wilber s shadow. The GIMP is trying to index each of these layers separately and so the dithering on those shadow layers looks very poor. There are two solutions. First, get rid of the layers. Image . Flatten Image combines all layers into one. That means it will be difficult to do any further editing on the image, so it may be best to make a copy of the image (Image . Duplicate). Then flatten and convert the copy so that you can keep all your layers in your original XCF. The flattened image looks much better when indexed, even with only eight colors (Figure 8-24). Figure 8-24. Flattening the image helps quite a bit. Redesigning for Better Indexed Results But sometimes, if the goal is to make a very small image, simplifying the design can help more. The sad fact is that those lovely, subtle color fades you can make in GIMP don t translate very well to indexed formats.
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CHAPTER 8 COLOR MANIPULATION, CHANNELS, AND LAYER MODES 319 Figure 8-21. An eight-color palette Yuck! The image looks awful! Time to undo and try a different setting. (Convert to Indexed, alas, does not have a preview.) Dithering an Indexed Image Take a look at those options again. An image that has shades of color can often benefit from dithering: combining several colors in the palette to simulate colors that aren t there. Dithering will increase file size significantly, but not as much as increasing the number of colors. The dialog offers several dithering options: The two Floyd-Steinberg options differ only subtly. Positioned dithering is usually best for animated GIFs. Enable dithering of transparency can help if you have any partially transparent layers. Figure 8-22 shows the effect of starting with that same 8-bit palette and adding Floyd-Steinberg (reduced color bleeding) dithering plus dithering of transparency. The shadows around the letters are now dithered with black dots, but they still don t look at all like a shadow. There s good news too: the transition in the color background looks much smoother now, as you can see in the zoomed image (Figure 8-23). Figure 8-22. An 8-color palette with Floyd-Steinberg (reduced color bleeding) dithering and dithering of transparency
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318 CHAPTER 8 COLOR MANIPULATION, CHANNELS, AND LAYER MODES Saving an image as GIF will automatically convert it to indexed mode, since GIF images are always indexed. But you usually won t get the smallest possible image that way: GIMP will use 256 colors, when four might have sufficed. You re much better off converting the image to indexed mode yourself, even for GIF. To save in indexed PNG format, converting to indexed mode is your only option. Since PNG images can be either indexed or full color, GIMP uses the image s mode to determine which form of PNG to use. A Typical Indexing Problem: Choosing the Palette Suppose you ve made a logo such as Figure 8-19, and want to save it in an indexed format. How do you make the image size as small as possible? To convert to indexed mode, use Image . Mode . Indexed…. Figure 8-20 shows the options. Figure 8-20. Options when converting to indexed mode By default, GIMP chooses Generate optimum palette, which creates a palette of the colors in the image, and no others. Most of the time, this is your best choice. The other options offer palettes for special cases. The black-and-white palette, which contains only those two colors, is useful if you re saving the results of a scan after you ve run Threshold on it. The web-optimized palette and the Web palette in the custom palette list use colors that used to be standard in web browsers. Today, nearly all web browsers and computers support 24-bit true color, so web palettes are much less important. You re usually better off optimizing a palette for each image. The trick in converting to indexed mode is in choosing the right number of colors: the smallest number for which the image still looks good enough. Start with a small number, like four or eight (make a guess based on how many colors you think the image should need), and click OK (Figure 8-21).
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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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316 CHAPTER 8 COLOR MANIPULATION, CHANNELS, AND LAYER MODES Figure 8-17. A scanned page from an old yellowed book For anything that really ought to be black and white (with no shades of gray in between), the tool of choice is Colors . Threshold… (Figure 8-18). Previous GIMP versions have it in Layer . Colors . Threshold…, and it can also be found in Tools . Color Tools . Threshold…. Figure 8-18. The Threshold tool
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CHAPTER 8 COLOR MANIPULATION, CHANNELS, AND LAYER MODES 315 Layer Modes Finally, you can use layer modes. First, set the foreground color to a medium sepia color: a28a65 (typed into the HTML Notation box) works well as a starting point. Now, create a new layer that is entirely filled with this color by clicking on New Layer in the Layers dialog. Set the Layer Fill Type to Foreground color. This covers your original layer. But don t panic! Go to the Mode menu in the Layers dialog and choose Color (Figure 8-16). Figure 8-16. With the sepia layer selected, choose Color in the Mode menu. This gives a sharper result than Colorize or Old Photo. Of course, you can use the Color layer mode to tint photos any shade, not just sepia; and you can create very impressive effects by drawing different colors on different parts of the color layer. You ll learn more about colorizing with layer modes in Chapter 10. Using Threshold to Clean Up Scans Have you ever tried to scan in a page from a book? It s especially tricky with an old yellowed book, or a page that has crumples and folds and dirt spots on it, or text showing through from the other side of the page. How can you clean up a page of text like Figure 8-17 into a nice black- and-white image?
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