Apple Shake 4 Tutorials - Manual (Page 104)

104 Chapter 3 Depth Compositing
The Add1 node applies a tint to the balloons and the sky, but doesn’t affect the
buildings from the background image. This is because the composite also includes Z-
channel masking, which prevents color correction on the closer elements (in Z space).
To fix this problem, you need to isolate the color-correction nodes for this branch of
the node tree using MDiv and MMult nodes. MDiv removes the alpha channel from
calculations, which prevents the alpha pixels from being color corrected. MMult brings
the alpha back after color correction is done.
To isolate color correction for a premultiplied image:
1 In the Node View, select the balloons node.
2 Insert a Color–MDiv node.
3 Select the Add1 node.
4 Insert a Color–MMult (not Mult) node.
If you add new color correction nodes for this element, make sure you insert them
between the MDiv1 and MMult1 nodes.
Now the color correction affects only the balloons image, without compromising its
alpha channel.