About LUT in Nuke

Viewer Input Toggle / LUT Toggle
A Nuke script may include a special node—sometimes called a viewer LUT or lookup table— that adjusts the viewer display to show how rendered output will look after transferred to film, video, or other devices or media.

Usually, the viewer LUT is a custom script that you create and import as a node. This must be named VIEWER_INPUT to have an effect on the viewers. Any image you view is passed through it when the IP button is activated.

Normally, the viewer will send the image it is going to display through the VIEWER_INPUT process node, then apply gain/gamma and LUT effects to the result prior to display. However, depending on what the input process node is doing, this may not be the correct order. Therefore, if your VIEWER_INPUT process node has float controls named "gain" and/or "gamma", then the Viewer will drive them from the corresponding viewer controls and not do that image processing itself. This allows you to implement the gain and gamma in your VIEWER_INPUT process gizmo/group node using whatever nodes and order you want. If your input process node does not have gain/gamma controls, then the Viewer will apply the effects in its normal way after running the image through the VIEWER_INPUT process node.