P.P.S. Fred wrote: << A circle is rationally parameterised by [w,x,y] = [1+t^2, 1-t^2, 2t], the Cartesian coordinates of the conic then being [x/w, y/w].

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Just now I'm realizing that the [w,x,y] probably represent *projective* coordinates, which I apologize for not grasping at first. --Dan Sometimes the brain has a mind of its own.