calibrating HDR pictures with real world luminance

hdri
1

Hi!

I am experimenting with HDR images and a luminance meter to see how tone
mappers can actually help reproducing human perception.

Here is what I am currently doing:

a)capture a serie of LDR images using a Canon Rebel XT, only changing the
shutter speed, with constant Fstop, saved as JPG with sRGB profile
b)creating a calibration curve in HDR Shop
c)measuring a few points in cd/m2 with a luminance meter (LS-110)
d)making an HDR image out of the LDR images using the calibration curve
e)experimenting different tone mappers available on the DVD of the HDRI book
from our favorite well known HDRI gurus.

Here are a few questions that puzzles me still:

1) I am trying to scale my HDR image to the measured physical values. I
found somewhere that luminance = 179 * (R*0.265 + G*0.067 + B*0.065). Can I
assume that this equation is the one to use to scale my HDR file properly?

2) In some references, it is mentionned that I should always capture LDRs
with a Daylight white balance. Is this the general agreement? Even if my
scene is lit with tungsten lights?

3)I found limited explanation on how the camera calibration curve would
affect the results in HDR Shop. I would have to think that this is to
linearize the color values (remove the effect of Gamma + CMOS response) so
luminance measurements can be actually applied to the colors later on.

4)HDR Shops refers to Fstops in the UI. However, I read that you want to
maintain Fstops constant and only change the shutter speed. The problem I
have is that the interpreted Fstops of HDR Shop turn into some "arbitrary"
numbers and I can't really type-in the values used by my camera.

I am not too sure if I am using the right methods. Some "cook book" would
be handy.

Best regards,

Pierre-Félix Breton
www.pfbreton.com <http://www.pfbreton.com/>

2

Hi,

You may want to look at the validation study at:
Inanici MN. “Evaluation of High Dynamic Range Photography as a Luminance Measurement Technique”, Journal of Lighting Research and Technology, Vol. 38, no. 2, June 2006, pp. 123-136.

Another version of this study is available at:

This study evaluates Photosphere rather than HDRShop, but you can find information regarding your first 3 questions.

Hope this helps... Regards,
Mehlika

···

On Tue, 26 Sep 2006, Pierre-Felix Breton wrote:

Hi!

I am experimenting with HDR images and a luminance meter to see how tone
mappers can actually help reproducing human perception.

Here is what I am currently doing:

a)capture a serie of LDR images using a Canon Rebel XT, only changing the
shutter speed, with constant Fstop, saved as JPG with sRGB profile
b)creating a calibration curve in HDR Shop
c)measuring a few points in cd/m2 with a luminance meter (LS-110)
d)making an HDR image out of the LDR images using the calibration curve
e)experimenting different tone mappers available on the DVD of the HDRI book
from our favorite well known HDRI gurus.

Here are a few questions that puzzles me still:

1) I am trying to scale my HDR image to the measured physical values. I
found somewhere that luminance = 179 * (R*0.265 + G*0.067 + B*0.065). Can I
assume that this equation is the one to use to scale my HDR file properly?

2) In some references, it is mentionned that I should always capture LDRs
with a Daylight white balance. Is this the general agreement? Even if my
scene is lit with tungsten lights?

3)I found limited explanation on how the camera calibration curve would
affect the results in HDR Shop. I would have to think that this is to
linearize the color values (remove the effect of Gamma + CMOS response) so
luminance measurements can be actually applied to the colors later on.

4)HDR Shops refers to Fstops in the UI. However, I read that you want to
maintain Fstops constant and only change the shutter speed. The problem I
have is that the interpreted Fstops of HDR Shop turn into some "arbitrary"
numbers and I can't really type-in the values used by my camera.

I am not too sure if I am using the right methods. Some "cook book" would
be handy.

Best regards,

Pierre-Félix Breton
www.pfbreton.com <http://www.pfbreton.com/&gt;

3

I'm not a researcher, so pls. take this into account when reading my considerations:

2) In some references, it is mentionned that I should always capture LDRs with a Daylight white balance. Is this the general agreement? Even if my scene is lit with tungsten lights?

In my operating experience, it is easier to control color (workflow) - even if there are no "outside" image areas.
Though, you can always correct WB in HDRShop (->Pixels->WB selection)

3)I found limited explanation on how the camera calibration curve would affect the results in HDR Shop. I would have to think that this is to linearize the color values (remove the effect of Gamma + CMOS response) so luminance measurements can be actually applied to the colors later on.

