Apple Shake 4 - Manual (Page 421)

Chapter 15 Image Processing Basics 421
Using the ClipMode Parameter of Layer Nodes
You can easily composite elements of any resolution. To set the output resolution of a
composite that contains images of multiple resolutions, go to the compositing node’s
parameters and use clipMode to toggle between the foreground or background (as the
output resolution). This applies to all layering commands. An element composited over
a differently sized background is one way to set your output resolution. For more
information on setting resolution, see Chapter 3, “
Adding Media, Retiming, and
Remastering ,” on page 107.
As outlined in the “About Channels” section above, you can easily combine 1-channel,
2-channel, 3-channel, 4-channel, or 5-channel images. For example, you can combine a
luminance image with an alpha channel (2 channels) over a 5-channel image, using an
Over node.
For more information on compositing math and the individual layer functions, see
Chapter 16, “
Compositing With Layer Nodes ,” on page 451.
About Premultiplication and Compositing
An understanding of premultiplication in compositing is essential to understanding
how to combine the tasks of compositing and color correction. This section details the
process that makes standard compositing functions work, and defines what
premultiplication actually is. Regardless of your compositing software, the concept of
premultiplication is important for understanding what can go wrong, and how to fix it.
The definition of a premultiplied image is simple—an image that has its RGB channels
multiplied by its alpha channel. Typically, images from a 3D renderer are premultiplied.
This means that the transparent areas are black in both the RGB channels and in the
the alpha channel. In premultiplied images, the RGB channels never have a higher
value than the alpha channel.
Premultiplication should always be considered whenever you have to modify a
foreground element and composite it over a background image. The premultiplication
of two or more composited images should be considered whenever you do one of the
following things:
• Perform color correction—in particular using nodes that raise the black level such as
Add, Clamp, ColorMatch, ColorCorrect, Compress, and Contrast
• When using filtering nodes
Some software packages take care of image premultiplication for you—they hide the
state of premultiplication. This works fine nine times out of ten, but for that last
problematic 10 percent, there is no practical solution. Still other compositing packages
pretend premultiplication doesn’t exist and encourage bad habits, such as chewing on
your matte, or modifying foreground/background alpha multiplication curves.