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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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