Greg, I apologize for so many questions! Are those executables for Linux OS? I use Windows 7 32-bit OS.
Message: 1 > Date: Mon, 20 Feb 2012 14:48:37 -0600 > From: "Tyukhova, Yulia" <[email protected]> > To: [email protected] > Subject: [HDRI] HDRI capture of LED > Message-ID: > <CAJYAbgpa_tQtuxLAvgFeOQXzC0X8F25D6haBdt1uOqT=4nh...@mail.gmail.com > > > Content-Type: text/plain; charset="iso-8859-1" > > Everybody, > > Thank you for fast responses/tools and suggestions! > > Greg, > > Thank you for your suggestions and files! > I am new to Radiance, and I assume that this is what I need to have > installed on my computer in order to use suggested Perl scripts. > If you can provide me with the link/info how to run it, that would be > really helpful! > > Let me restate the question about the compression of the curve in > Photosphere. > 1. Do manufactures compress the response curve or maybe it is limited by > camera/optics/sensor saturation itself on the upper end? > > 2. And I'm still curious, how CF is applied in Photosphere? > > I've been using ND filter t=0.0094 on the luminance meter, because > otherwise it is impossible to measure such high luminances. I assume, you > suggest to use it on the camera as well. > > I'm looking forward to analyze my images with the suggested > hdrgen. Luckily, I've been taken them in both formats jpeg and raw. > Greg, will you recommend to have regular calibration scene calibrated at > the grey card instead of using brighter scene? > > Thank you, > Yulia > > Hello Yulia, > > Seems your question has spawned quite a bit of interesting discussion... > > My main recommendation is to use camera RAW images for critical photometry, > especially when there are saturated colors involved. It is impossible to > correct the color of JPEG images and undo what the camera maker has done, > so you need to start from the sensor data. > > Photosphere does not accept camera RAW as input, but I have written a Perl > script that uses dcraw with the command-line HDR image builder hdrgen to > overcome this limitation. It also requires the use of another third-party > program, exiftool, which I have packaged together for you at: > > http://www.anyhere.com/gward/pickup/raw2hdr.tgz > > Unfortunately, I don't have a good set of documentation to go with it. > Typing "raw2hdr" by itself shows the basic syntax: > > Usage: raw2hdr [hdrgen opts][-h][-w][-C calib][-c cspace] -o > output.hdr input1.raw .. > > If your images are taken on a tripod (aligned exposures), you can use the > default settings: > > raw2hdr -o output.hdr expos1.cr2 expos2.cr2 expos3.cr2 ... > > The hdrgen settings can be found in the included HTML man page, and so can > the -h and -w option meanings in the included dcraw man page. The -C > option is to provide a linear factor to correct the overall exposure based > on previous calibrations. The -c option is to specify an output color > space. The default is "sRGB" which is actually linear CCIR-709 primaries. > The only other output color space I would recommend is AdobeRGB. There is > a CIE XYZ space supported by dcraw, but I have found it to be somewhat > unreliable, and I don't know where the fault lies in this. > > Regarding Axel's mention of camera flare, this is less of an issue for > sources that are brighter than the rest of the scene. It mostly affects > darker, surrounding regions. The -f option will attempt to estimate the > camera/lens PSF and remove it, but it cannot be relied upon to remove this > source of error completely. Your problem with the accuracy of the LED > sources is due no doubt (as others have said) to limitations in your short > exposures combined with the color issues inherent to JPEG processing. > > Other responses inline.... > > > From: "Tyukhova, Yulia" <[email protected]> > > Date: February 19, 2012 9:34:25 PM PST > > > > Hello everybody! > > I?ll provide the summary of my research and have questions within the > summary. I would appreciate any of your help! > > My research investigates if HDRI technique can precisely capture > luminances of small bright light sources (e.g. LED garage fixtures) with > narrow light distributions. > > I was able