Apple MA434Z/A User Manual - Page 418

Important: Premultiplication plays a vital role in compositing, and Shake gives you explicit control over premultiplying and unpremultiplying your images. For more information, see "About Premultiplication and Compositing" on page 421. Tree with Over node Result If the image is not premultiplied, it can be premultiplied in one of two ways: m Add a Color-MMult node before the Over node in the process tree. m Use the preMultiply toggle in the Over node's parameters. Example 2: Compositing Using the KeyMix Node KeyMix, the second most important compositing node, mixes a foreground input image and a background input image through a third, separate, input image-the mask. You can select which channel of the third image works as the mask. You can also invert the mask and control its intensity. As mentioned previously, a successful Over composite requires an alpha channel for the foreground and foreground RGB channels that are premultiplied by that alpha channel. 3D-rendered elements are almost always premultiplied. Scanned elements or other 2Dgenerated plates require an added alpha channel (also called the matte or mask channel) that is used to premultiply that image with the Color-MMult node. To get the necessary alpha channel, you have several options: • Pull a key with a Shake keying node (or combination of nodes). • Pull a key in a different software package and read the images into Shake. Copy the key into the alpha channel of the foreground image (with the Copy or SwitchMatte node). Finally, apply an MMult, and then composite. • Draw a mask with an Image-RotoShape node. • Paint a mask with an Image-QuickPaint node. • All of the above, combining masks with the IAdd, Max, Inside, or Outside node. 418 Chapter 15 Image Processing Basics