HLG ODT support

(Ssubramanian) #1

Looking for some help in creating HLG ODT based on Rec.2020 ST-2084 and BT.2100…
Is this right way to incorporate HLG in ACES version 1.0.3?


(Nick Shaw) #2

HLG and ACES is tricky. ODTs (as the name implies) produce display referred output, whereas HLG is (notionally, at least) scene referred. The picture rendering in HLG is supposed to happen in the display, whereas in ACES it happens in the combination of RRT and ODT.

All you can really do is create a transform for an HLG display of e.g. 1000nits, based on the ST.2084 1000nit ODT, and use the maths from ITU-R BT.2390-0 to add a PQ to HLG conversion so that (on that display at least) your HLG ODT produces the same display luminance in HLG mode as the PQ one does in PQ mode.

(Ssubramanian) #3

Thanks for quick response. That make sense.
One of the workflow we are trying address is to bring HLG footage generated by a camera ( for example HLG format by Panasonic GH5) into ACES color space for further processing. I think similar logic will work here too.

(Nick Shaw) #4

That one should be simpler. If the camera is genuinely producing scene referred HLG (I have no personal experience of GH5 HLG) then an IDT to convert that to ACES should just be a reverse OETF and a BT.2020 to AP0 matrix. You may well also need to add an exposure multiplier, since if the HLG was exposed to “look good” on an HLG display with no further processing, I suspect a grey card will map to significantly higher than 0.18 in ACES if you just use the BT.2100 maths. But that is just my speculation.