Nuke channels

Digital images generally consist of the four standard channels: red, green, blue, and alpha. Nuke allows you to create or import additional channels as masks, lighting passes, and other types of image data. A Nuke script can include up to 1023 uniquely named channels per compositing script. For example, you can combine multiple render passes from a 3D scene — an image from the red, green, and blue channels,
a depth mask (z-depth channel), a shadow pass, a specular pass, lighting passes, and multiple mattes all stored within one image sequence in your composite.Overview Before getting into the actual mechanics of channel management, it’s important to review some basic concept about how Nuke processes channels.

Understanding Channels

Think of a channel as a container that contains image data. Once created or read into your composite, the image data stored in a channel is available downstream in the network until the value is replaced with something else or the channel is removed. The channel may even be “empty” — depending on
where you reference it in the compositing network.Understanding Channel Sets (Layers)
All channels in a script must exist as part of channel set (also called a layer). You’re probably familiar with the default channel set — rgba — which includes the channels with pixel Creating Channels and Channel Sets values of red, green, and blue, and also the alpha channel for transparency. All channels in a composite must belong to at least one channel set. Some channels, like alpha, may be available in
other sets, too. Channel names always include the channel set name as a prefix, like this:set_name.channel_name.By default, every script has a channel set called rgba. When you first import an image element, Nuke automatically assigns its channels to the rgba set — that is, the image
channels are named rgba.red, rgba.blue, rgba.green, and rgba.alpha.The rgba set allows for the standard four-channel workflow of most node-based compositing systems. However, you’re not limited to these four channels. You can create new channels and assign them to new channel sets up to the limit of 1023 channels per script.