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# How does a 3D LUT map color gamuts if all it does is give an Output RGB value for a given RGB Input

Started 2 months ago | Questions thread
Re: gamut mapping with LUTs

spider-mario wrote:

Mandem wrote:

The common explanation(and, perhaps, overly simplified) online is that 3D and 1D LUT's simply give a certain RGB output for a given RGB input. This explanation doesn't seem to explain how then does a 3D Conversion LUT actually map a color gamut of a color space to a gamut of another color space.

It does if you remember that the input and output RGB triplets are to be interpreted differently. You seem to understand this at some level in your example. (1023, 0, 0) is maybe not the best numerical example to illustrate this as it may well be mapped to the same triplet in the target colorspace to be as saturated as it can be. But imagine that we are converting from Rec. 2020 to Rec. 709. In that case, the LUT could map (810, 236, 75) to (1023, 0, 0) for example. The first triplet, when interpreted in Rec. 2020, would represent the same color as the second triplet when interpreted in Rec. 709.

Now, as Jim mentions, the question of how to map “the color represented by (810, 236, 75) in Rec. 2020” to “a color that is included in Rec. 709” is another question with several possible answers. I was assuming that your question was about the “mechanical” effect of the LUT itself, the way that it represents that mapping numerically.

I'm trying to understand what the term "interpret" means here. Are you saying if I had a rec2020 and a rec709 screen right next to each other the (810,236,75) rec2020 would produce the same color as the (1023,0,0) rec709 which basically means the color shown by both screens has the same chromaticity coordinates on a CIE XYZ/1931 diagram.

Edit: And yes my question was about the way it represents a numerical remapping for a purely technical conversion LUT.

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