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.

Introduction

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.

Note:  When a script is saved, any channels that are not referenced in the script are discarded automatically.

When creating channels and layers, bear in mind these good practice guidelines:

• Ensure that all layers use the same channel names in the same order. This avoids complications with multilayer .exr files imported into Nuke.

• Always use proper names for channels, never just a single letter.

• Always create a custom layer for custom channels, don't add to the existing default layers.

• Never use more than four channels per layer. Nuke only has a four channel interface.

A current Channel Count is displayed in the bottom-right of the interface, which changes color as the number of channels increases. The default threshold is 1023, but you can set the limit in the Preferences under Project Defaults > Channel Management > Channel Warning Threshold.

The Channel Count turns yellow if you exceed the Channel Warning Threshold and red if the Channel Count is equal to or greater than the maximum channel value 1023.

Note:  Nuke does not remove unused channels until you close and reopen a script, so the Channel Count does not decrease when you remove Read nodes from the Node Graph.

Reserved Channels

Nuke reserves certain channel names for internal use to ensure the smooth running of scripts under the hood. If you create a custom channel with a reserved channel name, the custom name is renamed automatically to preserve this functionality. For example, if you create a custom channel called mask.a, the channel is renamed mask_extra.

The reserved channels are:

rgba.red, rgba.green, rgba.blue, rgba.alpha

depth.Z

forward.u, forward.v

backward.u, backward.v

deep.front, deep.back

disparityL.x, disparityL.y, disparityR.x, disparityR.y

mask.a

rotopaint_mask.a

mask_planartrack.a

mask_splinewarp.a

Quick Start

Here's a quick overview of the workflow:

1.

Channels in Nuke are always a part of a layer. You can create new channels and layers using the new option in the channel selection dropdown menus (such as output and mask) in a node’s properties panel. For more information, see Object Material Properties.

2.

Using the channel selection controls you can select which channels the node is processing and outputting, or using as a mask when color correcting for instance. For more information, see Calling Channels and .

3.

The channels can also be linked to other channel controls through the Link menu. For more information, see Linking Channels Using the Link Menu.

4.

Using the Shuffle node, you can rearrange your input channels and apply the result in the output. For more information, see Shuffling Channels.