This should be
(overlay/places "center" "middle"
(rotate 0 a)
(rotate 20 a)
(rotate 40 a)
(rotate 60 a))
not
.. "middle" "middle" ...
correct?
On Nov 14, 2009, at 1:18 AM, John Clements wrote:
>
> On Sep 26, 2009, at 6:29 AM, Matthias Felleisen wrote:
>
>>
>>
>> 1. htdp/world is deprecated, use 2htdp/universe instead.
>>
>> 2. we consider the pinhole approach to our image library as a mistake. To
>> solve the problem Robby is working on 2htdp/image, a replacement for the
>> image library. When it comes out (end of semester probably) it will support
>> rotate and other such operations, plus comparisons will be much faster.
>
> I'm surprised to say it, but right now I'm *really missing the pinhole*.
>
> I'm trying to write a function in 2htdp/image that draws, say, 36 copies of a
> shape arranged in a big "circle" around a center. With pinholes, this was
> easy. Move the pinhole way off to the left, then rotate the thing around the
> pinhole by 10 degrees, 20 degrees, etc. Overlay them all, and you're done. [*]
>
> With the new approach, I'm finding this nearly impossible. The natural
> approach involves, for instance, putting the thing "beside" a long horizontal
> space, and then rotating it. The problem is that it doesn't rotate about a
> fixed point. To see the problem, consider this program:
>
> (require 2htdp/image)
>
> (define a (beside (rectangle 150 2 "solid" "black") (ellipse 10 50 "solid"
> "purple")))
>
> (overlay/places "middle" "middle"
> (rotate 0 a)
> (rotate 20 a)
> (rotate 40 a)
> (rotate 60 a))
>
> The problem is that mapping a point in the pre-rotation shape to a point in
> the post-rotation shape is hard.
>
> ...
>
> After wrestling with this for quite a while, I have a solution of sorts; you
> have to overlay the shape you want to rotate on a large outline circle in
> such a way that the shape is entirely inside the circle. Rotating this shape
> does not change the bounding box at all, meaning that you can overlay them
> correctly. Needless to say, computing the size of the circle required is a
> big pain.
>
> E.G.:
>
> (define b (overlay/xy (circle 200 "outline" "blue") 200 200 a))
>
> (overlay
> (rotate 0 b)
> (rotate 20 b)
> (rotate 40 b)
> (rotate 60 b))
>
> Am I missing something obvious?
>
> John
>
> [*] with the obvious caveat that the old version didn't rotate shapes at all.
>
>
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