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CHAPTER 2 IMPROVING DIGITAL PHOTOS Figure 2-17. Lining up the sliders with the histogram. The bits of the histogram bunched at the far right are because of the bright sky. At the dialog s upper right are two buttons, Linear and Logarithmic. These control the way the histogram is drawn. They have no effect on the operation of Levels, but switching to Logarithmic can make the histogram easier to see by making it taller. One common point of confusion is that the sliders on input and output seem to work in opposite directions. If you move the leftmost input slider (at the dark end) to the right, the image gets darker; if you move the leftmost output slider (also at the dark end) to the right, the image gets lighter. What s going on here, and how can you remember which slider does what? Think about it this way: the input sliders let you expand the tonal range of an image that has a restricted range. The leftmost shadow slider controls the black level: anything in the original image that was darker than the position of this slider will be mapped to black. Similarly, anything to the right of (brighter than) the rightmost highlight slider will end up entirely white. Once the image has been remapped according to the positions of the input sliders, the output sliders do the opposite: they let you restrict the tonal range of the final image. Moving the leftmost output slider right means that nothing in the final image will be fully black: anything that would have been black will now be a little bit less dark. Conversely, moving the rightmost output slider to the left means that anything that would have been white will now be a little less bright.
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CHAPTER 2 IMPROVING DIGITAL PHOTOS Figure 2-16. The Levels dialog. Notice the values bunched up at the right (light) end: that s the white sky in Figure 2-12. Both input and output have sliders that you can play with to make the image brighter or darker, or give it more or less contrast. The three input sliders are referred to as shadow, midtone, and highlight, representing the darkest points in the image, a midpoint, and the brightest points. Sometimes you can use the histogram for guidance: drag the left input slider to where it lines up with the left edge of the thick part of the histogram, and line up the right slider with the right side of the histogram (Figure 2-17). Don t neglect to experiment with the middle input level slider, which is also called gamma value you can fine-tune the contrast of the image with it. You can also use the eyedropper buttons under the input levels slider to pick points in your image that should be all black or all white: click on the left eyedropper button, and then click on the darkest point in the image. Then do the same to match the right eyedropper to the lightest point in the image.
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CHAPTER 2 IMPROVING DIGITAL PHOTOS Figure 2-15. Both brightness and contrast have been increased. Levels The Brightness-Contrast tool is straightforward and useful. But the GIMP has two other brightness tools that are much more powerful: Levels and Curves. As with Brightness-Contrast, make sure that Preview is checked so that you can see the effects as you play with settings. Levels (Figure 2-16) has two sets of controls: Input Levels and Output Levels. Input Levels shows a histogram representing the current brightness, or tonal range of the image: this is a graph where the horizontal axis represents brightness, and the vertical axis represents the number of pixels that have that brightness. If the image is fairly well balanced, then the histogram should be spread fairly evenly across the image. Such an image is said to have full tonal range. If an image is way under- or over-exposed, the histogram is probably bunched up at one end or in the middle, or shows a spike over to the left or the right. Such an image has a reduced tonal range it is not using all the possible pixel values that can be displayed.
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CHAPTER 2 IMPROVING DIGITAL PHOTOS Figure 2-13. Just brightness increased. The image looks washed out. Figure 2-14. Just contrast increased. Notice how colors seem brighter, even though the image is still too dark.
