Hi Kevin, from my understanding, the exposure correction here is done by modifying the exposure, not the image data itself. pvalue -o would give you the raw pixel data, ignoring the exposure (and thus the correction you did). So I would not use the -o switch here, because you want the exposure correction (which you caluclated from your known luminance values) to be applied.
Three ways to handle uncertainty in my mail (in ascending order of wisdom): 1) Believe in my mail.[1] 2) Hope that Greg confirms (or rejects) these assumptions.[2] 3) Try out by e.g. using a 10 times higher value for the exposure correction then you get from your calibration. That should lead to 10 times higher values in the results, if the correction got applied, right? So if you get these 10 times higher values even with pvalue -o, my assumption was wrong. ;) Cheers, Lars. [1] strongly discouraged approach [2] promising to be on the safe side > HDRI list - > > Can someone help me better understand the pvalue –o option? > > http://radsite.lbl.gov/radiance/man_html/pvalue.1.html > > I have completed HDRgen for a large data set, and will apply exposure > correction, vignetting correction, then use pvalue (with or without –o) > to apply the spot calibration using a grey card mask (according to > below), and finally pvalue again (with or without –o) to get useful > results. > > This is in the context of the previous post for automating the spot > calibration for a large number of HDRs using luminance meter and grey > card in a scene. > > Do I let it at the default (+o)? > > Thank you, > > --- > > On Apr 14, 2011, at 12:52 PM, "Greg Ward" < [email protected] > wrote: > > > > > Hi Kevin, > > What you ask is a very tricky scripting operation. If you know > exactly where the point you want to calibrate is in every image, > you can script it with some combination of the Radiance ra_xyze, > pcompos, pvalue, and total programs. Let's say you know that > you have a value of 322 cd/m^2 in the square from > (x,y)=(300,450) to (380,530) -- y pixels measured from the > bottom of the image and x from the left, you could compute the > average using: > > set img_val=`ra_rxyze -o capture.hdr | pcompos -x 80 -y 80 - > -300 -450 | pvalue -h -H -d -pG | total -m` > > You could then use this to compute a correction (calibration) > factor to the image exposure: > > set meas=322 > > set ecorr=`ev "179*$img_val/$meas"` > > The 179 value is the standard lumens/watt conversion used in > Radiance, and the 322 value is your measured luminance value. > You then need to add the above to your image header, which can > be accomplished with getinfo like so: > > (getinfo < capture.hdr ; echo EXPOSURE=$ecorr ; getinfo - < > capture.hdr) > calibrated.hdr > > I hope this helps. > > > > -Greg > > * > > Kevin Van Den Wymelenberg > Integrated Design Lab – Boise, Director > *College of Art & Architecture, Assistant Professor > University of Idaho – Boise Center > /www.uidaho.edu/idl > / > ph. 208.724.9456 fx. 208.343.0001 > 306 S. 6th Street Boise, ID 83702 > / > / > > > > > > > > > _______________________________________________ > HDRI mailing list > [email protected] > http://www.radiance-online.org/mailman/listinfo/hdri
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