[HDRI] Illuminance and luminance values underestimation from calibrated HDR images
J. Alstan Jakubiec
alstan at jakubiec.net
Mon Feb 13 03:28:41 PST 2017
Dear Clotilde, List,
Well, I think this is not the case. The SIGMA 8mm f/3.5 has an equisolid projection.
I am quite sure as I received this information from Sigma UK. Moreover, I checked the information Sigma gave me by measuring myself the projection of the lens. I did it following David Geisler-Moroder method, that he presented at the 15th International Radiance Workshop in 2016. And I also got an equisolid projection. I could send you more info if you want.
The images have thus to be reprojected from equisolid to equidistant.
I wanted to follow up. Because of all of the questions about the lens
projection of the Sigma 8mm, I measured one of mine today using a
panoramic tripod attachment. I verified that the lens was rotating
correctly about the opening as per the standard parallax tests first. I
am including the measurements of the Canon 8-15mm fisheye as a reference
in addition although I measured it in 2015. You may find the results at,
https://dl.dropboxusercontent.com/u/3175325/lens_projection.pdf
It is notable that the Sigma 8mm is very close to an equisolid
projection, although not as nicely matching as the Canon 8-15mm at 8mm.
Zhe, Tobias and Claus -- I'm CC'ing you directly to make sure you see my
measurements related to the previous discussion.
Clotilde, there is one notable place where my workflow for calibration
factors is a little different than yours. I calibrate each image based
on a specific luminance measurement as per Inanici and Van Den
Wymelenberg's methods. Generally speaking, this brings my results
reasonably close to the measured illuminances in most cases where
overflow is not present. I keep the measurement point near the image
center (where vignetting~0) and perform vignetting correction after
calibrating the luminance at the near-center point.
Secondarily, I'm curious about your vignetting correction process before
cropping if you wouldn't mind sharing some more details. I imagine it is
difficult to correct when there is no valid Radiance view associated
with the image yet.
Best,
Alstan
On 2/10/2017 9:01 PM, Clotilde Pierson wrote:
> Dear Alex,
> Dear Alstan,
>
>> Are you sure your low dynamic range pictures are properly exposed from
>> which you construct the HDRI?
>>
>> Meaning, the fastest shutter speed picture has no saturation (i.e. all
>> white pixels) and your slowest shutter speed picture has some black
>> pixels? You need to make sure this is true else you won't get proper
>> illuminance calculation and also your bright spots (i.e. high
>> luminance) will be under reported.
> Well, I took 15 images with auto-bracketing (1EV between each image). I always check that the fastest shutter speed is almost all black pixels (no white pixels) and the slowest shutter speed is almost all white pixels (no black pixels). If I have too high luminance in my field of view, then I use a neutral density filter so that my darkest image does not have any white pixel.
>
>> I'd like to follow up with some other potential things to look out for in addition to what Alex suggested and questions based on your description. We dealt a lot
>> with this via the papers on HDR we published last year.
>
> Yes, I have read it.
>
>> * Full fisheye HDR images should always be cropped to a bounding
>> square and the view edited or input inline via Evalglare when doing
>> analysis. Are you cropping the images in all cases?
> Yes, all images are cropped at the end (after hdrgen and vignetting calibration) to a 1000x1000 square (with pcompos and pfilt) and the VIEW line in the header is modified to a -vta -vh 180 -vv 180.
>
>> * Where is your 1.39 calibration factor derived from? It is useful to
>> measure luminance for every HDR photo taken from an easy to identify
>> (and nearly-neutral) surface for use in calibrating images rather
>> than a constant. Typically the discrepancies I find when I
>> /don't///measure luminance this aren't as high as what you found
>> however.
> I derived my calibration factor when doing the vignetting calibration process. I did the vignetting calibration according to the method suggested in Cauwerts et al. (2013) paper. Therefore I have a calibration factor for each aperture:
> f/3.5 --> 1.32
> f/5.6 --> 1.38
> f/11 --> 1.48
> f/16 --> 1.59
> f/22 --> 1.84
>
> The reason why the f/3.5 calibration factor is not 1.39 anymore, is because I noticed that using hdrgen command with -e and -a options when creating my HDR images, gave me better results. It changes the exposure of the generated HDR images and I thus only got differences between measured and HDR-derived values of max 30% (it was 50% before). Therefore, I recalculated my calibration factors with the last generated HDR images.
>
>> * You should look up the recent discussion, "[HDRI] Convert
>> equisolidangular to equiangular projection" which strongly suggests
>> the Sigma 8mm f/3.5 to be a -vta / equidistant / equi-angle lens.
>> Reprojecting the image could add some error in this case.
> Well, I think this is not the case. The SIGMA 8mm f/3.5 has an equisolid projection.
> I am quite sure as I received this information from Sigma UK. Moreover, I checked the information Sigma gave me by measuring myself the projection of the lens. I did it following David Geisler-Moroder method, that he presented at the 15th International Radiance Workshop in 2016. And I also got an equisolid projection. I could send you more info if you want.
> The images have thus to be reprojected from equisolid to equidistant.
