CHAPTER 9 ADVANCED (Web hosting reviews) DRAWING 345 Taking the
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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