[HDRI] Re: hdrgen's response curve file

[Gregory J. Ward]

Hi Lars,

There are no units for the sensor response function, which isn't  
luminance, but individual responses for the three color channels  
(red, green, and blue).  Sadly, even these are mixed up, since there  
is a non-linear (and unknown) color transform between the sensors RGB  
and the image RGB values.

BTW, most of the accuracy from HDR sequences is unrelated to the  
derived camera response.  Rather, it's from the proper combining of  
closely-spaced exposures with accurately reported ISO, speed, and  
aperture.  If those things are not reported correctly by the camera,  
then all bets are off.  Other things that undermine accuracy are lens  
flare (trying to measure a dark area near a bright one or with direct  
light entering the camera), and vignetting for wide apertures.   
Absolute response is also a big issue, which is why you should  
probably have at least one luminance reading from each captured scene  
if the actual values (and not just relative values) matter to you.

-Greg

> From: "Lars O. Grobe" <grobe at gmx.net>
> Date: June 10, 2008 11:49:25 PM PDT
>
>> I like to keep my file formats as obvious as possible.
>
> While in general this works, in my case it was not obvious enough - at
> least I had to ask :-) Thank you for the explanation.
>
> Now, I see that I get a function for each channel. What I guess is  
> that
> the function is sensor response ( picture luminance ), is this
> assumption true?
>
> What I actually would like to do is, for a range of picture  
> luminances,
> plot a graph of the sensor output using the polynomal as given by
> hdrgen, and overlay it with the response curve as written by  
> pfstools. I
> am using both tools at the moment and would like to see the  
> differences
> for the given range of luminances caused by the different algorithms.
>
> Is this possible at all?
>
> Is there any kind of unit related to the sensor response values?
>
> TIA&CU Lars.