>
> Best,
>
> Clotilde
>
> -----Message d'origine-----
> De : hdri-request at radiance-online.org [mailto:hdri-request at radiance-online.org]
> Envoyé : jeudi 9 février 2017 23:47
> À : hdri at radiance-online.org
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> Today's Topics:
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> 1. Re: Illuminance and luminance values underestimation from
> calibrated HDR images (J. Alstan Jakubiec)
>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Fri, 10 Feb 2017 06:45:43 +0800
> From: "J. Alstan Jakubiec" <alstan at jakubiec.net>
> To: hdri at radiance-online.org
> Subject: Re: [HDRI] Illuminance and luminance values underestimation
> from calibrated HDR images
> Message-ID: <b29950ec-91c6-4789-8d7f-71c5db3a4e60 at jakubiec.net>
> Content-Type: text/plain; charset="windows-1252"; Format="flowed"
>
> Hello Clotilde,
>
> I'd like to follow up with some other potential things to look out for in addition to what Alex suggested and questions based on your description. We dealt a lot with this via the papers on HDR we published last year.
>
> * Full fisheye HDR images should always be cropped to a bounding
> square and the view edited or input inline via Evalglare when doing
> analysis. Are you cropping the images in all cases?
> * Where is your 1.39 calibration factor derived from? It is useful to
> measure luminance for every HDR photo taken from an easy to identify
> (and nearly-neutral) surface for use in calibrating images rather
> than a constant. Typically the discrepancies I find when I
> /don't///measure luminance this aren't as high as what you found
> however.
> * You should look up the recent discussion, "[HDRI] Convert
> equisolidangular to equiangular projection" which strongly suggests
> the Sigma 8mm f/3.5 to be a -vta / equidistant / equi-angle lens.
> Reprojecting the image could add some error in this case.
>
> Best,
> Alstan
>
> On 2/9/2017 6:09 AM, Alex Mead wrote:
>> Clotilde:
>>
>> Are you sure your low dynamic range pictures are properly exposed from
>> which you construct the HDRI?
>>
>> Meaning, the fastest shutter speed picture has no saturation (i.e. all
>> white pixels) and your slowest shutter speed picture has some black
>> pixels? You need to make sure this is true else you won't get proper
>> illuminance calculation and also your bright spots (i.e. high
>> luminance) will be under reported.
>>
>> - Alex
>>
>> On Wed, Feb 8, 2017 at 11:10 AM, Clotilde Pierson
>> <clotilde.pierson at uclouvain.be <mailto:clotilde.pierson at uclouvain.be>>
>> wrote:
>>
>> Dear all,
>>
>> I am using a Canon EOS 5D Mark II with a Sigma EX DG 8mm f/3.5
>> fisheye lens to capture LDR images. I then use hdrgen with the
>> response curves I defined previously to generate HDR images from
>> these LDR images. I calibrate the HDR images for the vignetting
>> effect and the distortion (equisolid to equidistant projection).
>> Finally, I apply a calibration factor of around 1.39.
>>
>> When comparing the vertical illuminance and luminance values of
>> the HDR images with the real measures I took (Minolta LS-110
>> luminancemeter and Hagner EC1-X luxmeter), I noticed that the HDR
>> images are underestimating the illuminance (e.g. 1217lux instead
>> of 1993lux) and the high luminance values (e.g. 208 cd/m? instead
>> of 402.3cd/m?). I determined the illuminance value of an HDR with
>> Evalglare ?V and the luminance values with ximage in Radiance.
>>
>> I also tried only applying default hdrgen (+cropping & header
>> modification to set the VIEW to vta to use in Evalglare) but I
>> still got big differences between the HDR-derived and the measured
>> luminance and illuminance values.
>>
>> Is somebody using the same instruments I am? If yes, do you also
>> happen to have this issue? Or does anybody have already encounter
>> this problem?
>>
>> Thank you for your insights !
>>
>> Best,
>>
>> Clotilde
>>
>> *Test*
>>
>> **
>>
>> *Clotilde Pierson*
>>
>> /FNRS PhD Fellow | Arch. Eng./
>>
>> *Architecture et Climat**
>> *Facult? d?architecture, d?ing?nierie architecturale et
>> d?urbanisme (LOCI)
>>
>> Universit? catholique de Louvain (UCL)
>> Place du Levant, 1 bte L5.05.04 B-1348 - Louvain-la-Neuve (Belgique)
>> clotilde.pierson at uclouvain.be
>> <mailto:clotilde.pierson at uclouvain.be>
>>
>> T?l. 32 (0)10 47 91 52 - Fax 32 (0)10 47 21 50
>> http://www.uclouvain.be/architecture-climat.html
>> <http://www.uclouvain.be/architecture-climat.html>
>>
>>
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>>
>>
>>
>>
>> --
>> Alex Mead
>> (616) 901-2479, UC Berkeley Systems Engineering Ph.D. (expected 2017),
>> www.alex-mead.com <http://www.alex-mead.com>
>>
>> www.CEEphotos.com <http://www.ceephotos.com> - web master, creator
>>
>>
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