Apple Shake 4 - Manual (Page 420)

420 Chapter 15 Image Processing Basics
In the following example, MDiv and MMult nodes are added to color correct a 3D-
rendered element. Again, you can alternatively omit the MMult, and enable preMultiply
in the Layer–Over node.
For an example of color correcting premultiplied elements, see Tutorial 3, “Depth
Compositing” in the Shake 4 Tutorials.
Combining Images in Other Ways
Shake also has a set of mathematical and Boolean layering operators. The IAdd, IDiv,
IMult, ISub, and ISubA nodes add, divide, multiply, or subtract two images together. The
“I” stands for “Image.” The second subtracting node, ISubA, returns the absolute value of
the image. If you place a dark gray image in the first input (value .2, .2, .2), and then a
white image (1, 1, 1) in the second input, the ISubA operation, it returns a light gray
image (.2 - 1 = -.8, take the absolute value of, returning .8, .8, .8). This is a quick way to
compare two images.
The more flexible tool for generating difference mattes is the Common node, which is
used to isolate common or different elements between two images. The other
mathematical operators are Min and Max, which return the lower or higher values of
two images, respectively. The Boolean operators Inside, Outside, and Xor are also useful
for masking images:
• Inside places the foreground image inside of the background alpha.
• Outside places the foreground image outside of the background alpha.
• Xor reveals only areas without a common alpha area.
Shake contains several effect operators:
• ZCompose for Z-based compositing
• Screen, to composite light elements and preserve highlights
• Atop, to place an element only on the foreground image. (For example, you can use
the Atop node to place a smoke element over a CG character, matching smoke in the
background plate.)
Color correcting Result