Hello,
I'm investigating since quite a long time for a good hdr tone-mapping
formula
For the time being, we're using the following:
a) a classic global operator defined by:
LinearLDR.rgb = pow(1 - exp(-UserScale*HDR.rgb*AutoScale), UserGamma)
SrgbLDR.rgb = LinearToSrgb(LinearLDR.rgb) (display gamma convertion)
With AutoScale = clamp(MiddleGray/AverageLuminance, MinScale, MaxScale)
This formula works quite well with dynamic adaptation of AverageLuminance.
b) Yet, we've some situations where dynamic range of hdr input is too
high and tone mapping
output burns too much (for instance when getting out of a dark tunnel,
outside locally burns)
I've a big belief that a local operator could behaves better in case (b).
I've made an old try with a self-made multi-scale gaussian blur of
intensity
(with downsizes/upscales to reach big scales) which was triggering
another "AutoScale" factor,
yet results were not so good.
c) I've recently implement gpu histograms first to visualize hdr input,
then i've tried a naive histogram equalization in log space which
results is very poor.
I'm appealed by some histogram mapping like
c.1) "black & white 2" , (minIntens(<1%)=>0, maxIntens(>99%)=>1,
midIntens(50%) controling a gamma)
yet minIntens=>black is quite odd
c.2) smarter histogram equalization (ward histogram adjustement)
Yet, I've the feelling histogram won't really solve case (b)
(pdf: 2007 "Efficient Histogram Generation Using scattering on GPUs")
d) I've more recently implemented a approximation of Reinhard local
operator (with box filters computed by summed area tables)
result beging to be interesting yet, I've some artifacts, the biggest
being banding & aliasing, I fear theses are inhere in the operator...
and even if Alpha & Teta parameters default value works (0.05 & 8),
others values (like 0.025 proposed by the paper) don't work so well.
in addition very high luminance colors don't go to white & made "pure"
saturated colors (but this defect may be tweaked)
(pdf: 2008 "Real-Time Photographic Local Tone Reproduction Using
Summed-Area Tables" Slomp/Oliveira CGI2008)
Have you any advices or know local operators which really works for big
hdr range images ??
Many thanks
JSeb
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