This is also my understanding.

4)HDR Shops refers to Fstops in the UI. However, I read that you want to maintain Fstops constant and only change the shutter speed. The problem I have is that the interpreted Fstops of HDR Shop turn into some "arbitrary" numbers and I can't really type-in the values used by my camera.

I personally would be interested to see, how far Photomatix would support you with this task:
The new version reads EXIF data, resulting in "normalized" HDR images. Basic idea behind is to make tone mapping in a panorama workflow easier, but it should also make your task easier reproduceable.
I didn't invest much time to test this functionality, but as far as i found out, it turned out to be useful when using panos for IBL (i don't have to fiddle that much with exposure correction)...

Best regards
Bernhard

···

--
Bernhard Vogl
Vienna, Austria

http://www.austria-360.at
Tutorials: http://www.dffe.at

4

Hi,

That study is a very interesting read. Thank you for the link. And thank you for taking the time to do this elaborate analysis in the first place.

It makes me wonder how closely your Mathlab algorithm for calculating the luminance is related to the Photosphere's algorithm. Does the determined 10% average accuracy apply to Photosphere's luminance readout as well, provided we work off a good calibration curve?

Also, I find it interesting that the error is higher for primary colors, with Red going up to 40% sometimes. Those really seem to drive up the average a lot. Could the limited gamut of the Radiance format itself have an influence here? Or is it really all due to the cameras internal demosaicing and JPEG compression? Could this error be minimized by shooting RAW pictures instead?

Regards,
Christian

···

Am 26.09.2006 um 13:33 schrieb Mehlika Inanici:

Hi,

You may want to look at the validation study at:
Inanici MN. “Evaluation of High Dynamic Range Photography as a Luminance Measurement Technique”, Journal of Lighting Research and Technology, Vol. 38, no. 2, June 2006, pp. 123-136.

Another version of this study is available at:
http://repositories.cdlib.org/lbnl/LBNL-57545/

This study evaluates Photosphere rather than HDRShop, but you can find information regarding your first 3 questions.

Hope this helps... Regards,
Mehlika

On Tue, 26 Sep 2006, Pierre-Felix Breton wrote:

Hi!

I am experimenting with HDR images and a luminance meter to see how tone
mappers can actually help reproducing human perception.

Here is what I am currently doing:

a)capture a serie of LDR images using a Canon Rebel XT, only changing the
shutter speed, with constant Fstop, saved as JPG with sRGB profile
b)creating a calibration curve in HDR Shop
c)measuring a few points in cd/m2 with a luminance meter (LS-110)
d)making an HDR image out of the LDR images using the calibration curve
e)experimenting different tone mappers available on the DVD of the HDRI book
from our favorite well known HDRI gurus.

Here are a few questions that puzzles me still:

1) I am trying to scale my HDR image to the measured physical values. I
found somewhere that luminance = 179 * (R*0.265 + G*0.067 + B*0.065). Can I
assume that this equation is the one to use to scale my HDR file properly?

2) In some references, it is mentionned that I should always capture LDRs
with a Daylight white balance. Is this the general agreement? Even if my
scene is lit with tungsten lights?

3)I found limited explanation on how the camera calibration curve would
affect the results in HDR Shop. I would have to think that this is to
linearize the color values (remove the effect of Gamma + CMOS response) so
luminance measurements can be actually applied to the colors later on.

4)HDR Shops refers to Fstops in the UI. However, I read that you want to
maintain Fstops constant and only change the shutter speed. The problem I
have is that the interpreted Fstops of HDR Shop turn into some "arbitrary"
numbers and I can't really type-in the values used by my camera.

I am not too sure if I am using the right methods. Some "cook book" would
be handy.

Best regards,

Pierre-Félix Breton
www.pfbreton.com <http://www.pfbreton.com/&gt;

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

5

Hi,

The luminance calculation given in the paper (as Matlab algorithm) should be exactly the same as Photosphere uses to calculate luminance. Greg is the only person to verify this I guess :slight_smile:

The calculations are based on CIE chromaticities for the reference primaries (sRGB) and CIE Standard Illuminant D65:

         R (x, y ,z) = (0.64, 0.33, 0.03)
   G (x, y, z) = (0.30, 0.60, 0.10)
   B (x, y, z) = (0.15, 0.06, 0.79)
   D65 (x, y, z) = (0.3127, 0.3290, 0.3583)

In Radiance on the other hand, calculations are based on equal energy white point. Equal energy (x, y, z) = (0.33 0.33 0.33).

