I have spent some time doing tests myself!
But I have been focusing on the 1D tone curve or Tone Mapping Operator, ignoring colors altogether for now. There will always be a curve for mapping scene intensities to a displayable range regardless of the employed color preservation method.
I kind of went all-in on the task and made myself an interactive tool, please try it yourself here:
https://share.streamlit.io/jandren/tone-curve-explorer
I found it super helpful to play around with the tone curves like this as it gave me a better understanding of the properties of the curves rather than the look they produce. Here are some of my observations:
The Log-Logistic Curve
- Models most of the average base curve very well with contrast=1.65, except the brightest part where it gives more room for highlights. I think the average base curve is very relevant here as a kind of meta-study of multiple camera companies, their engineers, testers, and managers.
- Always returns a slope with a single peak + smooth tails independent of contrast setting.
- Works well for any target black < target grey and target white > target grey (actual look on an HDR monitor TPD).
- Contrast < 1 will have peak contrast at luminance zero. Might seem weird at first but make sense when you study how the value curve changes with lower contrast.
- Possible improvement: Introduce a skewness parameter for shifting the peak contrast towards shadows or highlights!
The Filmic Curve
- Is not a superset, for any contrast setting, of the Log-Logistic Curve.
- Does not guarantee: slope >= 0, slope peaks = 0, or target white >= output >= target black
- White and black relative exposure seems to be defined mostly to make the spline model work.
- Every other setting is covariant with the white and black relative exposure. Contrast is for example the slope in the normalized computational [0,1] range and not related to the absolute image contrast.
- Latitude and Balance are defined as relative to the dynamic range and usually needs to be retuned if the contrast setting is changed.
- Auto hardness does not work well when, target black != 0 or when target white != 1
- The purpose of hardness, sometimes called gamma in the documentation, seems to mostly be about making the spline setup work for that particular combination of display and scene luminances.
- My general feeling is that I have 8 parameters to tune, but only a narrow tunnel of actual good/correct settings in this 8-dimensional space. I would personally say that the actual degree of freedom of the filmic curve is about two, contrast + skewness, but with manual overhead and traps. So I will not blame anyone for saying that it is hard to use!
Please share your experiences as well!