[ptx] Re: Brightness/colour correction in pano12 and nona

[Gellule]

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" <pablo.dangelo at web.de> wrote in message 
news:41EA3340.6000809 at web.de...
> 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
>
>