That is the reason for minor difference between original Luminance calculations in Radiance (R*0.265 + G*0.067 + B*0.065) and the calculation in the paper (R*0.2127 + G*0.7151 + B*0.0722). My two cents is that the difference in between those two equations is quite minor.

It is true that the saturated colors in Macbeth chart yielded increased errors. RAW format might result in better results, but I don't have enough data to claim that... My limited trials with RAW formats looked quite promising though.

Some of the folks in this group may comment on their experience with RAW format, may be?

Regards,
Mehlika

···

On Tue, 26 Sep 2006, Blochi wrote:

Hi,

That study is a very interesting read. Thank you for the link. And thank you for taking the time to do this elaborate analysis in the first place.

It makes me wonder how closely your Mathlab algorithm for calculating the luminance is related to the Photosphere's algorithm. Does the determined 10% average accuracy apply to Photosphere's luminance readout as well, provided we work off a good calibration curve?

Also, I find it interesting that the error is higher for primary colors, with Red going up to 40% sometimes. Those really seem to drive up the average a lot. Could the limited gamut of the Radiance format itself have an influence here? Or is it really all due to the cameras internal demosaicing and JPEG compression? Could this error be minimized by shooting RAW pictures instead?

Regards,
Christian

Am 26.09.2006 um 13:33 schrieb Mehlika Inanici:

Hi,

You may want to look at the validation study at:
Inanici MN. “Evaluation of High Dynamic Range Photography as a Luminance Measurement Technique”, Journal of Lighting Research and Technology, Vol. 38, no. 2, June 2006, pp. 123-136.

Another version of this study is available at:
http://repositories.cdlib.org/lbnl/LBNL-57545/

This study evaluates Photosphere rather than HDRShop, but you can find information regarding your first 3 questions.

Hope this helps... Regards,
Mehlika

On Tue, 26 Sep 2006, Pierre-Felix Breton wrote:

Hi!

I am experimenting with HDR images and a luminance meter to see how tone
mappers can actually help reproducing human perception.

Here is what I am currently doing:

a)capture a serie of LDR images using a Canon Rebel XT, only changing the
shutter speed, with constant Fstop, saved as JPG with sRGB profile
b)creating a calibration curve in HDR Shop
c)measuring a few points in cd/m2 with a luminance meter (LS-110)
d)making an HDR image out of the LDR images using the calibration curve
e)experimenting different tone mappers available on the DVD of the HDRI book
from our favorite well known HDRI gurus.

Here are a few questions that puzzles me still:

1) I am trying to scale my HDR image to the measured physical values. I
found somewhere that luminance = 179 * (R*0.265 + G*0.067 + B*0.065). Can I
assume that this equation is the one to use to scale my HDR file properly?

2) In some references, it is mentionned that I should always capture LDRs
with a Daylight white balance. Is this the general agreement? Even if my
scene is lit with tungsten lights?

3)I found limited explanation on how the camera calibration curve would
affect the results in HDR Shop. I would have to think that this is to
linearize the color values (remove the effect of Gamma + CMOS response) so
luminance measurements can be actually applied to the colors later on.

4)HDR Shops refers to Fstops in the UI. However, I read that you want to
maintain Fstops constant and only change the shutter speed. The problem I
have is that the interpreted Fstops of HDR Shop turn into some "arbitrary"
numbers and I can't really type-in the values used by my camera.

I am not too sure if I am using the right methods. Some "cook book" would
be handy.

Best regards,

Pierre-Félix Breton
www.pfbreton.com <http://www.pfbreton.com/&gt;

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

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

6

As Mehlika points out, there are slight differences in destination color spaces to account for, but the major one is actually in the camera itself. Most manufacturers spend a fair amount of time "tuning" their cameras' colors to have a more pleasing, photographic look, without regard to color accuracy. In general, I have found the Canon DSLRs to stick fairly close to the sRGB specification, especially if a "Neutral" color setting is used. Nevertheless, working from RAW images is a reasonable way to get around most of the in-camera processing that undermines absolute CIE color accuracy.

