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CHAPTER 9 ADVANCED DRAWING 345 Taking the difference of two numbers is the same as subtracting, isn t it? Usually, that s true. But in the GIMP, Difference mode is, well, different. Difference is a subtraction too: but after subtracting, if any color values are negative, GIMP uses the absolute value of that color instead of truncating it to zero. | B F | (Difference) This means that instead of a lot of dark areas, the regions that would have been black become lighter again especially in the colors that would have been absent. You can see that in Figure 9-11: areas of the trunk that were dark blue in Subtract mode become magenta in Difference mode. Difference sometimes creates strange, garish colors and unusual transitions between colors. It can be useful when you need that sort of effect. In Chapter 10, you ll see several other good uses for Difference mode. Multiply and Divide Multiply multiplies the value of the pixels in the two layers (bet you already guessed that). This generally makes the image darker. Why should that be? Multiplying two numbers should make a bigger number, shouldn t it? And bigger numbers are lighter? The trick is that the result is normalized to 255 (in other words, divided by 255). Otherwise, too much of the image would end up being too bright. So, the actual equation for Multiply is: F * B / 255 (Multiply) Note This ends up being darker than the front layer because B / 255 is less than 1 (except where B is white). So when it s multiplied by F, you get a number that s usually less than F. Divide is a little more complicated. As you learned in grade school, you can t divide by 0 (at least without getting infinity). In order to guard against division by 0, GIMP adds 1 to the front layer s pixel values. Like Multiply, Divide is normalized, though this time it s normalized to 256 (1 is added to the normalization because of the 1 that was added to the front layer s pixels). B * 256 / (F + 1) (Divide) Even after normalizing the result, Divide mode often results in nearly white pixels that look burned out. Dodge and Burn, Screen and Overlay These four modes are closely related to Multiply and Divide. Burn and Overlay are similar to Multiply and make the image darker; Dodge and Screen are more like Divide, and make an image brighter.
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344 CHAPTER 9 ADVANCED DRAWING Dissolve and the two special drawing tool modes Behind and Color Erase were discussed in Chapter 4. Now I ll cover the other modes, what they do, and some ways to use them. You ll learn even more uses for the layer modes in Chapter 10. MATHEMATICALLY SPEAKING In the discussions of the individual modes, I ll give equations for what each mode is actually doing under the hood. For the mathematically inclined, this can help to understand better what the modes do. If that doesn t describe you, don t panic! You don t have to understand the equations to get layer modes. Many talented artists use the GIMP s layer modes to create beautiful art without ever worrying about the mathematical underpinnings. If you re not comfortable thinking about colors as vectors, or with the mathematical operations, just skip the equations (and the rest of this sidebar). When equations are given, rather than using the (R, G, B) notation used above, I ll use a shorthand notation, where each pixel is referred to as a single vector with three color components. The two layers will be referred to as F, for the layer in front, and B, for the layer in back. For instance, in the Subtract mode example above, a shorter way of writing the operation is: B F This is just a shorthand way of saying For each of the three colors in B and F, subtract the F color from the corresponding B color. Colors must always range from 0 to 255. If not otherwise stated in an equation, you can safely assume that if the result of an operation results in a negative color, 0 will be substituted; if an operation results in a value greater than 255, GIMP will use 255. So the result of subtracting bright red from medium gray is: (128, 128, 128) (255, 0, 0) = (0, 128, 128) or, no red, half green, and half blue: a dark cyan. Addition, Subtract, and Difference Addition mode is straightforward: the two pixel values are added together. F + B (Addition) Of course, if that makes any of the colors greater than 255, then that color is set to 255. Addition will nearly always make the image lighter. Subtract is the opposite: the front layer is subtracted from the layer below it. B F (Subtract) This will make colors darker, naturally. You ll often end up with a lot of black or near-black from using Subtract mode. Note Why is it Addition, but Subtract rather than Subtraction ? Historical reasons. Which usually means, No one remembers any more why they re named that way.
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CHAPTER 9 ADVANCED DRAWING 343 A lot of the modes don t do anything interesting when the background is white. For instance, in Addition mode, the colors of the two layers are added together. But white already contains a full complement of each color. You can t get any whiter than white, so in Addition mode, the red swirl just becomes white and disappears in the background. For the same reason, other modes won t seem to do anything useful if the background is black. A Quick Tour Through All of the GIMP s Layer Modes Figure 9-11 shows all of the GIMP s layer modes, using the tree from Chapter 4 against a background of light blue/cyan. Figure 9-11. All of the GIMP s layer modes Be careful not to draw too many conclusions from this one diagram, though. The effects of each layer mode can vary considerably depending on the colors of the layers involved, as you ll see. Also, though it may not seem like it as you read through, all these modes have their uses which are often not what you d expect at all! But more on that later.
