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[Bruno Postle]

On Wed 22-Oct-2003 at 12:22:06AM +0200, Pablo d'Angelo wrote:
> 
> 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).

> 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.

The ptools gimp and photoshop plug-ins have a radial luminance
correction tool, it works by an additive process rather than
multiplication/division.

Helmut had some reasoning for this that I can't remember (I can look
it up).

I works quite well, but there is no single parameter that you can
use for any one lens.

I wrote a similar 'multiplicative' version that works ok too:

   http://bugbear.blackfish.org.uk/~bruno/imager-test/

-- 
Bruno