I have just worked out a stable and rather simple way to determine the lens correction parameters. I uploaded the example image, the corrected image an the Hugin project file to www.unfallrekonstruktion.de/imagemagick/Olympus_C2500L.zip
The Olympus C2550L is an older prosumer zoom camera with a quite pronounced barrel distortion at shorter focal lengths -- which are now almost perfectly corrected. The basic idea is to use modern faccades as targets, which is in fact not my own idea, but that of Tom Niemann, author of PTlens, see http://epaperpress.com/ptlens In my examples, the faccade consists of square "tiles", such that we can use the corner points of the "tiles" as targets. I chose 32 target points, i.e. 4 horizontal lines with 8 points each. I then determined the coordinates of these points by the help of WinMorph (www.debugmode.com/winmorph), which relieves me from the workload of having to pick every point twice, like in Hugin. I then constructed the point list for the PTO file (by the help of Excel), combining the 32 source points to 4 horizontal and 8 vertical lines. For the target points, I just reversed the ordering of the horizontal points, as I found that if the source and target points are totally identical, the optimiser just doesn't work (?). The results for the parameters a, b, c yielded by Hugin's optimiser were then used to perform a distortion correction with ImageMagick (see http://www.imagemagick.org/Usage/windows/#vb_example and http://www.imagemagick.org/Usage/distorts/#barrel), which turned out to be very good. So I basically suggest 1) to use modern faccades as suitable targets 2) to generate the point list for the optimisation not with Hugin, as this generates a needless overhead. I would also suggest to rewrite the Simple Lens Calibration example in regard to this. I am however not sure 1) whether the ordering of the horizontal points really has to be reversed for the target image 2) how to initiate the optimisation without using Hugin, which now more or less just creates a superfluous overhead. 3) whether it would be more efficient to use horizontal and vertical lines instead of general lines -- the horizontal lines however being not strictly horizontal in the image, due to rotation of the camera. Could you help me in this regard? I'm planning to check the results of my manual optimisation against those achieved by the calibrate_lens program, thus I'm looking forward to the Windows EXE file of that program. Wolfgang Hugemann
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