Apple MA434Z/A User Manual - Page 417

Node Color-Set Effect Adds or removes R, G, B, A, or Z. Layer- SwitchMatte Adds A. Operation A value set to 0 removes the specified channel. A channel parameter set to something other than 0 adds that channel. Copies in any channel from the second input for use as the new alpha channel for the first input. Many operations allow you to select which channel is used as the modifying channel. For example, the SwitchMatte, KeyMix, and IBlur nodes give you the option to select the R,G,B, or A channel as your control or alpha channel. This often removes the need to swap your channels before you do many operations. Two exceptions to this are the Inside and Outside nodes, which always depend on the second image's alpha channel. To convert a BW (or BWA) image into an RGB (or RGBA) image without changing its values, use the Command Line field to specify the following operation: shake myBlackAndWhiteImage.iff -forcergb -fo myRGBImage For more information on channel/compositing functions, see Chapter 16, "Compositing With Layer Nodes," on page 451. Compositing Basics and the Alpha Channel The Shake compositing nodes are located in the Layer tab. The primary compositing nodes are the Over and KeyMix nodes. For more information on how to use these nodes, see Chapter 16, "Compositing With Layer Nodes," on page 451. Example 1: Compositing Using the Over Node The following images are used in this discussion to demonstrate how the primary compositing nodes work: Foreground (with mask) Foreground alpha channel (part of foreground image) Background As its name implies, the Over node places a foreground image with an alpha channel over a background image. The foreground RGB channels should be premultiplied by the alpha channel. In a premultiplied image, the RGB channels are multiplied by the alpha channel. Chapter 15 Image Processing Basics 417