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256 CHAPTER 7 FILTERS AND EFFECTS Filters for Images Filter is a general term for anything that can operate on an existing image. Filters will always be found in the menus of an image window, but they aren t all in the Filters menu. First, in GIMP versions prior to 2.4, scripts (filters written in GIMP s built-in Script-Fu language, or certain other languages such as Perl or Python) appeared in their own top-level menus named for each language, such as Script-Fu. In addition, certain plug-ins may also create new menus: for instance, GAP, the GIMP Animation Plug-In, creates a top-level Video menu. Finally, there are some filter operations in other image window menus, such as Colors (in pre-2.4 versions, these might appear in Layer . Colors or Image . Colors). Tip Many image filters work only on RGB images, because they need a full range of color. Other filters will only work on a layer with an alpha channel. If you want to use a filter and can t figure out why it s grayed out, check your image s mode (in the title bar of the image window) and make sure it s RGB, not indexed or grayscale. Also check the Layers dialog to see whether the current layer has transparency (right-click on the layer and see if Add Alpha Channel is still available). In Chapter 11, you ll learn about how to find out a plug- in s requirements using the Plug-In Browser. Tools vs. Plug-Ins Most filters are implemented as plug-ins. If you install a plug-in you ve downloaded from the web (you ll learn more about that in Chapter 11), it will most often appear in the Filters menu, and will look just like the filters that come installed with GIMP. The Colors menu contains a mixture of plug-ins and tools. You know about tools: they re operations that have buttons in the Toolbox, like the selection and drawing tools. But did you know that some of the other operations you ve used, such as Brightness-Contrast, Levels, and Curves, are also tools? What s the difference between a tool and a plug-in? Tools can respond to clicks in the image window. When you use a drawing tool, you drag your mouse across the window and leave a trail behind you. When you use a selection tool, you click or drag in the image to indicate where you want the selection to go. Plug-ins can t do that: everything must be done from within the plug-in s own dialog. Equally important, tools can give you a full-size preview of their effect in the image window, while plug-ins can only offer a miniature preview in the plug-in s dialog. Finally, tools display their options in the Tool Options dialog (usually docked below the Toolbox), while plug-ins must use a separate dialog. In menus, tools generally have an icon next to them. Plug-ins usually don t.
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Filters and Effects CHAPTER 7 The GIMP has a huge collection of special effects, tools, and techniques you can use to modify your image or create something brand new. You ve probably already experimented with some of GIMP s plug-ins, but there may be some interesting toys that you haven t seen yet. In this chapter, I ll take you on a tour of some of the GIMP s filters and special effects. I won t try to cover everything there s always more to explore but this chapter should give you a taste of the sorts of effects you can create. You ll learn about the following: Image window filters versus Toolbox Xtns Filters for images Scripts to make new images Image Window Filters vs. Toolbox Xtns Most of the GIMP s fun special effects are located in two places: the image window s Filters menu (plus a few in other menus, such as the Colors menu), and the Toolbox window s Xtns menu. What s the difference? Simply, filters operate on an existing image, while items in the Toolbox s Xtns menu will create a new image. The Toolbox options generally create buttons, arrows, bullets and other items usable in web pages or applications, fancy text that you can use for logos or posters, and patterns that you can use as the background to an image or in effects in other images. Plus, there are a few miscellaneous tools to create items, such as a shaded sphere. Filters can change an image in all sorts of ways. There are straightforward operations you ve already used such as blurring, sharpening, or adding drop shadows. There are various artistic filters that can make your photo look like a painting or sketch. There are filters to handle edge detection or noise reduction, and distortion filters that can bend your image like a pretzel. I ll cover filters first, and then move on to the operations in the Toolbox that can create new images.
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CHAPTER 6 ERASING AND TOUCHING UP 253 Figure 6-34. The right curve can make color adjustment easier, even if you adjust only one color channel. Summary By now you should have a fairly good collection of tools for touching up any problems you might see in your photos. You can dim bright highlights, and enhance areas that are too dark. You can sharpen or blur specific points, or large regions of an image. You know how to paint unwanted objects out of a photograph or copy patterns from another part of the image (or even from a different image). If a photograph has an odd color cast, you ve seen several ways to correct it. These techniques can keep you busy for quite a while, especially if you have a large collection of photographs that could use some minor touch-up work of one sort or another. But this is only a tiny sample of the toys the GIMP has to offer! In the next chapter, let s explore some of the other filters, plug-ins, and tools buried within the GIMP.
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252 CHAPTER 6 ERASING AND TOUCHING UP Figure 6-33. Using Levels to adjust color balance Curves Curves is even better. It lets you control the amount of each color at each point in the curve. Figure 6-34 shows the effect of a curve that reduces the amount of red in the brightest areas of the picture, while keeping red the same or even increasing it slightly in darker areas. This gets rid of the red in the clouds without making the rest of the photo blue, a trick that was difficult with Levels. I didn t need to change the other two color channels; making a curve on the red channel was enough.
