346 CHAPTER 9 ADVANCED DRAWING Dodge divides (Web design course)
346 CHAPTER 9 ADVANCED DRAWING Dodge divides the back layer by the inverse of the front layer (subtracting the front layer from white, just like Colors . Invert would do). This usually lightens the image about the same amount as Divide mode does, though sometimes it doesn t whiten the image quite as much . . . but it inverts some colors. (Unfortunately, that s hard to tell from Figure 9-11.) B * 256 / (256 F) (Dodge) In Burn mode, it s the back layer that is inverted rather than the front layer; then after the division, the result is inverted again. 255 ((255 B) * 256) / (F + 1) (Burn) Burn ends up looking a lot like Multiply, though there are some differences. In Figure 9-11, you can see that they both got darker, but Burn took on the blue color of the background layer, while Multiply turned black. With other color combinations, though, Burn might end up darker. In Screen mode, the values of the visible pixels in the two layers are inverted, multiplied, and the product inverted again. (255 ((255 B) * (255 F))) / 255 (Screen) This sometimes results in a washed out look and a bright image with muted colors. Overlay is a combination of Multiply and Screen modes. The back layer is inverted, multiplied by twice the front layer, added to the original rear layer, and then multiplied by the original rear layer. (If you didn t follow that, don t worry about it.) B * (B + (2 * F * (255 B)) / 255) / 255 (Overlay) It usually doesn t darken nearly as much as straight Multiply, though in this case, it has taken on so much of the background color that you can scarcely see the tree any more. Hard and Soft Lights Hard light is another combination of Multiply and Screen modes. Depending on the color of the rear layer, you can sometimes think of it as an opposite of Overlay mode. The formula is complicated because it has two parts: one for dark colors, and another for bright ones. if (F > 128) (Hard light) 255 ((255 B) * (255 (256 * (F 128))) / 256) else (B * F * 2) / 256 Hard light can sometimes be a useful mode for combining two photographs when you want bright colors and sharp edges. Soft light is not the opposite of Hard light, but it does tend to soften edges and dim colors. It s yet another mixture of Multiply and Screen modes, and its effect is most similar to Overlay mode. In fact, in some versions of GIMP, Soft light and Overlay modes are identical. If Rm is the result from Multiply mode, and Rs is the result from Screen mode, the equation for Soft light is: ((255 B) * Rm) + (B * Rs)) / 255 (Soft light)
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