A color lookup is what Photoshop's Curves tool does. It is a very powerful tool, capable of replicating the function of many other color nodes. Its disadvantage is that it requires more processing than many of those nodes. The curve of a color lookup can also be a bit more difficult to read than nodes featuring sliders.
Reading the ColorLookup
It looks like a graph because it is. It graphs the value range change that you impose upon the image. Here's how you read it. The Curve on the left is its default state and represents no change to the image. The curve on the right represents a multiplication value of 0.5 (1 x 0.5 = 0.5).
Curve-type color lookups are common to most digital image editors. An experienced digital artist you will be able read a curve: to know what its effect upon an image will be. Examples of common curves can be found in Color Operations Visualised. They were made using the technique outlines here.
Points on the curve
To set a point on the line you 'Command Option' click on it. By default a curve point can only be moved vertically or horizontally, but if you hold the 'Command' key movement on all axis is possible. The 'shape' of each curve point is extremely editable through the contextual menu (right click on a point then the Interpolation sub-menu). Of all the options that you see there Smooth interpolation is the most useful (and also the default). However, Linear interpolation can also be useful. A Linear Interpolation presents as a sharp point and can be used to tame unruly tight curves. You can also pull the handles of the point around to change the shape of the curve. This is useful as the ColorLookup curves can sometimes be a bit feisty.
Source and target
Beneath the curve is a source and target pair. Target these two points in your image and press Set RGB. This is most useful for matching the middle points of one image with another but if used on upper or lower values the resultant curve can be a bit extreme and will need a tidy.
A Photoshop curve never extends beyond the black and white (zero to one) limits of Photoshop’s clipped color space. A Nuke ColorLookup curve can extend into any value you can dream of. Your job will often be to ensure that the shape of the curve is ‘nice’ within this zero to one constraint. A curve should not double back on itself (below right). Such a curve will give your images an unpleasant solarized look. A flatline curve (shown on the left) should also be avoided as this will crunch all the dark values into black (though sometimes you might want this).
- Important: A very common curve type is the Gamma. Unless handled carefully, gamma adjustments can produce un-nice curves. If you pull a value in the lower registers up or down then the part of the curve that extends past the 1:1 value (i.e. top right) will shoot off in the opposite direction which will badly affect all your super whites (values above one). This can be dealt with by adding an anchor point just above the 1:1: point.
- Remember: you can zoom in infinitely to all Nuke curves which means that you can make some very targeted adjustments.
Master / RGB
The master curve manipulates the entire range of values within the image, the other curves (RGBA) perform channel specific operations to the image. You can select more than one point on more than one curve at a time and move them together. It is, however, not recommended to combine a master curve adjustment and a channel-specific adjustment in one operation as this can make the color adjustment difficult to manage, edit and troubleshoot.
This node is not without its troublesome quirks:
- In Windows, the curve can not be edited unless you click once on it.
- If you do not de-select a point after you have finished with it (by clicking away from the curve in the curve view) then you run the risk of accidentally adding a point to it when you move to another curve.
- The space that this curve lives in is infinite (unlike Photoshop's that is limited to lying between zero and one). As a result it can somtimes present a confusing world view to the user. They may find themselves thinking that they are looking at the zero to one data curve when in fact they are looking at a zero to one hundered curve. Cure: press H' to centre the curve in the window and keep your eye on the number values of the curve.