From: ehgasm2@uts.mcc.ac.uk (Simon Marshall)
Subject: How do I compensate for photographic viewpoint and distortion?
Reply-To: S.Marshall@dcs.hull.ac.uk
Organization: Manchester Computing Centre, Manchester, England
Lines: 42

Hi to all out there.  We have this problem, and I'm not certain I'm solving it
in the correct way.  I was wondering if anyone can shed light on this, or point
me in the right place to look...

We have an X-ray imaging camera and a metallic tube with a cylindrical hole
passing through it at a right angle to the tube's axis:

                                                          |
                                     ||                   [ image
                                                          |
  X-ray source ]                     ||                   | screen
                            metallic || tube              |
                                     ||                   |
                                                          |

We know source--screen centre distance, radius of the tube, radius of the hole.

We do some calculations based on the image of the hole on the screen.  However,
the calculations are mathematically highly complex, and must assume that the
object's hole projects an image (resembling an ellipse if the tube is not
parallel to the screen) in the centre of the screen.  However, it is unlikely
that the object is placed so conveniently.  

Firstly, we must transform the major and minor axis of the ellipse.  I cannot
know what the angle between the tube and screen is.  Do I have to assume that
they are parallel to do the transformation?  How do I do this transformation?

Secondly, there is a distortion of the image due to the screen being planar
(the source--screen distance increases as we move away from the centre of the
screen).  How can I compensate the ellipse's axis for this image distortion?

So, please can anyone give us a few pointers here?  How do we transform the
image so it appears as it would if it were in the centre of the screen, and how
do I deal with distortion due to the shape of the screen?

We'd appreciate any help, either posted or emailed.

Thanks in advance, Simon.
-- 
Simon Marshall, Dept. of Computer Science, University of Hull, Hull HU6 7RX, UK
  "Football isn't about life and death.  It's more important than that." Bill
Email: S.Marshall@cs.hull.ac.uk   Phone: +44 482 465951  Fax: 466666   Shankley