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some edits to nitty-gritty section and fix up the koch curve names

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(and actually draw the snowflake)
This commit is contained in:
Robby Findler 2012-12-28 10:53:10 -06:00
parent e212c1c279
commit 7b9879c0e6
1 changed files with 17 additions and 19 deletions
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36
collects/teachpack/2htdp/scribblings/image-guide.scrbl
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@ -120,20 +120,26 @@ library, e.g.:
@image-interaction[(sierpinski-carpet 4)]
@image-interaction[(define (koch-snowflake n)
@image-interaction[(define (koch-curve n)
(cond
[(zero? n) (square 1 "solid" "black")]
[else
(define smaller (koch-snowflake (- n 1)))
(define smaller (koch-curve (- n 1)))
(beside/align "bottom"
smaller
(rotate 60 smaller)
(rotate -60 smaller)
smaller)]))]
@image-interaction[(koch-snowflake 5)]
@image-interaction[(koch-curve 5)]
@section[#:tag "nitty-gritty"]{The nitty gritty of pixels, pens, and lines}
@image-interaction[(above
(beside
(rotate 60 (koch-curve 5))
(rotate -60 (koch-curve 5)))
(flip-vertical (koch-curve 5)))]
@section[#:tag "nitty-gritty"]{The Nitty Gritty of Pixels, Pens, and Lines}
The image library treats coordinates as if they are in the upper-left corner
of each pixel, and infinitesimally small (unlike pixels which have some area).
@ -163,8 +169,7 @@ building a grid like this:
@image-interaction[
(define s1 (square 20 'outline 'black))
(define r1 (beside s1 s1 s1 s1 s1 s1))
(define q1 (above r1 r1 r1 r1 r1 r1))
q1
(above r1 r1 r1 r1 r1 r1)
]
The reason interior lines in this grid are the same thickness as the lines around the edge
@ -184,30 +189,23 @@ that is not possible. Instead, the same pixels are lit up as with the 2 pixel wi
with only 1/2 of the intensity of the color. So a 1 pixel wide black @racket[pen] object draws
a 2 pixel wide outline, but in gray.
@image-interaction/margin[2
(rectangle
20 20 "outline"
(make-pen "black" 1 "solid" "round" "round"))]
@image-interaction[(define p1 (make-pen "black" 1 "solid" "round" "round"))]
@image-interaction/margin[2 (rectangle 20 20 "outline" p1)]
When combining pens and cropping, we can make a rectangle that has a line that is one pixel
wide, but where the line is drawn entirely within the rectangle. This rectangle has a two-pixel wide
black pen, but we can crop out the outer portion of the pen.
@image-interaction[(crop
0 0 20 20
(rectangle
20 20 "outline"
(make-pen "black" 2 "solid" "round" "round")))]
@image-interaction[(define p2 (make-pen "black" 2 "solid" "round" "round"))
(define s2 (crop 0 0 20 20 (rectangle 20 20 "outline" p2)))
s2]
Using that we can build a grid now too, but this grid has doubled lines on the
interior.
@image-interaction[
(define pn (make-pen "black" 2 "solid" "round" "round"))
(define s2 (crop 0 0 20 20 (rectangle 20 20 "outline" pn)))
(define r2 (beside s2 s2 s2 s2 s2 s2))
(define q2 (above r2 r2 r2 r2 r2 r2))
q2
(above r2 r2 r2 r2 r2 r2)
]
While this kind of rectangle is not useful for building grids, it