Illuminance and luminance values underestimation from calibrated HDR images

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

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]> 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 ‚ÄďV 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

*[image: 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]

Tél. 32 (0)10 47 91 52 - Fax 32 (0)10 47 21 50
http://www.uclouvain.be/architecture-climat.html

_______________________________________________
HDRI mailing list
[email protected]
http://www.radiance-online.org/mailman/listinfo/hdri

--
Alex Mead
(616) 901-2479, UC Berkeley Systems Engineering Ph.D. (expected 2017),
www.alex-mead.com

www.CEEphotos.com <http://www.ceephotos.com> - web master, creator

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 ‚ÄďV 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>

    _______________________________________________
    HDRI mailing list
¬†¬†¬†¬†[email protected] <mailto:[email protected]>
    http://www.radiance-online.org/mailman/listinfo/hdri
    <http://www.radiance-online.org/mailman/listinfo/hdri>

--
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

_______________________________________________
HDRI mailing list
[email protected]
http://www.radiance-online.org/mailman/listinfo/hdri

1 Like

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 : [email protected] [mailto:[email protected]]
Envoyé : jeudi 9 février 2017 23:47
√Ä : [email protected]
Objet : HDRI Digest, Vol 88, Issue 7

Send HDRI mailing list submissions to
¬†¬†[email protected]

To subscribe or unsubscribe via the World Wide Web, visit
  http://www.radiance-online.org/mailman/listinfo/hdri
or, via email, send a message with subject or body 'help' to
¬†¬†[email protected]

You can reach the person managing the list at
¬†¬†[email protected]

When replying, please edit your Subject line so it is more specific than "Re: Contents of HDRI digest..."

Today's Topics:

   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" <[email protected]>
To: [email protected]
Subject: Re: [HDRI] Illuminance and luminance values underestimation
  from calibrated HDR images
Message-ID: <[email protected]>
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 > <[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/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)
¬†¬†¬†¬†[email protected]
<mailto:[email protected]>

    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>

    _______________________________________________
    HDRI mailing list
¬†¬†¬†¬†[email protected] <mailto:[email protected]>
    http://www.radiance-online.org/mailman/listinfo/hdri
    <http://www.radiance-online.org/mailman/listinfo/hdri>

--
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

_______________________________________________
HDRI mailing list
[email protected]
http://www.radiance-online.org/mailman/listinfo/hdri

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1 Like

Hi Clotilde,

Have you tried using the raw2hdr script, which calls dcraw and exiftool as well as hdrgen? I have found this to be a more reliable method than using hdrgen alone. However, you will need to download and install dcraw.c and exiftool to get it working. These tools and the raw2hdr script are included in the Mac OS X command-line HDR image builder available from www.anyhere.com.

If you are on Linux, you can download and install the other tools from the web and use the attached script.

Best,
-Greg

raw2hdr.pl (2.82 KB)

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 : [email protected] [mailto:[email protected]]
Envoyé : jeudi 9 février 2017 23:47
√Ä : [email protected]
Objet : HDRI Digest, Vol 88, Issue 7

Send HDRI mailing list submissions to
¬†¬†[email protected]

To subscribe or unsubscribe via the World Wide Web, visit
  http://www.radiance-online.org/mailman/listinfo/hdri
or, via email, send a message with subject or body 'help' to
¬†¬†[email protected]

You can reach the person managing the list at
¬†¬†[email protected]

When replying, please edit your Subject line so it is more specific than "Re: Contents of HDRI digest..."

Today's Topics:

    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" <[email protected]>
To: [email protected]
Subject: Re: [HDRI] Illuminance and luminance values underestimation
  from calibrated HDR images
Message-ID: <[email protected]>
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 >> <[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/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)
¬†¬†¬†¬†¬†[email protected]
<mailto:[email protected]>

     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>

     _______________________________________________
     HDRI mailing list
¬†¬†¬†¬†¬†[email protected] <mailto:[email protected]>
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     <http://www.radiance-online.org/mailman/listinfo/hdri>

--
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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Dear Alstan,
Dear all,

  > 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.

Well, I also take several luminance measurements each time I capture a LDR series. I was planning to use these measurements as a means of verification (rather than a means of calibration) to be sure I got the right luminance values with my HDR.
But anyway, I imagine that using an in-situ luminance measurement instead of a previously derived calibration factor to calibrate an HDR images does not have a great impact on the results. Both calibration factors (the one derived beforehand and the one derived for each HDR image) should be similar, shouldn't they?

