[ptx] HFOV, focal distance...

[Terje Mathisen]

schimmer at cg.cs.tu-bs.de wrote:
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> Terje Mathisen wrote:
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> 
>>After literally days of calibration, I came up with the following
>>parameters:
>>
>>Max wide angle (7,5 mm, Portrait mode):
>>  a: 0,006312, b: -0,028406, c: 0,011234, HFOV: 39,654
>>
>>Max zoom (19,8 mm, Portrait):
>>  a: -0,000391, b: -0,002956, c: 0,003308, HFOV: 15,554
>>
>>
>>The tele parameters are particularly good, mosaics taken using those
>>usually show very close to zero residual control point errors after
>>optimization.
> 
> So, now the question: WHAT are the a,b,c parameters exactely and how did you get
> those numbers?

As others have replied, the (a,b,c) values correspond to a third-order
radial polynomial describing lens distortion. It is never exact, but can
give a pretty good approximation to the real behaviour of a given lens.

I measured it by printing a really big rectangular spreadsheet on an A0+
printer, i.e. the printout is about 1.4 x 1.0 m.

I then took a photo of this printout, after placing the camera on a
tripod and using a small mirror to make sure that the camera was
pointing at the actual center of the image.

On the single image I placed 100+ horizontal and vertical line control
points, then optimized everything (y,p,r,a,b,c) (remembering to select
rectiliear output!) except HFOV.

The HFOV value was retrieved by putting the (a,b,c) values from above
into a 360-degree pano, then optimizing only HFOV. This tweaked HFOV
value was of course imported back into the calibration project, where it
resulted in some very small changes in the (a,b,c) triplet.

> And at least, what meanings do they have for panorma creation?

It makes it much easier and faster to get good stitches, since you have
less variables left to optimize.

Terje

-- 
- <Terje.Mathisen at hda.hydro.com>
"almost all programming can be viewed as an exercise in caching"