05lines.scm

;; transformation matrix
(define m11@ 64) (define m12@ 0)  (define m13@ 0)
(define m21@ 0)  (define m22@ 64) (define m23@ 0)

(define (main)
  (define speed 4)
  (define angle 0)

  (while 1
    (fill-rect 15 0 0 W H)

    (dotimes (i COLOR:COUNT)
      (make-rotation-matrix (+ angle i))
      ;; compensate Mode 13h distortion by scaling the y axis by a factor of 5/6
      (set! m21@ (/ (* m21@ 5) 6))
      (set! m22@ (/ (* m22@ 5) 6))
      ;; move to screen center
      (set! m13@ (>> W 1))
      (set! m23@ (>> H 1))
      ;; draw two line segments
      (line i 0 0 35 0)
      (line i 60 0 90 0))
    (pause-frames 1)

    (set! angle (add1 angle))))

(define (make-rotation-matrix angle)
  ;; sin@ and cos@ return result in 10.6 fixed-point format
  ;; as for the input, 128 ~ pi (with wrap-around)
  (define the-sin@ (sin@ angle))
  (define the-cos@ (cos@ angle))

  ;; rotation matrix is:
  ;;   [cos -sin  0]
  ;;   [sin  cos  0]
  ;; TODO: coordinate system to be reviewed
  (set! m11@ the-cos@)
  (set! m12@ (- 0 the-sin@))
  (set! m13@ 0)
  (set! m21@ the-sin@)
  (set! m22@ the-cos@)
  (set! m23@ 0))

;; draw a transformed line;
;; matrix values are 10.6 fixed-point so we use the fixed-point mul@ operator
(define (line color x1 y1 x2 y2)
  (draw-line color
    (+ (+ (mul@ m11@ x1) (mul@ m12@ y1)) m13@) (+ (+ (mul@ m21@ x1) (mul@ m22@ y1)) m23@)
    (+ (+ (mul@ m11@ x2) (mul@ m12@ y2)) m13@) (+ (+ (mul@ m21@ x2) (mul@ m22@ y2)) m23@)))