Hi Axel, This is interesting. I hadn't realized that the Sigma lens used something other than the equidistant projection. I thought this was the more normal type, and our casual measurements seemed to indicate that it followed this projection. Perhaps I wasn't as careful as I ought to have been with my observations. The horizon on the Sigma is enough of a mess that it's difficult to gauge things accurately at the outer rim of the circular image.
Looking at the ever-helpful Wikipedia page on the topic <http://en.wikipedia.org/wiki/Fisheye_lens#Mapping_function>, I see that the equidistant and equisolid-angle projections are fairly similar except at the outer reaches (near +/-90°). Of course, errors could still be large assuming the wrong projection if your only light sources is out in that part of the view. In lieu of reprojecting the image with pinterp, which is as you say unsupported, it is possibly to apply a correction to the image values to account for the difference in solid angle at each pixel. Given that the solid angle of the equisolid-angle projection is the same for each pixel, we really only need the solid angle for the equidistant projection. This can be computed with a simple expression, which is sin(theta)/theta. Does this help? -Greg > From: Axel Jacobs <[email protected]> > Date: February 3, 2012 6:27:59 AM PST > > Dear list, > > I've been exerimenting with Radiance's findglare and glarendx, trying > to get UGRs from photographic HDRs. I'm using the Sigma 4.5mm on a > D200, which seems to be quite a popular choice amongst you. > > Unlike the FC-E8/Coolpix combo, which produces an equidistant > projection (-vta), the Sigma 4.5mm results in a 180deg equisolidangle > view. I gather from this post to the rad-gen list: > http://www.radiance-online.org/pipermail/radiance-general/2010-April/006709.html > that the NYT cart was based on a Sigma lens (4.5mm ?), operated at > F5.6. The code snippet in that post suggests that the HDRs were > vignetting corrected. > > An overall calibration of the image luminance can be carried out (I > think) by measuring the vertical illuminance at the lens when the > exposure-bracketed sequence is taken, and then running findglare and > glarendx -t ver_illu on the HDR, which should give a calibration > factor that can then be used to fiddle with the EXPOSURE= line. This > is probably more accurate than calibrating against spot meter > readings. So far, so good. > > What I don't seem to be able to find in the googleable literature, nor > in the HDR book, is any words of wisdom regarding the impact of the > lens projection on glare metrics. Radiance doesn't have an > equisolidangle view type, so using pinterp as detailed in this post: > http://www.radiance-online.org/pipermail/radiance-general/2011-August/008141.html > is not an option. > > It might be possible to utilse ImageMagick to re-project the JPGs > prior to running hdrgen, but I'd rather not go there. > > The deviation between equisolidangle and -vta is most noticeable for > high off-axis angles, which is also where glare sources have less of > an impact (Guth position index). I'm therefore wondering whether > people just tend to go with the vignetting-corrected and > luminance-calibrated HDR without worrying too much about re-projecting > the fisheye. The same question would apply to evalglare's DGP rating, > which relies on the HDR coming in -vta. Has anybody looked into this? > > Cheers > > Axel _______________________________________________ HDRI mailing list [email protected] http://www.radiance-online.org/mailman/listinfo/hdri