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CHAPTER 2 IMPROVING DIGITAL PHOTOS Figure 2-12. The Brightness-Contrast dialog. The original image is too dark. The basic controls in the dialog are simple: two sliders, one for brightness and one for contrast. You can also type in numbers (the values go from 127 to 127, with 0 as the current image), but most of the time you ll probably just drag the sliders until the image looks the way you want it. In addition, the dialog has a checkbox marked Preview. With Preview on, any changes you make to the sliders will be shown in the image window, so you can see exactly how your changes will look. No actual changes will be made until you click OK. Many GIMP dialogs have a Preview option; it s a very useful feature that you ll want to enable unless you have a very slow machine. If you get the settings too far off, the Reset button at the bottom of the dialog will set them back to 0 so you can start over. Cancel will dismiss the dialog without making any changes to the image, while OK will take the current settings and change the image accordingly. Figures 2-13, 2-14, and 2-15 show some examples of the changes brightness and contrast can make. Play around with the sliders to get a feel for what they do. You ll probably find that when you make an image brighter, you ll want to increase contrast a bit, or else it will look washed out. You may also be surprised at how well a slight contrast increase can bring out brighter colors in an image. Be careful of pushing contrast too far, though, or you can make the image look grainy.
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CHAPTER 2 IMPROVING DIGITAL PHOTOS Tip If you already have an area selected, there s a faster way to crop: use the Image . Crop Image item in the image window menus. The dialog also offers a button for Auto Shrink, which attempts to shrink the crop rectangle to a hard boundary. On photographs, this usually won t do anything because there are no obvious edges to tell the GIMP which parts can be removed. The Crop dialog s X, Y, Width, and Height fields can be useful for making small or very specific corrections. If you know the size you eventually want the image to be for example, 640 480 you can simply type those numbers into Width and Height, and then use the Move handles to drag the rectangle to the right place. Or you can use the arrow buttons next to one of the fields to adjust that value by small increments. You may find the arrow buttons confusing; in particular, the fact that clicking up on the Y position makes the crop rectangle go down, and vice versa. It happens because up on the Y position increases the Y coordinate, and pixel coordinates increase downward: the (0, 0) position is the upper-left corner of the image. If this confuses you, don t worry too much about it; just watch the crop rectangle when you make changes and notice in which direction it goes, and you ll be fine. The arrow keys on your keyboard adjust the X and Y origin of the crop rectangle by single pixel values. The Control key combines with the arrow keys to change the width and height. Adding the Shift key to any of these combinations changes the values by a larger increment. When you have the rectangle adjusted precisely to where you want it, hit Enter or click inside the rectangle to make it final. In 2.2 and earlier, you can also use the Crop button in the dialog. The Crop tool s options are fairly simple, though the Resize option doesn t do what you probably expect it to. What it actually does is to reduce the size of the image canvas while leaving the layer size unchanged. These terms will become clearer when you read about layers in Chapter 3; for now, use Crop rather than Resize, and leave the other options at their defaults. Current Layer Only restricts the crop to only the selected layer. You may find that useful once you begin to use multiple layers. Fixed Aspect Ratio is the only tricky option. It only works once a crop rectangle is already active in the image. In order to crop an image and retain the image s existing aspect ratio, first click in the image, then when the Crop dialog appears, click on From selection to set the crop rectangle to the whole image. Then select Fixed aspect ratio in tool options. After all that, dragging on one of the resize handles will constrain the shape of the rectangle. With any luck this will become easier in future versions of the GIMP. Brightening and Darkening A common problem with camera images is their being too light or too dark. The GIMP has three very powerful level adjustment tools to correct such problems: Brightness-Contrast, Levels, and Curves. You can find these tools in two different places in the menu system: the top-level Colors menu (Layer . Colors in earlier versions of the GIMP), or Tools . Color Tools. Brightness-Contrast The simplest level adjustment tool is Brightness-Contrast (Figure 2-12).