I do not believe that HDRShop does anything to the camera's color space, other than an attempt to linearize the three channels. The same is true of Photosphere, although it does attempt to get an absolute luminance calibration, and provides features for inputting your own per-camera or per-image luminance calibration factor. If you use dcraw.c or Photoshop's RAW converter in "don't touch" sRGB calibration mode, you should get out something reasonably close to the sRGB primary color space, provided you have shot with a daylight white balance. (See note below on WB settings.) I have had good luck using dcraw myself, and found it's color transformations to be fairly reliable. (See Dave Coffin's website on dcraw.c at <http://cybercom.net/~dcoffin/dcraw/&gt; for code and related links.) Neither HDRShop nor Photosphere can unscramble the eggs once the colors have been messed around in a typical digital camera, and I suspect this is behind the large delta's in Mehlika's report for primary colors.

Regarding white balance, shooting in daylight mode assures that your measurement condition matches the sRGB color space, so you have some home of getting out absolute colorimetry, which naturally will include any coloration due to the light source. In other words, the camera will measure something like what you would measure with a chroma meter. If instead you apply an appropriate white balance setting, e.g. incandescent under tungsten lighting, the camera performs some sort of von Kries transform (one would hope), bringing neutral colors back to the D65 white point of sRGB. The problem then is that you would have to know exactly what transform was applied to get back to absolute colors, and in general you cannot know. For my work, I shoot with the appropriate white balance when what I care about is appearance in my HDR results, and I shoot with D65 when I'm going for color (and luminance) measurements. Remember that white balance will affect the luminance values as well.

-Greg

···

From: Blochi <[email protected]>
Date: September 27, 2006 4:52:00 AM BDT

Hi,

That study is a very interesting read. Thank you for the link. And thank you for taking the time to do this elaborate analysis in the first place.

It makes me wonder how closely your Mathlab algorithm for calculating the luminance is related to the Photosphere's algorithm. Does the determined 10% average accuracy apply to Photosphere's luminance readout as well, provided we work off a good calibration curve?

Also, I find it interesting that the error is higher for primary colors, with Red going up to 40% sometimes. Those really seem to drive up the average a lot. Could the limited gamut of the Radiance format itself have an influence here? Or is it really all due to the cameras internal demosaicing and JPEG compression? Could this error be minimized by shooting RAW pictures instead?

Regards,
Christian

7

GRegory,

Thanks for your elucidation about WB! Now i'm able to understand my 'observations' :slight_smile:

Regards
Bernhard

8

No problem. I also meant to respond to Christian's question about color gamut. While it's true that some colors may fall outside the RGB gamut of sRGB and/or Radiance RGBE files, this is mostly restricted to saturated purples and aquamarines. Red, orange, and fairly saturated green are well-covered in an HDR image. If you want to capture a larger color gamut, I recommend shooting in Adobe RGB if your camera supports it or shooting RAW and converting to Adobe RGB in Photoshod, then combining exposures with the latest version of Photosphere (1.6.5), writing the file out either as OpenEXR or TIFF to avoid gamut clamping. (Versions before 1.6.3 dated 8/19/2006 didn't support Adobe RGB properly.)

-Greg

···

From: "Bernhard Vogl" <[email protected]>
Date: September 27, 2006 9:47:33 AM BDT

GRegory,

Thanks for your elucidation about WB! Now i'm able to understand my 'observations' :slight_smile:

Regards
Bernhard

9

Thank you for that explanation.
My Nikon does support Adobe RGB, and that sounds like a good compromise of speed and quality. I will do some testing with RAW, too.

Christian

···

Am 27.09.2006 um 02:15 schrieb Gregory J. Ward:

No problem. I also meant to respond to Christian's question about color gamut. While it's true that some colors may fall outside the RGB gamut of sRGB and/or Radiance RGBE files, this is mostly restricted to saturated purples and aquamarines. Red, orange, and fairly saturated green are well-covered in an HDR image. If you want to capture a larger color gamut, I recommend shooting in Adobe RGB if your camera supports it or shooting RAW and converting to Adobe RGB in Photoshod, then combining exposures with the latest version of Photosphere (1.6.5), writing the file out either as OpenEXR or TIFF to avoid gamut clamping. (Versions before 1.6.3 dated 8/19/2006 didn't support Adobe RGB properly.)

-Greg

From: "Bernhard Vogl" <[email protected]>
Date: September 27, 2006 9:47:33 AM BDT

GRegory,

Thanks for your elucidation about WB! Now i'm able to understand my 'observations' :slight_smile:

Regards
Bernhard

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