Hi Axel, I tend not to use the -x option for critical work, as it's mostly a time-saver as opposed to a way to improve accuracy.
That said, the value range I consider "safe" extends from 27 to 228 in the 8-bit domain. I have found this empirically to be above the noise floor (provided the ISO setting is not too high) and below where some cameras introduce highlight roll-off. I cannot use a strict cut-off for image values. Up to 0.2% of the pixels below the minimum are ignored and 0.05% of pixels above the maximum, likewise. This avoids issues with stuck pixels, which would otherwise make the -x option useless. This is also why it is better not to use -x for critical work, because you may lose the peak highlight in your image if it happens to be very small. Note also that hdrgen would never discard an input exposure off the end because it was "in range." Only the exposures shorter than first one below the safe maximum and the exposures longer than last one above the safe minimum that are considered superfluous. I have a Lubuntu installation running under VMWare with gcc version 4.6.1-9ubuntu3. I could use it to recompile hdrgen if you like, but I'm not sure what machines it would run on.... Cheers, -Greg > From: Axel Jacobs <[email protected]> > Date: February 27, 2012 3:49:40 PM PST > > Dear list and Hi Greg, > > In the recent raw2hdr bundle you packaged for Yulia > http://www.radiance-online.org/pipermail/hdri/2012-February/000363.html > there's a man page for hdrgen that list some options which do not appear in > the man page that comes with the LINUX download you have on > http://anyhere.com/ > > I have just discovered that the -x option does actually exist in the LINUX > version of hdrgen, which is from around 2006, I think. > > In the more recent (MacOS) version, the -x option is described as > "-x Toggle over- and under-exposed image removal. Normally “off,” > this option causes unnecessary exposures that are too light or too dark to > contribute useful information to be automatically ignored." > > I have always lived under the impression that 'useful information' is limited > by pixel values of 200 in the darkest JPEG, and 20 (out of 255) in the > brightest. I really don't remember where I took this from, but it must have > been a post on the hdri list which I am unable to find now. Sorry. > > In your message to hdri just recently, > http://www.radiance-online.org/pipermail/hdri/2012-February/000365.html > you stated > "Specifically, all values in the short exposure's histogram should be be > below 245." > which is different to the 200 threshold I mentioned above. > > I have been experimenting with HDR photography for glare studies (think UGR), > and have noticed some discrepancies in the results that one gets if hdrgen is > run with and without the -x option. I was therefore wondering what hdrgen > considers as > "too light or too dark to contribute useful information". I would think that > this is decision is made based on the value of the darkest/brightest pixel in > the image. Is this assumption correct, and if so, what are the threshold > values that are used with the -x option? > > Kind regards > > Axel _______________________________________________ HDRI mailing list [email protected] http://www.radiance-online.org/mailman/listinfo/hdri
