[ptx] Colour/brightness correction using histograms

[Bruno Postle]

The original PTStitcher uses some kind of histogram comparison of
the overlapping areas of a pair of images to calculate the
transformation required to correct colour and brightness
differences.

At least I think so, we have never had the source code to look at.

The 'gunroll' tool which is part of the comparametric toolkit seems
to do something very similar:

  http://sourceforge.net/projects/comparametric/

As far as I can tell, it takes histograms of two images with
different exposures and uses the differences between them to place
each into linear colourspace.

Maybe some of this could be useful for nona.

-- 
Bruno