I got some interesting results after adding optional Michaelis-Menten/Naka-Rushton to @nick 's DCTL implementation of the ZCAM model and comparing our whole set of frame captures with SSTS under different settings :
- Default settings for everything
- SSTS + Viewing Conditions = dim
- MM + Viewing Conditions = dim
- SSTS + Highlight Desat = 1.75 + GC Threshold = 0.7 + Ref lum = 200 + Y Mid = 8 + Y Max = 120 + Viewing Conditions = dim
- MM under same modified settings
- MM under modified settings but with Y Mid back to 10
- OpenDRT 0.90b4 with sRGB gamma 2.2 preset
- OpenDRT 0.90b4 with sRGB gamma 2.2 preset but corrected with an additional node to use piecewise sRGB EOTF
Please note that I’m using the version from this commit : Add full Scharfenberg ZCAM DRT with partially implemented inverse and that I’ve fixed the DCTL+Cuda errors myself. I stuck to it because it gave better results with our subtle red volumetric fog in a very dark area than the later one with high boundary gamut compression.
Please also note that I add to tweak blues to remove the cyan hue shift as it ruined sky colour (a memory colour) and also tweak the saturation in the high ranges because the DRT was desaturating way too fast at high EVs and that led to a small exposure compensation so the saturation increase wouldn’t kill the brightness sensation too much.
Final note is that I haven’t had time to fully test the model in HDR yet as I need to test it on multiple monitors with different Y_MAX and different Rec.2020 coverage.
With those caveats out of the way, here are the results I got :
- Dark viewing conditions is too dark for game content in SDR because games are usually played in bright environments but this is not new and we already knew that so that’s why I moved to dim for further tests.
- Dim viewing conditions with default settings and SSTS gives a good result with almost all of my footage except the scenes which have too much bright VFX in them (especially fire). By viewing our test footage under this DRT, I actually learned that there was a subtle red volumetric fog in our tutorial area that I didn’t know about because it was completely lost under OpenDRT (crushed to gray under gamma 2.2 and crushed to black under piecewise sRGB).
- In general, Michaelis-Menten/Naka-Rushton tone scale makes everything darker but can sometimes make brights brighter.
- The tweaked settings with SSTS hit a very sweet spot. Lowering Y_MID to 8 allows us to increase chroma (through increasing ref white and Y_MAX) and reduce highlight desat which, in turn, makes scenes with very bright VFX look way better. It also matches blacks to the darker piecewise sRGB curve which is something that is non-negotiable with our art department.
- As it makes things darker, MM tone scale completely crushes blacks under the same conditions.
- However, darks can be matched with MM by raising back Y_MID to 10. With this setting, we end up with more contrast when using Michaelis-Menten/Naka-Rushton tone scale from OpenDRT due to the different Y_MIDs.
- OpenDRT 0.94b2 with sRGB preset and pure gamma 2.2 kinda matches default settings with SSTS in brightness but loses our subtle red volumetric fog (it kinda makes it gray).
- OpenDRT 0.94b2 with sRGB preset corrected to use piecewise sRGB EOTF crushes darks to black a lot. SDR fire VFX also look way too pink. I tried correcting that with a hue shift in an experimental branch but it unfortunately twisted all red assets to orange.
Final conclusion from someone who actually used OpenDRT which was seen as a pre-alpha of ACES 2 in a shipped product : ZCAM model with SSTS looks very very promising. Next step for me is running that by our leads.