P: Add Rec 709 to LUT color spaces in new enhanced profiles

[johnrellis]

The new enhanced profiles can include LUTs, but the most common color space for existing LUTs, Rec 709, is omitted: 



There are thousands of Rec 709 LUTs available for video color grading, and allowing Rec 709 LUTs in enhanced profiles would let people achieve the same look for their video and still images.

Note that it would be necessary to include two versions of Rec 709 for gamma 2.2 and 2.4, since those are both widely used.

[Rikk Flohr: Photography]

Updating Status

[default5atvrwz4xhrv]

It is incorrect to say that Rec 709 has a gamma of either 2.2 or 2.4. Rec709 does not have a simple exponent for its transfer function.

Rec 709 has a specific transfer function defined as follows: 

V  = If Y < 0.018, 4.5Y Else 1.099Y^(1/2.2) - 0.99

Where V is the non-linear encoded value for R/G/B, and Y is the luminance of the channel (which is just the scene-linear R/G/B value, not to be confused with non-linear luma). 

The chart below shows the relationship between the scene-linear value and the non-linear encoding for the actual Rec709 opto-electrical transfer function, and the encodings using simple gamma functions. As you can see the encodings are not interchangable. I don't mean to be a stickler, but if you have LUTs encoded with a simple gamma OETF and Adobe implemented Rec709 properly, your LUTs would still not appear correctly if you convert them to camera raw presets.