Apple MA434Z/A User Manual - Page 424

If you ignore the premultiplication of your composites, you may have problems with edges, or with raised global levels. Many people see these types of errors, and assume it is a mask problem, so they pinch in the mask a bit. Even more extreme people have erroneously assumed you cannot color correct premultiplied images. These problems are easily solved through proper management of premultiplication. The Math of Over and KeyMix To understand why premultiplication problems occur, it is best to cut straight to the heart of compositing by understanding how a standard composite operator (foreground through a mask over a background) works. This has nothing to do with Shake, but in fact was worked out in the 1970s by two clever fellows named Porter and Duff. The following is the math equation they developed. In this equation, A is the foreground's alpha channel: Comp = (Fg * A) + ((1-A)*Bg) Rather than go into this too deeply, look briefly at the significant part, (Fg*A). This is the definition of premultiplication-"RGB multiplied by its alpha." To avoid the math, you can build the composite with other nodes, in effect constructing an Over node from scratch. The following example (continued from the above section) shows how to build the compositing equation. To build the compositing equation: 1 Read in the munch_unpremult.jpg and munch_mask.iff images from the /Tutorial_Misc/ premult directory. The munch_unpremult image is an unpremultiplied image. It has no mask, so there is no correspondence between black pixels in the RGB channels and black pixels in the mask to describe transparency in the image. munch_unpremult.jpg munch_mask.iff 2 The first step in the formula is (Fg*A). To duplicate this using nodes, attach an IMult node to munch_unpremult and connect the munch_mask node to the IMult background input. 424 Chapter 15 Image Processing Basics