[magick-users] RE: Barrel / pincushion distortion

[Wolfgang Hugemann]

> The forward equation is not easily achieved, but I would think there 
> will not be such extreme distortions as in perspective and arc.  If 
> there are, then one would  just have to live with point or better 
> bilinear interpolation. However, my main issue is how one goes about 
> figuring out what the a,b,c,d constants should be!!!!

Yes, that's part of the problem. Again, PTlens can help with the solution:

First of all you can read these values for a lot of cameras from its 
database via Clens. Secondly, 
http://epaperpress.com/ptlens/calTargets.html gives hints on suitable 
targets, though it does not tell us the calibration algorithm itself, as 
this work is always performed by Tom Niemann himself. I think that is 
the trick he really sells with PTlens.

I though don't believe this task to be too tricky. It's probably 
straightforward: you have to determin the four polynominal coefficients 
from a suitable image. This means that you need the real coordinates of 
just two points in the image.

And I forgot: GIMP is also Open Source, so one might look at its code as 
well: http://www.gimp.org/downloads/

Greetings from Germany
Wolfgang Hugemann