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342 CHAPTER 9 ADVANCED DRAWING Where the swirl is red, you re seeing just that red. The layer mode is Normal, so GIMP takes each pixel in the top layer (the red swirl), determines that it s not transparent, and shows you only that. Outside of the swirl, the top layer is transparent, so GIMP shows you the next layer down. Simple! But what happens if you set the layer mode to Subtract (Figure 9-10)? Figure 9-10. The red swirl with Subtract mode Why did the red swirl turn that color? In Subtract mode, the color you see is the result of subtracting the front layer (the swirl) from the back layer (the background). Each color channel is subtracted separately. Remember that white is the combination of the three primary colors: red, green, and blue. Each color has a range from 0 to 255 (8 bits, in computer terminology). A white pixel is (255, 255, 255) to the GIMP. A red pixel is (255, 0, 0): all the red, with no green or blue. So: white red = (255 255, 255 0, 255 0) or (0, 255, 255). The result has no red at all, but full green and blue: it s cyan. That s why the red swirl on top of a white background becomes cyan in Subtract mode.
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CHAPTER 9 ADVANCED DRAWING 341 Layer Modes By now, you should be very comfortable with adding layers to an image. Generally, when you put one layer on top of another, the top layer is what you see. Lower layers only show through when the upper layer is at least partially transparent. But layers can be combined in many other ways, using layer modes. You ll find them in the Mode menu in the Layers dialog. GIMP will use a layer s mode to determine how to combine each pixel from the top layer with the pixel in the same location in the layer below. Layer modes are also called blending modes, because they specify how to blend the colors in a layer with the colors in the layer below it. Note The Mode drop-down menu in the drawing tool options, which you ve already seen in Chapter 4, works basically the same way that layer modes do. It also adds a few special modes that are only used for drawing tools. Confusing? Let s look at an example. Figure 9-9 shows a red swirl on a white background. Of course, the swirl is in a separate layer from the background. Figure 9-9. A red swirl layer on a white background layer
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340 CHAPTER 9 ADVANCED DRAWING Figure 9-8. You can make all sorts of frames using layer masks. Interpreting the Mask Border Color Sometimes it s hard to tell the mode of a layer mask. But it s a lot easier if you learn to decode the colors of the mask thumbnail s border in the Layers dialog: Black: Not selected. White: Selected. If you draw, you will draw on the mask. Green: Displayed. GIMP will display the contents of the layer mask, rather than applying it as a mask. When in this mode, the border color doesn t help you tell whether the mask is selected for drawing; to find out, check the color of the layer preview next to the mask preview. If the layer preview is black (unselected) but the layer is highlighted in the dialog, the layer mask is selected. Red: Disabled. Again, you can t tell whether or not the layer mask is selected if it s disabled, so check the layer preview s border color.
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CHAPTER 9 ADVANCED DRAWING 339 Figure 9-7. Blurring beyond the size of the image creates squared-off edges. This happens because the oval frame gets bigger when it s blurred, and the image isn t big enough to contain it. For a really fuzzy border, you have to leave quite a bit of room around your original selection. If you think you might want a very fuzzy boundary, you re best off not cropping much until the very end. Note You can do the same sort of operation on GIMP s QuickMask: select the image with a fuzzy border, and then paste it onto a white background. Why use a layer mask? Because that makes it easier to see the effect of the operation. With the QuickMask, you have to modify the mask, change it to a selection, copy it, and paste it somewhere else only then will you see whether you need to go back and modify the QuickMask. Of course, since the layer mask is just a black-and-white image itself, you can modify it in a variety of ways to make all sorts of borders, like the ones in Figure 9-8. Experiment! You may not like most of the effects, but you might also find the perfect look.
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338 CHAPTER 9 ADVANCED DRAWING Figure 9-6. Blur the layer mask to get a more gradual border. But wait what s wrong with Figure 9-6? Notice what happens at the edges. All this blurring has made the border extend beyond the edges of the image. If you put the image on a web page, you ll see squared-off edges, as in Figure 9-7.
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CHAPTER 9 ADVANCED DRAWING 337 If you re happy with the result, make the selection visible again (that way you won t be surprised the next time you try to select something), and then cancel it (Select . None). Save the image as JPEG since it s full color unless you absolutely need the transparency. If you do, make the background layer invisible (you won t want to save that part), and then save in a format that allows transparency, such as GIF or PNG (refer back to Image File Types in Chapter 2 for a discussion of the trade-offs). Even Bigger Fuzzy Borders What if you want more feathering than the 100 pixels that the GIMP allows in its selection tools? Well, while you still have the selection, you can use Select . Feather…. But that doesn t let you preview what the border will look like. A better solution is to make sure the layer mask is selected (it will have a white border, as in Figure 9-5) and then blur it with Filters . Blur . Gaussian Blur…. Figure 9-5. A selected layer mask has a white border. Blurring the existing 100-pixel feathering by another 250 pixels gives a more gradual border, as shown in Figure 9-6.
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336 CHAPTER 9 ADVANCED DRAWING Figure 9-3. Make a new layer mask, initialized to Selection. Figure 9-4. Adding a white background layer and making the selection invisible gives you a much better idea of how the border will look.
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