This article aims to explain the difference between Layers and Channels in Nuke.
In the Nuke Viewer you can control the 'Layers', 'Channels' and the 'Channel selector' using the first three Viewer knobs as shown below:
A layer is a group of 'Channels' which are displayed in the Viewer according to the 'Channel Selector'.
The 'Layers' select the RGB 'Channels' used in the 'Channel selector' and the 'Channels' select the alpha channel used in the 'Channel selector'. This allows comparing a fourth channel to channels one, two and three easily, when the first three channels in a layer are used in the channel selector as R, G, and B.
You should aim at using only four channels in a layer, as although Nuke does support more channels per layer, using a higher number is not recommended.
NOTE: Due to a known bug, both the knob which controls the 'Layers' and the knob which controls the 'Channels' are called channels, as shown below:
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