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
<[email protected] <mailto:[email protected]>>
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/m² instead
of 402.3cd/m²). 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**
*Faculté d’architecture, d’ingénierie architecturale et
d’urbanisme (LOCI)
Université catholique de Louvain (UCL)
Place du Levant, 1 bte L5.05.04 B-1348 - Louvain-la-Neuve (Belgique)
[email protected] <mailto:[email protected]>
Tél. 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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