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 .