The coordinate system is centered at the origin to simplify symmetry handling. All layout objects are generated relative to this centered frame.
(x0 (- (/ odLen 2.0)))
(y0 (- (/ odWid 2.0)))
(x1 (+ (/ odLen 2.0)))
(y1 (+ (/ odWid 2.0)))
This approach ensures that mirroring, abutment, and arraying operations behave predictably.