I still have tried to calibrate my HDR image with an in-situ luminance measure:
With the Canon EOS 5D Mark II and a Sigma fisheye EX DG 8mm (aperture set on f/11, WB on daylight and all other settings on neutral), I took a JPG series of 17 images (shutter speed going from 1/8000 to 5s). Then I picked a sequence of 12 images out of these 17 jpg images (until there is no pixel <20 in the lightest exposure and no pixel >200 in the darkest exposure). To derive the HDR image, I used hdrgen -a -e -f -g and a response curve I derived previously. I applied the vignetting calibration and the reprojection with the pcomb command in Radiance. I cropped the image to a square (1000x1000) and modified the header (view and exposure).

Using Photosphere, I applied a calibration factor on the central grey card (the HDR gave 77.1cd/m² and I measured 111.9cd/m²). The calibration factor used is 1.45, which is not so far from the 1.48 I was using before. For the other luminance values I measured in the scene, I got these values with the calibrated HDR:
2660 cd/m² (HDR) VS 2817 cd/m² (real)
42.4 cd/m² (HDR) VS 41.04 cd/m² (real)
1740 cd/m² (HDR) VS 1516 cd/m² (real)
And for the illuminance value, I measured 550lux and I got 605lux with the calibrated HDR.
These are still big differences in my opinion, and not always on the same side. What do you think?

  > 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.

Well, I derived my vignetting curves exactly the same way that Cauwerts used (we are from the same lab). Then, from these curves, I created .cal files (one for each aperture I use). For instance, I have this .cal file for aperture f/11

sq(x)=x*x;
r=sqrt(sq(x-2825)+sq(y-1879))/1775; (where I define the radius of the circular image where to apply the vignetting using the pixel coordinates of the centrum) sf=1/(1+(K_1*r^1)+(K_2*r^2)+(K_3*r^3)+(K_4*r^4)+(K_5*r^5)+(K_6*r^6)+(K_7*r^7)+(K_8*r^8)); (with K_x the factors I defined) ro=sf*ri(1); go=sf*gi(1); bo=sf*bi(1);

Then, I simply use the command: pcomb -f calFile input.hdr > output.hdr

Also, I am not sure how the x option works in hdrgen but it seems it is not 100% reliable. I thus decided to manually pick the sequence of input JPG images, removing too light or too dark images myself.

Best,

Clotilde

-----Message d'origine-----

···

De : [email protected] [mailto:[email protected]]
Envoyé : lundi 13 février 2017 12:29
√Ä : [email protected]
Objet : HDRI Digest, Vol 88, Issue 9

Send HDRI mailing list submissions to
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When replying, please edit your Subject line so it is more specific than "Re: Contents of HDRI digest..."

Today's Topics:

   1. Re: Illuminance and luminance values underestimation from
      calibrated HDR images (J. Alstan Jakubiec)

----------------------------------------------------------------------

Message: 1
Date: Mon, 13 Feb 2017 19:28:41 +0800
From: "J. Alstan Jakubiec" <[email protected]>
To: [email protected]
Cc: Zhe Kong <[email protected]>, [email protected],
¬†¬†[email protected]
Subject: Re: [HDRI] Illuminance and luminance values underestimation
  from calibrated HDR images
Message-ID: <[email protected]>
Content-Type: text/plain; charset="windows-1252"; Format="flowed"

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 : [email protected]
[mailto:[email protected]]
Envoy? : jeudi 9 f?vrier 2017 23:47
? : [email protected]
Objet : HDRI Digest, Vol 88, Issue 7

Send HDRI mailing list submissions to
¬†¬†[email protected]

To subscribe or unsubscribe via the World Wide Web, visit
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You can reach the person managing the list at
¬†¬†[email protected]

When replying, please edit your Subject line so it is more specific than "Re: Contents of HDRI digest..."

Today's Topics:

    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" <[email protected]>
To: [email protected]
Subject: Re: [HDRI] Illuminance and luminance values underestimation
  from calibrated HDR images
Message-ID: <[email protected]>
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 >> <[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/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)
¬†¬†¬†¬†¬†[email protected]
<mailto:[email protected]>

     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>

     _______________________________________________
     HDRI mailing list
¬†¬†¬†¬†¬†[email protected] <mailto:[email protected]>
     http://www.radiance-online.org/mailman/listinfo/hdri
     <http://www.radiance-online.org/mailman/listinfo/hdri>

--
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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