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JRVR > Calibration > 3DLUT Gamma (tone curve) - what is it?
Hendrik:
At least on the screenshot, 17 is pure black. :)
Can you share the .cube 3DLUT file you are using? Maybe I can spot something in the way its being applied, or by analyzing the raw values.
Hendrik:
I had a peek at the 3DLUT cube files you shared, and this is what the raw data says. Just for the first data points.
Normalized to a 0-1 floating point scale, we get this. Since we are dealing with greyscale in these examples, going to simplify a bit and leave out all but the first color component.
The LUT is 65x65x65 (0-64), so normalizing its input is dividing by 64, and video in this example would come from 0-255, divided by 255.
--- Code: ---Input Output
0.0000 0.0000 < actual LUT value (LUT index 0/0/0), pixel value 0
0.0039 0.0119 < pixel value 1 (1/255), LUT interpolated => Output pixel value 3
0.0078 0.0239 < pixel value 2 (2/255), LUT interpolated => Output pixel value 6
0.0118 0.0361 < pixel value 3 (3/255), LUT interpolated => Output pixel value 9
0.0156 0.0477 < actual LUT value (LUT index 1/1/1)
--- End code ---
Looking at the corresponding screenshot above (1886 in this case), these values match exactly what the screenshot captured.
The 17 bar has an average RGB value of 3 (some dithering applies), the 18 bar 6, and the 19 bar ~10 (since the original was limited range and expansion happens, the bars are not perfectly 1 value apart either).
So ... from this it appears the LUT is applied properly. The question that remains would be if the madVR 3DLUT is just made different by the tool, or if madVR does something very different when applying it.
Can you also send me a madVR 3DLUT of the same measurement, ideally? Might be able to look at the first few values at least in a hex editor, not sure there are any tools to convert 3DLUT formats.
I was also checking ArgyllCMS for clues on how it writes LUTs, and apparently madVR LUTs are always 256x256x256. I don't think the precision makes a huge difference, but still curious.
mattkhan:
I sent the madvr versions to you
I still don't understand why the 2 cube LUTs differ in their output *if* the correct course of action is to match the displaycal tonecurve gamma with the corresponding setting in JRVR. The fact that changing this option does produce different output seems to suggest matching these two values is not correct (or there is something else going on).
Hendrik:
Looking at the madVR 3DLUT file, the values seem to also match your madVR screenshot. For some reason the LUTs are just very different. I can try to add support for madVR 3DLUTs, but there was a reason I didn't do it in the past .. i forgot exactly what it was. Maybe I'll remember when I work on it. Them being limited range is a bit annoying, but maybe something I can work around.
mattkhan:
I remeasured using the LUTs I sent you just to be sure what I was seeing
I retract my statement about JRVR LUT handling differing with the gamma setting. At least just from looking at gamma, I think they are the same (pics attached). I'm working on some method to automate measurement using JRVR but, until then, this will have to suffice.
The response is totally different to the madvr lut though and I think that one is correct (as the no LUT measurement has a gamma similar to this response so the cube LUTs are "bad" in some way even if they are being applied accurately)
Unfortunately I'm not aware of another tool that generates a cube lut or can convert from one format to another so it's a bit hard for me to dig deeper on this.
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