Channels and layers

A pipe carries an image made of named layers (Nuke's channel sets). Each layer has up to four channels: rgba is the main image; others hold depth, motion vectors, normals, render passes, IDs and so on.

Where layers come fromCopy Anchor Link

A Read groups a file's channels by name:

Parts of a multi-part EXR take the part name. A Read only decodes the layers the comp actually uses, so a heavy multi-layer EXR stays fast when you only look at rgba.

Choosing what a node works onCopy Anchor Link

• Colour and filter nodes have the standard Nuke knobs: channels (rgba, rgb, alpha, all, none, or a layer name), mask, invert, (un)premult by and mix.
• Shuffle moves channels between layers; Copy copies channels from another input; ChannelMerge and Remove combine or drop layers.
• Layers a node does not touch pass through from its main input (the B input on a Merge).
• The Write node's channels knob writes rgb, rgba or all (every layer, as a multi-layer EXR).

In the ViewerCopy Anchor Link

The layer menu in the Viewer header shows any layer without adding a Shuffle. Page Up / Page Down step through the layers; R, G, B, A, Y, M show one channel (press again for RGB).

ID layersCopy Anchor Link

Cryptomatte and object / material ID layers store hashed values in the float bits. Nodes that resample the image pass these layers through untouched, so the IDs stay exact.

Merge and transformsCopy Anchor Link

• Merge has A channels, B channels, output and also merge knobs, so it can merge any layer, not only rgba. The output layer is replaced; the rest come from B.
• Transform, Crop and Blur default to channels: all, so every layer moves, crops or blurs together. ID layers are moved or cut without blurring, so the IDs stay exact.