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Gmane
From: Rik Littlefield <rj.littlefield <at> computer.org>
Subject: Re: Re: Brightness/colour correction in pano12 and nona
Newsgroups: gmane.comp.misc.ptx
Date: Sunday 20th November 2005 21:08:34 UTC (over 12 years ago)
I agree with Gellule's interpretation. 

In practice, I think it should not be much problem that N_A is not 
strictly increasing.  By definition, flat sections in N_A will occur 
only in regions where no pixels have those values in A.  So if the basic 
model is right, no pixels in B (at least not many) will end up mapping 
to those sections of N_A.  Using some average over the flat region seems 
OK.  Maybe less risk of banding if for each pixel in a flat region of 
N_A you pick a random value from among the possibilities..

Bear in mind that this whole model assumes that matching the histograms 
within the whole overlap region will match colors on a pixel by pixel 
basis.  This is true under idealized conditions, but it breaks down if 
different portions of the overlap region had different shifts.  For 
example, you can expect less than perfect correction if radial falloff 
causes a left-to-right gradient in one image but right-to-left in the 
other.  Radial falloff should be corrected  in each image separately, 
before attempting to correct one image against another.  (And accurately 
correcting for radial falloff requires knowing the actual 
light-level-to-pixel-value gradation curve, not just some idealized 
gamma, but that's another story.)

--Rik

Gellule wrote:

>Hello,
>
>I have a group of pictures for which the best result is achieved with 
>PTStitcher with brightness correction and then enblend. It seems to me
that 
>it would be interesting to have the brightness/color correction in nona.
Is 
>that a wanted/planned feature? Would an explanation of what H.Dersch might

>mean help?
>
>The gradation curves N(I) seem to be the integration of the histograms
n(I):
>   N_A(I) = Sum_{i in [0;I]} n_A(i)    for n_A(i) the histogram of image A
>   N_B(I) = Sum_{i in [0;I]} n_B(i)    for n_B(i) the histogram of image B
>
>The correction from image B to image A would then follow:
>A pixel of intensity I in image B should become a pixel of intensity 
>N_A^-1(N_B(I)).
>
>Basically, you end up with (almost) exactly the same histrogams. 
>Unfortunately, N_A is a discrete function and not strictly increasing
(don't 
>know if those are the proper mathematical terms), and there must be some 
>subtleties to add in the implementation in order to get the thing working.
>
>Thank you for Hugin.
>
>-Gellule
>
>
>"Pablo d'Angelo"  wrote in message 
>news:[email protected]
>  
>
>>Hi!
>>
>>Just stumbled upon this when searching for another mail...
>>
>>Bruno Postle wrote:
>>    
>>
>>>While I was looking around in adjust.c, there seems to be all the
>>>code used by PTStitcher to do brightness/colour correction of image
>>>pairs:
>>>
>>>  void GetColCoeff ( Image *src, Image *buf, double ColCoeff[3][2] )
>>>  void ColCorrect ( Image *im, double ColCoeff[3][2] )
>>>  void DoColorCorrection ( Image *im1, Image *im2, int mode )
>>>
>>>Could this be useful in nona?
>>>      
>>>
>>I believe this is an old version of the correction algorithm, not the one

>>described on
>>
>>http://www.path.unimelb.edu.au/~dersch/cbcorrect/cb.html
>>
>>Regarding that page, the description is a bit sparse on implementation 
>>details.
>>
>>Quote from H.Dersch:
>>================================
>>    
>>
>>>If two images are perfectly colour balanced, their histograms are
>>>identical. The algorithm now included in PTStitcher calculates
>>>gradation-curves for each colour channel which match the corresponding
>>>histograms. These gradation curves are then used to correct image B.
>>>There are several advantages of this method:
>>>[...]
>>>The curves used for correction are calculated using a transformation
>>>of the histograms, and are exact (at least under usually applicable
>>>assumptions), not mere optimizations. They refer to the best fit
>>>obtained using 256 variables for each channel. In other words: If the
>>>image can be adjusted, this method will find the correct curves for
>>>performing this adjustment.
>>>      
>>>
>>Does anyone know what kind of transformation he uses? My first idea would

>>have been minimisation of the error between the histograms, using 
>>numerical optimisation.
>>
>>ciao
>>  Pablo
>>
>>
>>    
>>
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