[ptx] HFOV, focal distance...

[Bruno Postle]

On Fri 03-Jun-2005 at 10:51 +0200, Lars Schimmer wrote:
> 
> So, now the question: WHAT are the a,b,c parameters exactely

They represent the radial distortion of the input images, typically
they are used to correct barrel distortion introduced by the camera
lens.  Here is a picture showing exaggerated barrel distortion:

  http://www.panotools.info/mediawiki/index.php?title=Barrel_distortion

> and how did you get those numbers?

There are several ways of doing it (somebody ought to do tutorials
for all this stuff):

- Taking a single photograph of a rectangular object, selecting lots
  of horizontal and vertical control points, then optimising roll,
  pitch, yaw, fov, a, b & c.

- Taking two or more overlapping photographs and selecting lots of
  normal control points, then optimising roll, pitch, yaw, fov, a, b
  & c.

  To get a really accurate measure of the fov (field of view), you
  have to take a full 360 degree panorama.

- Probably others I can't think of right now.

The fov, a, b & c parameters are fairly consistent between shots
with the same camera/lens combination so you can use them again and
again.  Though they can vary depending on the aperture size and the
focus distance, so I tend to recalculate them for every stitch.

The a & c parameters control distortion at the centre and edges of
the picture so are not very important, you can normally just zero
them and use the b parameter by itself.

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

They help the photos line up better.

-- 
Bruno