flower.scm

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

(define (main)
  (define angle 0)
  (define speed 256)

  (define num-petals 12)
  (define angle-step 5461)    ;; 65536 / num-petals
  (define half-length (/ H 4))
  (define half-width 15)

  (while 1
    (fill-rect COLOR:WHITE 0 0 W H)

    (dotimes (i num-petals)
      (make-rotation-matrix (>> (+ angle (* angle-step i)) 8))
      ;; compensate Mode 13h distortion by scaling the x axis by a factor of 6/5
      (set! m11@ (/ (* m11@ 6) 5))
      (set! m12@ (/ (* m12@ 6) 5))
      ;; move to screen center
      (set! m13@ (>> W 1))
      (set! m23@ (>> H 1))
      ;; draw petal using a palette of pastel colours (56 to 79)
      (define color (+ 56 (* 2 i)))
      (tri color half-length        half-width
                 half-length        (- 0 half-width)
                 (* 2 half-length)  0)
      (tri color half-length        half-width
                 half-length        (- 0 half-width)
                 0                  0))
    (pause-frames 1)

    (cond (key-held? KEY:RIGHT) (set! half-length (+ half-length 1))
          (key-held? KEY:LEFT)  (set! half-length (- half-length 1)))
    (cond (key-held? KEY:UP)    (set! speed (+ speed 16))
          (key-held? KEY:DOWN)  (set! speed (- speed 16)))

    (set! angle (+ angle speed))))

(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 triangle;
;; matrix values are 10.6 fixed-point so we use the fixed-point mul@ operator
(define (tri color x1 y1 x2 y2 x3 y3)
  (fill-triangle 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@)
    (+ (+ (mul@ m11@ x3) (mul@ m12@ y3)) m13@) (+ (+ (mul@ m21@ x3) (mul@ m22@ y3)) m23@)))