DPR's DR test plot - JPEG - why 'S' shaped?

Started 9 months ago | Questions thread
Dave_J_E Regular Member • Posts: 139
Re: DPR's DR test plot - JPEG - why 'S' shaped?

Entropy512 wrote:

Dave_J_E wrote:

I believe there are two tone curves at play here, The first is an S shaped TC applied during raw conversion to improve global contrast (linear looks a bit flat), The second is the sRGB tone curve (gamma) which would have been applied when the jpeg was created. You won't see the effect of this when viewing the image but it will show up in the measurements of the jpeg values.

I'm not sure if any camera on the market outputs a bare sRGB or Rec709 TRC - they all preprocess with some sort of (usually) S-shaped tone curve. (I have not yet tried a Sony camera with gamma explicitly set to Rec709 though).

Interesting timing - I've been experimenting with using OpenCV's calibrateRobertson to reverse engineer the transfer functions of an A7III:

These are the effective tone curves that would need to be applied to linear data prior to an sRGB transfer function to get the measured camera response.

cine2 was omitted because it has a cap lower than 255 which leads to glitches from calibrateRobertson, similarly code value 255 was omitted from all reverse engineered curves for the same reason.

sRGB, HLG, and rec709 are calculated from published formulas instead of measured data. Same for RawTherapee's Standard Film Curve - the aggressiveness of the bottom of the S for RT, along with RT's AMTC not quite matching Robertson-derived data, has me questioning whether there might still be some accuracy issues here. I plan on publishing my analysis scripts and capture script to github in the next week or two.

Interesting. Some of these curves are typically used for video which is an area of some vagueness to me. My understanding is that in a video situation, these log based curves would be used instead of say Rec 709. After grading, the video may be exported with a Rec 709 transfer characteristic?


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