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CHAPTER 2 IMPROVING DIGITAL PHOTOS Figure 2-11. The 2.2 Crop tool in action In GIMP 2.4, dragging any corner lets you resize the rectangle; dragging inside the rectangle lets you move it. In earlier versions of the GIMP, dragging the upper-left or lower-right drag handles lets you make the crop rectangle larger or smaller, while dragging the upper-right or lower-left drag handles moves the rectangle without changing its size. Watch the mouse cursor carefully as you move it over the drag handles. The cursor will change as the mouse covers the handle, and will give you a clue as to what the GIMP will do if you begin dragging at that position. Figure 2-11 shows the GIMP 2.2 Crop tool: the cursor itself is a cross, plus a copy of the Crop tool s scalpel symbol and a diagonal arrow indicating that you can change the rectangle s size. The cursor for the other two corners is a pair of crossed arrows, which is used throughout the GIMP to indicate moving an object around within the window. GIMP 2.4 s Crop tool has simpler cursors. At the same time, in GIMP 2.2 and earlier, the Crop dialog will appear. You ll find precise information about the location of the upper-left corner, the width and height of the cropped image, and the aspect ratio. Or you can use the From Selection button, if you have already selected an area where you want to crop. In GIMP 2.4, there s no more Crop dialog; the information shows up in tool options in the Toolbox. (If you find that the Crop dialog gets in your way in earlier versions, hold the Shift key down when you first drag in the image to start the crop. That will prevent the dialog from coming up.)
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CHAPTER 2 IMPROVING DIGITAL PHOTOS Cropping Fairly often, a photo has more in it than you want. Perhaps you took a picture of something far away, and your zoom wasn t quite enough to get up close, so the subject only occupies a small part of the center of the photo. Or perhaps there s some unwanted object intruding on part of the frame, such as another person, a garbage bin, or your finger. (Not that you d ever take a photo with your finger in front of the lens! At least, with the GIMP, you don t ever have to admit to doing such a thing.) The solution to this problem is cropping. Click on the Crop tool button in the Toolbox to select it; it looks like a scalpel blade (Figure 2-10). Figure 2-10. The Crop button in the Toolbox To use the Crop tool, press the left mouse button down in the image window at the upper- left point of where you want the cropped image, drag down and right to where you want the lower-right point to be, and then release the mouse button. (You can actually start at any corner, as long as you drag diagonally to the opposite corner of the rectangle you want cropped.) You will see a preview of the cropped image (Figure 2-11) with drag handles so that you can adjust the crop rectangle.
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CHAPTER 2 IMPROVING DIGITAL PHOTOS Interlace is like JPEG s progressive option: it makes images load in a different way, so that someone viewing them in a web browser can see more of the image before it has loaded completely. GIF comment is like a JPEG comment, a place where you can store copyright information or details about where or how the image was taken. (GIF doesn t include EXIF, so the GIF comment is the only place such information can be recorded.) Animated GIF Options controls ways of making an animation, as you ll see in Chapter 3. Saving an RGB image as PNG will save it as a full-color PNG. If you want indexed PNG, you must convert the image to indexed mode before saving it. That said, the Save as PNG dialog offers yet another set of options. Interlacing is similar to GIF s interlacing or JPEG s progressive options. Save background color relates to images with transparency. Since not all web browsers handle PNG transparency, those that don t can use the GIMP s current background color instead. Save gamma saves information about your monitor. Some viewing programs can compensate for this, and try to adjust the PNG image so that it looks the same on the viewer s monitor as it did on yours. (Most viewing programs probably won t get this right, so for most people, there s no point in checking this option. It increases file size very slightly.) Save layer offset is only meaningful if you re saving a layer that has been moved relative to the rest of the image (you ll learn about layers in Chapter 3). Most viewers probably won t handle this correctly either. Save resolution records the current resolution (dots per inch or dpi) of the image, and is only useful if you re trading files with someone whose software cares about dpi. Save creation time records the time the image was last modified. Save comment is like the GIF or JPEG comment, but alas, you can t set it in this dialog. Save color values from transparent pixels is similar to Save background color, intended mainly for browsers that don t handle transparency properly. Finally, PNG offers a settable Compression level. Unlike JPEG, PNG s compression is not lossy. The only disadvantage to using high compression levels is that it will take slightly longer to save the file. Most of the time difference happens when saving; it doesn t take a viewer much longer to uncompress a highly compressed PNG image. So usually it s best to use the largest amount of compression possible.
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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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