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
