;;linear interpolation
;;given: 2d points p1 = (x1 y1) & p2 = (x2 y2)
;;       x3 value to interpolate x1<x3<x2
;;return y3 interpolated y value
(defun linterp (p1 p2 x3 / x1 x2 y1 y2)
  (setq x1 (float(car p1)) x2 (car p2) y1 (float(cadr p1)) y2 (cadr p2))
  (if (or (< x1 x3 x2) (< x2 x3 x1))
    (+ y1 (*(/(- y2 y1)(- x2 x1))(- x3 x1)))
  );if
);linterp