[ptx] Brightness/colour correction in pano12 and nona

[Pablo d'Angelo]

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