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CHAPTER 6 ERASING AND TOUCHING UP 251 Figure 6-32. Using the Color Balance tool Using Curves or Levels for Balancing Colors You ve already used the Levels and Curves tools for adjusting brightness and contrast. But they re also powerful tools for adjusting color. The key is the Channel menu at the top of the Levels and Curves tools. By default, it s set to Value, meaning that the tool will adjust the brightness of the image. But you can also adjust levels or curves of the individual color channels: Red, Green, or Blue. You can even adjust the transparency, or Alpha, channel. However, that is a more advanced technique and isn t generally useful for photographs. Levels Adjust each of the colors independently. Using the Levels tool, I decreased the red and blue highlights, and increased the green shadows (Figure 6-33).
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250 CHAPTER 6 ERASING AND TOUCHING UP Figure 6-31. Sliding the Hue slightly to the left eliminates a lot of the magenta sky. Color Balance Layers . Colors . Color Balance is another tool for correcting color problems in photographs. Choose to modify Shadows, Midtones, or Highlights, then slide the sliders in the direction you want to go: in this case, away from Magenta (toward Green) and away from Blue (toward Yellow). Figure 6-32 shows the result. Caution A serious limitation of the Color Balance tool is the need to choose Shadows, Midtones,or Highlights. You must choose one, not a combination, and it s not always obvious which is best. In this case, Highlights seemed like the obvious choice, because the strongest magenta cast was in the brightest part of the clouds. Wrong! Choosing Highlights created a grainy texture in that region, whereas choosing Midtones gave nearly the same color balance but without the unwanted texture. When in doubt, use Midtones.
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CHAPTER 6 ERASING AND TOUCHING UP 249 You can get an idea of which Hue values represent which color by bringing up the color selector (click on the foreground or background color swatch in GIMP s Toolbox window). Slide the S and V sliders all the way to the right, and look at the colors in the H (Hue) slider. The effect of adjusting the Hue slider slightly to the left shows up well in Figure 6-30. A subtle change can help eliminate a color cast in a photo. Figure 6-30. A small Hue shift fixes the magenta sky. However, you usually have to be very sparing with the Hue slider. Move it more than a trifling distance, and you ll have huge color changes in your image, as Figure 6-31 shows.
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248 CHAPTER 6 ERASING AND TOUCHING UP Figure 6-29. Long exposures can create strange color casts, such as this purple sky. To be honest, I rather like the purple sky in this photograph. But what if I wanted to make the colors more normal? Hue-Saturation The GIMP has several ways of adjusting color balance. The simplest is Colors . Hue-Saturation… (in versions prior to 2.4, find the Colors menu inside Layer). You ll learn about the various color models in detail in Chapter 8. For now, all you need to know is that Hue represents the color of a pixel: whether it s red, green, blue, or some other shade. Saturation is how strong the color is: whether it s intense and bright, or washed out and pale. Value (shown as Lightness in the dialog) is how bright or dark the color is. For correcting color casts, Hue is the only slider you need.
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CHAPTER 6 ERASING AND TOUCHING UP 247 When you re happy with the blur amount, click OK (Figure 6-28). You ll probably want to toggle the selection off temporarily (View . Show Selection) or eliminate it completely (Select . None) to see the effect. Make sure you don t have any halos around the selected object! Figure 6-28. The butterfly stands out more when the grass is blurred. Correcting Color Balance Cameras don t always record the same colors your eye sees. Photographs taken indoors under incandescent lights often come out looking yellow (though some cameras have settings to compensate for this effect). Fluorescent lights can turn photos blue. Long exposures can have strange effects as well, especially with film cameras. For instance, Figure 6-29 was a relatively long exposure on color slide film. Although it was a beautiful storm (I was trying to capture lightning, but didn t manage it), the clouds were just ordinary gray, not the purple shade that showed up in the slide.
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246 CHAPTER 6 ERASING AND TOUCHING UP Figure 6-27. The Gaussian Blur filter Gaussian Blur can use either of two Blur methods: IIR and RLE. It really doesn t matter which one you use. They produce the same results, but switching to RLE may be slightly faster for images that are computer-generated or have large areas that are all the same color. IIR, the default, is faster for photographs. Adjust the Blur Radius (which controls the amount of blur) while watching the preview. Make sure the Preview button is checked (it should be, by default). You can scroll around in the preview window if you need to. As with most effects, the right amount of blur depends on how large (in pixels) your image is, as well as how strong you want the effect to be. Sometimes blurring too much can create an unrealistic, dreamy effect. This could be either good or bad, depending on your goal. Usually you ll want to keep the Horizontal and Vertical numbers the same (they re linked together by default), but if you ever need them to be different, you can unlink them. Tip Sometimes that tiny preview window is annoying and you want a bigger one. With Blur it s particularly obvious, since it s hard to tell the overall effect of blurring the background from viewing only a small piece of an image. GIMP can increase preview size, but it takes some fiddling. See the section on gtkrc in Chapter 12 to learn how to change preview size.
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