[racket] 2htdp/image Sierpinski

From: Robby Findler (robby at eecs.northwestern.edu)
Date: Sun Oct 31 10:24:30 EDT 2010

There are a few things going on here. When you say (carpet 300 "red"),
you're actually asking Racket to draw more than 42,000 squares, which
can take some time (plus more than 20,000 intermediate overlaid shapes
to work thru to get to the squares). Also, the redrawing you're seeing
isn't specific to this image. It happens for all of them, but you just
don't see it because the drawing typically finishes faster than you
can tell. (FWIW, the redrawing aspect of it will be slightly better in
Gracket2 in that it will do all that drawing offscreen instead of
letting you see those intermediate states.)

Overall, tho, we'd like to be able to support this kind of thing, but
we need to do a better job. As I'm writing this, I wonder if we could
exploit the sharing in the image. In your example, there are only 11
calls to 'square' that turn into those 42,000 different squares spread
around. If we could detect and exploit the sharing somehow that could
lead to a big speed up. (But this is just a half-formed thought;
nothing may come of it.)


On Sat, Oct 30, 2010 at 11:18 PM, Gregor Kiczales <gregor at cs.ubc.ca> wrote:
> In 2htdp/image the implementations of Sierpinski triangle and carpet are nice and simple, which is great for using them in class.
> (define (carpet size c)
>  (if (<= size 2)
>      (square size "outline" c)
>      (local [(define sub (carpet (/ size 3) c))
>              (define ctr (square (/ size 3) "solid" "white"))]
>        (overlay (square size "outline" c)
>                 (above (beside sub sub sub)
>                        (beside sub ctr sub)
>                        (beside sub sub sub))))))
> But...  (carpet 200 "red") produces some strange behavior on my Mac. (carpet 300 "red") is even worse.  The image draws very slowly, which I assume comes from the way the image is actually composed of all the composed images rather than a single bitmap. But, stranger than that, it seems to need to draw itself several times.
> Also I would have thought that 'forcing' the image by actually drawing it would save the bitmap so that future drawing was faster. But I may not understand how this is actually working.
> Is this style of using above, beside and friends to make something this big intended to work? Or is there a different style I should be using?
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