We will assume that the stereo rig is calibrated, that is the
PPMs are known. This assumption is not strictly necessary
[5, 8], but leads to a simpler
technique. The idea behind rectification [2] is to define
two new perspective matrices which preserve the optical centres but
with image planes parallel to the baseline. In addition we want the
epipolar line of the point 
in the right
image to be the horizontal line 
in the left image.
Consider Fig. 4 where the old retinal plane 
and the new one
are depicted: image rectification is the operation of
transforming from to .
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