   Automated Tone Mapping for Skylight
  Has any one implemented the tone reproduction operator given in the paper"Tone Reproduction and Physically Based Spectral Rendering" by Kate Devlin, Alan Chalmers, Alexander Wilkie and Werner Purgathofer?I am trying to render realistic skylight. I have used luminance and sky colour models from "Astronomy and the Limits of Vision" by Bradly Schaefer and "A Practical Analytic Model for Daylight" by Preetham et al. My difficulty is converting the resulting high range of luminance values (0.0001 cd/m^2 for starlight to 100000 cd/m^2 for sunlight) into something realistic on a LCD monitor.Manually adjusting the parameters for a given scene (mainly World Adaptation Luminance although there are others), I can get good looking results from the Devlin et al. operator, but I can't see how to automate this adjustment and get realistic output whatever the sun position. Is the adapted luminance similar to the average or the maximum luminance of a scene, or some weighted value?Maybe the operator has too many parameters, but even switching to a simple exposure model, like Y = 1 - exp(exposure*Y), I have the same issue - what exposure to use as the value around sunrise needs to be different to that at noon or midnight (not always pitch black especially at polar latitudes).Can anyone advise on this?   
  Unfortunately I'm not really familar with that paper...I'll have to read it through later. It sounds like the "world adaptation luminance" might just be what other papers define as "background intensity", which is typically approximated using an arithmetic or geometric mean of luminance values. Geometric mean tends to work well, since outliers don't have as much effect and it also maps well to human logarithmic response to intensity.  It's defined as exp(avg(ln(L(x, y) + epsilon)), where epsilon is a very small value to avoid the singularity at 0. You'll see this done all of the time in HDR implementations, and it's usually done by first taking the natural log of luminance of your scene and then repeatedly downsampling until you get to 1x1. Then in the last step you just apply the exp to get a luminance value again.Either way I find the geometric mean of luminance is a good starting point for any kind of auto-exposure adjustments.   ;  Geometric mean, thanks for that.But I'm not sure if world adaptation luminance is like background intensity or not, and generally for daylight manually setting LWa to a value nearer maximum luminance looks better. But this breaks down at sunset/sunrise leaving the sky looking too dark. Night sky then needs a low but not average value to reveal the twilight.I guess that I just don't understand the paper or adaptation. If you happen to find time and interest to have a look at it then I would be intersted in what you think.I'll post some pictures when I figure out how, they might reveal something I am doing wrong.   