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03-08-2009, 09:41 AM   #1
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de-fishing theory question

Hi all,

I have a DA 10-17 and I'd like to train Bibble to de-fish it. It's not currently in Bibble's lens database, but I though that was easily solved: just find the a, b, and c parameters for the polynomial correction model and tell them to Bibble.

However, initial results are quite bad; I used Hugin to generate a, b, c (using this tutorial), and while Hugin corrects the image just fine, Bibble doesn't with the same a, b, c.

I poked around a little on ye olde Internet and found a hint that the fisheye image mapping is so different from that of rectilinear lenses that the polynomial correction model doesn't apply and won't work.

True? Before debugging my Bibble problem further, I want to figure out if there's a fundamental reason it won't work (and I should pursue a different de-fish path).

Thanks,

Reid

03-08-2009, 06:35 PM   #2
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QuoteOriginally posted by rpriedhorsky Quote
I poked around a little on ye olde Internet and found a hint that the fisheye image mapping is so different from that of rectilinear lenses that the polynomial correction model doesn't apply and won't work.

True? Before debugging my Bibble problem further, I want to figure out if there's a fundamental reason it won't work (and I should pursue a different de-fish path).

Thanks,

Reid

There are two problems you might be running into:

(1) Bibble and Hugin may apply parameters differently:
a, b, and c specify a polynomial describing how your lens differs from either an ideal rectilinear lens or an ideal fisheye lens, depending on the lens type you specify. If you're using hugin to calculate a, b, and c for the fisheye model, and Bibble applies a, b, and c to the rectilinear model, you will get a different result.

(2) Polynomial approximations of trigonometric functions suck:
So why can't you just find a, b, and c to do the fisheye-to-rectilinear mapping, and avoid having a special fisheye lens model?

De-fishing and barrel distortion correction both require computing the distance r_src from the center of the input image that corresponds to r_dest in the output image. For simple barrel distortion correction, you can usually find a polynomial that works, but for de-fishing the mapping function is

r_src = 2 sin(arctan(r_dest)/2).

(Here I'm assuming that the units are scaled so that the focal length of the lens is 1).

You can approximate this function with a polynomial for a small range of values of r_dest near zero, but any polynomial approximation will fail for larger values (think about what happens to r_src when r_dest gets large).

Last edited by troyz; 03-08-2009 at 08:16 PM.
03-10-2009, 08:37 AM   #3
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Original Poster
troyz, thanks for the help. It seems the conclusion, then, is that Bibble's stock distortion correction will not work, and that I need to get a fisheye correction plugin (e.g. Percy, IIRC) or mess with correction in an external program if I want to un-fish the images.
03-10-2009, 10:01 AM   #4
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I have used PTLens for simple de-fishing, but the results are not great unless you are only doing partial de-fishing. I have discovered fisheye hemi and love the results, though it is not producing a rectilinear image.

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