The standard set of parameters required to define a CCD camera are;

- focal length (
**f**) - image centres ()
- aspect ratios ()
- radial distortion terms

In order to define a stereo camera system two such sets of parameters
are required (one for each camera **L** and **R**) together with
additional parameters describing the between
camera transformation which is best described using a minimum of
six degrees of freedom. For this we use the last three parameters of the
quaternion where
describing the camera rotation, ( the last free parameter is recovered from
the normalisation constraint ), plus the interoccular
separation **I** plus two components of the translation direction vector
.

However, the relatively simple geometric behaviour of an SEM stereo system makes it possible to describe the allowable epi-polar geometry in terms of a small number of free parameters:

- relative image centres ().
- affine image rotation ().
- focal ratio ( ).

Using a two-dimensional Hough technique, candidate matches from a stereo corner matcher (Figure 11a) can be used to identify the remaining degrees of freedom (Figure 10). The final set of matches which are consistent with this solution are shown in Figure 11b.

**Figure 10:** Output from the Hough matcher

**Figure 11:** Reprojected candidate matches a) before b) after hough filtering

** Acknowledgements**

The authors would like to thank A.Hill, A.Brett and J.Schumm for their help with the LaTeX.

Fri Mar 28 14:12:50 GMT 1997