Code Immersion

Once you have defined the better-draw stuff (see this post) you can do some simple animation by first doing some drawing, then erasing, then drawing again, etc. You should call sleep/yield between each draw and the next erase, both to allow the OS to actually draw the image and to make it stay visible for a bit before you erase it and redraw it elsewhere. Something like (sleep/yield 0.001) might be a good first stab (that’s supposed to sleep for 1/1000 of a second). There are of course fancier ways to make animations that will be smoother, etc., but this is simple and can be fun.

As an example, let’s first define a procedure that draws a guy:

(define draw-guy
  (lambda (x y)
    (ellipse x y 50 50 88 156 174 1.0)
    (ellipse (+ x 15) (+ y 10) 10 10 20 50 150 1.0)
    (ellipse (+ x 30) (+ y 10) 10 10 20 50 150 1.0)
    (ellipse (+ x 12) (+ y 25) 30 15 220 50 50 1.0)))

To run this you first want to show the window — (show-window) — and if you’ve previously drawn in it clear it — (clear). Then call (draw-guy).

Now let’s write a procedure to erase the guy. Notice that I make this circle a little bigger, because the “smoothing” in the drawing code, which gets rid of jaggies around circles, etc., “leaks” a little bit of color into adjacent pixels:

(define erase-guy
  (lambda (x y)
    (ellipse (- x 5) (- y 5) 60 60 255 255 255 1.0)))

Now I can make a guy move in a diagonal line using a “for” loop like this:

(define move-guy
  (lambda ()
    (for ((n (in-range 0 100)))
      (draw-guy (* n 2) (* n 3))
      (sleep/yield 0.01)
      (unless (>= n 99)
        (erase-guy (* n 2) (* n 3))))))

Again, you should show and clear the window, and then call (move-guy).

Of course we can move him in fancier ways too, by doing something like this:

(define wavy-guy
  (lambda ()
    (for ((x (in-range 0 500)))
      (let ((y (+ 200 (* 200 (sin (/ x 50.0))))))
      (draw-guy x y)
      (sleep/yield 0.0005)
      (unless (>= x 499)
        (erase-guy x y))))))

This entry was posted on October 16, 2009 at 3:34 pm and is filed under Uncategorized.