[HDRI] calibrating HDR pictures with real world luminance

[Blochi]

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/>
>>
>>
>>
>>
>
>
>
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