User Manual

The following nodes are used to recombine channels in different ways:
 Channel Boolean: This is a 3D node used to remap and modify channels of 3D
materials using a variety of simple predefined math operations.
 Channel Booleans: Used to shuffle or rearrange YRGB and auxiliary channels within a
single input image, or among two input images, to create a single output image. If you
connect only a single image to this node, it must be connected to the background input
to make sure everything works.
 Matte Control: Designed to do any combination of the following: (a) recombining
mattes, masks, and alpha channels in various ways, (b) modifying alpha channels using
dedicated matte controls, and (c) copying alpha channels into the RGB stream of the
image connected to the background input in preparation for compositing. You can copy
specific channels from the foreground input to the background input to use as an alpha
channel, or you can attach masks to the Garbage Matte input to use as alpha channels
as well.
Understanding Premultiplication
Now that you understand how to direct and recombine image, alpha, and auxiliary channels in
Fusion, it’s time to learn a little something about premultiplication, to make sure you always
combine RGB and alpha channels correctly.
Premultiplication is an issue whenever you find yourself compositing multiple images together,
and at least one of them contains RGB with an alpha channel. For example, if you have an
animated title file with transparency rendered into it, or if an animator gives you an isolated VFX
plate of a spaceship coming in for a landing with the transparency baked in, you need to
consider the state of the alpha channel as you use these images.
Most computer-generated images are premultiplied. A premultiplied alpha channel means that,
for every pixel of an image, the RGB channels are multiplied by the alpha channel. It guarantees
that translucent parts of the rendered image, such as flares, smoke, or atmospheric effects, are
correctly integrated into the black background areas so that the image appears correctly when
you view that layer by itself.
TIP: Computer-generated 3D images that were rendered anti-aliased are almost
always premultiplied.
Non-premultiplied images, or so-called “straight” alpha channels, have RGB
channelsnotmultiplied by the alpha channel. These images appear weirdly bright in translucent
areas, which tells you that you probably need to multiply the RGB and A channels before doing
specific tasks.
Chapter – 69 Understanding Image Channels 1395