[Radiance-general] HDRI - Camera Response Curve

[Kirk Thibault]

Makes sense.  I was curious about this simply because I wondered how  
robust the Camera Response Curves are in accounting for camera-based  
pre-processing.  Some cameras do things to the RAW data to enhance  
contrast, boost saturation etc. and then write the JPEG.  Some  
cameras allow the user to define the amount of this processing (I'm  
thinking of the Canon Digital Rebel that I shoot with, but I imagine  
that many cameras now offer user control of these parameters in some  
form).  Do the camera response curves essentially account for these  
settings inasmuch as they affect the way the camera "exposes" the RAW  
image data from the sensor (referring to your statement below about  
second-guessing what the camera is doing)?  I know that generating  
HDR images from multi-exposure LDR images "demands" that auto-white  
balance not be used - are there similar suggestions for the camera- 
based pre-processing parameters like contrast, saturation, etc. that  
may affect HDR generation or is that essentially what the curves  
compensate for (assuming that the same exact pre-processing camera  
parameters are applied to each image)?  Because these pre-processing  
algorithms are automated, would they necessarily be applied  
consistently across the entire range of exposures or could they  
introduce some sort of changing response similar to a changing white  
balance that might affect the combination of the LDRs and the  
extraction of a response curve?

I suppose I could experiment by setting the parameters on my Digital  
Rebel to combinations of extrema in a relatively controlled lighting  
situation and see if it matters, but I'm not sure I would know what  
to look for to measure any differences that might occur.  Clearly my  
limited understanding of the physics is being exposed here (my  
limited understanding has a very large apparent dynamic range) -  
which may mean that in my dimwitted approach, it may not matter anyway!
:-)

Thanks,

kirk
------------------------------

Kirk L. Thibault, Ph.D.
kthibault at biomechanicsinc.com

p.  215.271.7720
f.   215.271.7740
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skype. kirkthibault




On Jan 10, 2006, at 11:56 AM, Greg Ward wrote:

> Hi Kirk,
>
> There generally isn't much difference between camera RAW and JPEG  
> in terms of resolution.  The main difference is that we don't need  
> to second-guess what the camera is doing or figure out the response  
> function (in most cases).
>
> Working on the NYSERDA project again, I've developed a prototype  
> script based on Dave Coffin's dcraw.c that converts camera RAW  
> directly into 24-bit BMP with a known gamma, then takes this into  
> hdrgen to compute the final HDR image.  I have done numerous  
> experiments with this process and compared them to dealing only  
> with JPEGs, and there's not a lot of difference between the two  
> methods.  In general, I would say that the RAW translation offers  
> better color fidelity, but suffers from greater noise.  The color  
> difference is attributable to the odd color manipulations that  
> happen in the camera.  The noise difference no doubt has a lot to  
> do with the noise reduction that happens in many digital cameras,  
> which is bypassed using RAW directly.
>
> -Greg
>
> P.S.  Bear in mind that although most RAW image files are 12 bits,  
> they are 12 bits of linear range, which isn't really any better  
> than 8 bits using a standard 2.2 gamma.
>
>> From: Kirk Thibault <kthibault at biomechanicsinc.com>
>> Date: January 10, 2006 7:58:47 AM PST
>>
>> Greg et al -
>>
>> Regarding Photosphere and the creation of HDR images from multiple  
>> exposure LDR images:  is there any advantage to the process by  
>> using higher bit RAW or TIFF (12-16 bit) linear LDRs when  
>> combining them into an HDR, or does it not matter or actually make  
>> the process harder or less efficient, etc.?  Just curious.
>>
>> I've been reading and rereading the Tone Mapping chapters in the  
>> new book trying to get a theoretical and physical basis for the  
>> machinery working behind the scenes in Photosphere/hdrgen.  Pretty  
>> cool stuff.
>>
>> Thanks,
>>
>> kirk
>
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