This correspondence discusses computing the pose of a model from three
matching point pairs under weak-perspective projection. A new approac
h to the problem that is motivated geometrically is described. Like pr
evious methods, the method here involves solving a biquadratic equatio
n, but here the biquadratic's solutions, comprised of an actual and a
false solution, are interpreted graphically. The final equations take
a new form, which leads to a simple expression for the image position
of any unmatched model point.