All LUTs are literally a table of numbers, representing input and output values. They are very dumb mechanisms, have no idea what your image is.
But, ALL LUTs are design for a particular input.
Some are designed for a particular shot, some for a look for a film, and some for a general color space.
The LUT has no idea what it’s designed for, so you’re supposed to.
Most (but not all) LUTs will output to a display referred color space. That is, it’s meant to be viewed.
But many LUTs don’t expect the input to be in that display referred state.
Best practice, is to use LUTs you know the targeted input for, and what it’s targeted output is. Ex: ARRI’s Alexa to Rec709 LUT. We know it’s expecting ARRI LogC input and a Rec709 g2.4 out.
The biggest mistakes I see people making are 1) using LUTs that they have no idea what the intended input/output is and 2) trying to grade their image to suit the LUT or 3) applying the LUT and then reducing it’s intensity (output gain).
PROTIP: If a LUT looks wrong to you, unless you reduce it’s gain… you’re probably using it incorrectly. It’s a bit like not knowing how to put a car in drive, so you drive it in reverse but really slowly.
So, first order of business… know your LUTs… know what colorspace it’s input and output are meant to be in.
Now “Juan,” you say “what if I know the right color space and want to apply the LUT, but my image isn’t in that color space?”
Good question!
Modern color grading applications generally offer color management tools. So you can just use a fancy thing like OCIO, or Resolve Color Space Transform tool, or Baselight’s Truelight Color Space controls, to transform your image into the color space the LUT craves.
Frequently, FPE or Film Print Emulation LUTs – which are quite popular – expect a cineon log/p3 input, and will produce a P3-gamma2.6 or Rec709 gamma 2.4 output. For one example.
Do remember that the output will be whatever the LUT is transforming to (generally a display referred space : Rec709 or P3 for two examples) so if that’s what you want, you’re done. Otherwise you need to adapt after.
Thanks for coming to my TED talk, please LUT responsibly.
-Juan