[ptx] Vignetting correction in nona

[pablo.dangelo at informatik.uni-ulm.de]

Hi all,

I'm currently implementing a vignetting correction during remapping. The
following things will be implemented:

Gamma correction prior to the remapping. This is similar to the gamma value
supported by PTStitcher.

Two vignetting correction modes:
 1. correction by addition: i_new(x,y) = i(x,y) + corr(x,y)
 2. correction by division: i_new(x,y) = i(x,y) / corr(x,y)

 The first case is similar to the vignetting correction implemented in panotools
 plugin and suitable for gamma corrected images (like the typical gamma 2.2
 (or something similar) of the images output by scanners or a digicam.)

 The correction by division should work best with linear images (gray/color
 values proportional to light captured by the sensor)

The correction offset/factor can be specified by a polynomial which depends on
the normalized distance r (0..1) between point (x,y) and the image center.

corr(x,y) = j + k*r^2 + l*r^4 + o*r^6

 where j, k, l and o are variables specified in the stitcher script file.

or a flatfield image (flat):
 1. for additive correction
     corr(x,y) = flat(x,y)
 2. for correction by division:
     corr(x,y) = flat(x,y) / max(flat)

I decided to use different interpretations of the flatfield image because it
should then be possible to just feed a captured flatfield image into nona.

Any further wishes or thoughts?

ciao
  Pablo

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