to figure out luminance values for a single LED, which can be > compared to the ones from HDR images. But I have a couple of > questions/concerns on HDRI technique and Photosphere. > > At first, I?ve used ?regular? scene to retrieve response curve of the > camera (large smooth gradients with very dark and bright areas, and had > reflectance standards for the absolute calibration). > > Camera: EOS T1i Rebel with 28-105mm lens, at 28mm > > Calibrated at the grey reflectance sample 186.45 cd/m2 > > CF=0.957 > > > > I?ve got the following RC for RGB: > > red(r) = -6.434199e-03+ 4.518039e-01*r + 1.291426e+00*r^2 + > 1.802896e+00*r^3; > > green(g) = -5.804720e-03+ 4.175837e-01*g + 1.176582e+00*g^2 + > 1.721643e+00*g^3; > > blue(b) = -4.376831e-03+ 3.784418e-01*b + 1.075695e+00*b^2 + > 1.658471e+00*b^3 > > If I look at the histogram of the scene, maximum luminance within the > scene is 60,291 cd/m2. > > Then I use this RC to analyze HDRI of a captured LED. The value is > 230,000 cd/m2 for a single LED, which is low (it?s has to be around 7*106 > cd/m2). So, it underestimates the luminance. > > It seems like calibration point is critical here. I?ve decided to try to > capture a different scene for deriving RC with a wider range. It would make > sense that camera has to see higher luminance values in order to accurately > measure them later. The dynamic range has to cover measured values. > > 1. How does Photosphere deals/approximates/calculates the upper end of > the curve? I assume it gives more weight to mid tone values? But what > happens with high luminance values? > Photosphere (and hdrgen) use all the brightest pixels from the shortest > exposure and all the darkest pixels from the longest exposure. Middle > exposures have their brightest and darkest pixels downgraded. > > So, the new brighter scene was picked with the direct sun! But in order > to avoid the damage of the camera?s sensor, measurements were taken before > the sunset. > > In the new brighter captured scene without the calibration all values for > reflectance standards were overestimated, while the value for the sun > underestimated. Then I decided to calibrate my scene at the sun! > > But when I apply absolute calibration, it simply multiplies CF to all > values. > > 2. I assumed when CF is applied, it does not equally change all > values, but does it proportionally to RC (since it is not linear). Why > does it do it equally for the whole range? > > Lsun=80*106 cd/m2. And of course CF is very big 391. > > New RC: > > red(r) = 3.219064e+00+ -2.655078e+01*r + 9.351069e+02*r^2 + > -2.115052e+03*r^3+1.594538e+03*r^4; > > green(g) = 2.094164e+00+ -1.468109e+00*g + 7.306838e+02*g^2 + > -1.720743e+03*g^3+1.380693e+03*g^4; > > blue(b) = 1.049078e+00+ 1.591820e+01*b + 5.848958e+02*b^2 + > -1.461635e+03*b^3+1.251033e+03*b^4 > > But then something interesting happened. When I analyze LED, it gives a > value of 79*106 cd/m2. So, it jumps to this upper limit calibrated with the > sun previously. > > (I had similar results for EOS 7D with the lens 16-35mm, at 16mm) > > > > I don't think your shortest exposure properly captured the LED, and maybe > didn't capture the sun, either. > > 3. Does photosphere compress the response curve, so at the upper end > all values above certain threshold will have the same number? > > Photosphere does not compress the curve. > > 4. Any additional suggestions on properly obtaining and calibrating > HDRI for this purpose? > > I would only reiterate others' suggestion to use a neutral density filter, > and using raw2hdr rather than Photosphere. > > > -- > > Thank you, > > Yulia Tyukhova > > Best, > -Greg > > -- > Thank you, > *Yulia Tyukhova* > * > * > Fulbright Scholar, "Intern LC" > Architectural Engineering Graduate Student, UNL-Omaha, NE, USA > B.E. and M.E. in Lighting Engineering (MPEI), Moscow, Russia > [email protected] > [email protected] > +1 (402) 996 0910 > PKI 247 > -------------- next part -------------- > An HTML attachment was scrubbed... > URL: < > http://www.radiance-online.org/pipermail/hdri/attachments/20120220/3cf4d57d/attachment-0001.html > > > > ------------------------------ > > Message: 2 > Date: Mon, 20 Feb 2012 13:14:15 -0800 > From: "Gregory J. Ward" <[email protected]> > To: High Dynamic Range Imaging <[email protected]> > Subject: Re: [HDRI] HDRI capture of LED > Message-ID: <[email protected]> > Content-Type: text/plain; charset="us-ascii" > > Responses inline... > > > From: "Tyukhova, Yulia" <[email protected]> > > Date: February 20, 2012 12:48:37 PM PST > > > > Everybody, > > > > Thank you for fast responses/tools and suggestions! > > > > Greg, > > > > Thank you for your suggestions and files! > > I am new to Radiance, and I assume that this is what I need to have > installed on my computer in order to use suggested Perl scripts. > > If you can provide me with the link/info how to run it, that would be > really helpful! > > Actually, you don't need to have Radiance installed. You just need to > move the executables (non-HTML files) from the unpacked directory to > /usr/bin or /usr/local/bin or some other directory in your shell's PATH > variable. These are command-line tools that must be run from the Terminal > application under /Applications/Utilities. I.e., start Terminal and copy > the files from your Downloads folder with: > > cd Downloads > tar xzf raw2hdr.tgz > cd raw2hdr > cp raw2hdr dcraw exiftool /usr/bin > cd > raw2hdr > > > This should give you the usage message I wrote you earlier if it all goes > well. Some basic commands and pointers for Unix are available many places > online. Googling "basic unix tutorial" gave this page at the head of the > list: > > http://www.ee.surrey.ac.uk/Teaching/Unix/ > > > Let me restate the question about the compression of the curve in > Photosphere. > > 1. Do manufactures compress the response curve or maybe it is limited by > camera/optics/sensor saturation itself on the upper end? > > Some camera makers do compress the top end of the response curve, and do > funny things at the bottom as well. Photosphere attempts to discover the > tone curve and correct for these manipulations, but it isn't perfect and if > the camera is changing the tone curve dynamically, it's pretty hopeless. > There are settings you can use on a DSLR to disable such manipulations, > but using RAW files bypasses the problems entirely because the data is > linear. > > > 2. And I'm still curious, how CF is applied in Photosphere? > > A calibration factor is applied equally to all coefficients in the > polynomial, which is exactly the same as applying a linear scale factor > after the HDR merge operation. > > > I've been using ND filter t=0.0094 on the luminance meter, because > otherwise it is impossible to measure such high luminances. I assume, you > suggest to use it on the camera as well. > > Whatever gives you a short exposure that is past the integration time of > your source (1/60th second is acceptable) and not saturated is OK. > Specifically, all values in the short exposure's histogram should be be > below 245. > > > I'm looking forward to analyze my images with the suggested hdrgen. > Luckily, I've been taken them in both formats jpeg and raw. > > Greg, will you recommend to have regular calibration scene calibrated at > the grey card instead of using brighter scene? > > The best scene for calibration is a white card in a scene with no bright > sources directed at the camera. The calibration should hold in other > scenes where lens flare is not problematic. > > > Thank you, > > Yulia > > Certainly, > -Greg > -------------- next part -------------- > An HTML attachment was scrubbed... > URL: < > http://www.radiance-online.org/pipermail/hdri/attachments/20120220/276e1ec6/attachment.html > > > > ------------------------------ > > _______________________________________________ > HDRI mailing list > [email protected] > http://www.radiance-online.org/mailman/listinfo/hdri > > > End of HDRI Digest, Vol 46, Issue 7 > *********************************** > -- Thank you, *Yulia Tyukhova* * * Fulbright Scholar, "Intern LC" Architectural Engineering Graduate Student, UNL-Omaha, NE, USA B.E. and M.E. in Lighting Engineering (MPEI), Moscow, Russia [email protected] [email protected] +1 (402) 996 0910 PKI 247
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