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[Pablo d'Angelo]

On Wed, 08 Oct 2003, Peter Suetterlin wrote:

> > hurrican ;-) ). Most of the times I use colour/brightness correction
> > it's just to improve little disharmonies at overlaps - maybe very
> > small vignetting effects from the lens?!
> 
> Definitely, depending on the Lens.
> For my old Powershot A50 I ended up doing a 'flatfield', i.e., taking
> many images of clear sky (or white paper?  don't really remember).
> Then you can see the intensity gradient over the field.  I wrote a
> small C program that corrected this gradient, and it largely improved
> the stitching results.

I've played around with gimp a little bit, and found that a technique like
you describe is quite effective for images created by my Canon A40 as well.

what exactly did you do to correct the vignetting? I simply added the
brighness difference I got from a white wall shot to the picture (using a
layer).

Before addition, it was heavily smoothed (with a gauss filter), to
remove the small structured details on the wall.

I'm quite sure that a simple addition, like I did is the wrong way. Probably one should
transform the picture to HSV and divide the V channel of though a brightness
factor image.

This brighness factor image would be created from a (smoothed) shot of a subject
with equal brighness by calculating: V(x,y) / V_center for all x,y.

Hmm, probably this depends on the apperature, focal length and lots of other
stuff.  so it might be useful to try and shoot a reference images under similar 
conditions (with the same exposure lock used for shooting the panorama
pictures).

I might add a feature to do this processing into hugin as an optinal
preprocessor step, before runing PTStitcher.

this might also make the pano tools color correction work better, since
these effects are strongest in the outer, overlapping regions, that are used
by PTStitcher to do the color correction.

ciao
  Pablo
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