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add a section to the image guide about rotation and centers

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Robby Findler 2013-07-02 05:14:40 -05:00
parent ebc6a6618e
commit 349072c623
2 changed files with 50 additions and 2 deletions
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46
pkgs/htdp/teachpack/2htdp/scribblings/image-guide.scrbl
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@ -324,6 +324,52 @@ And then put three of them together to form the Koch snowflake.
(rotate -60 (koch-curve 5)))
(flip-vertical (koch-curve 5)))]
@section[#:tag "rotate-center"]{Rotating and Image Centers}
When rotating an image, some times the image looks best when it
rotates around a point that is not the center of the image. The
@racket[rotate] function, however, just rotates the image as
a whole, effectively rotating it around its center.
For example, imagine a game where the hero is represented
as a triangle:
@image-interaction[(define (hero α)
(triangle 30 "solid" (color 255 0 0 α)))
(hero 255)]
rotating the hero at the prompt looks reasonable:
@image-interaction[(rotate 10 (hero 255))
(rotate 20 (hero 255))
(rotate 30 (hero 255))]
but if the hero has to appear to spin in place, then it will not look
right, as you can kind of see if we use α-blending to represent
old positions of the hero:
@image-interaction[(overlay (rotate 0 (hero 255))
(rotate 10 (hero 125))
(rotate 20 (hero 100))
(rotate 30 (hero 75))
(rotate 40 (hero 50))
(rotate 50 (hero 25)))]
What we'd really want is for the hero to appear to rotate around
the centroid of the triangle. To achieve this effect, we can put
the hero onto a transparent rectangle such that the center of the whole
image is the centroid of the triangle:
@image-interaction[(define (hero-on-blank α)
(define the-hero (hero α))
(define w (image-width the-hero))
(define h (image-height the-hero))
(define d (max w h))
(define dx (/ w 2)) ; centroid
(define dy (* 2/3 h)) ; centroid
(define blank (rectangle (* 2 d) (* 2 d) "solid" (color 255 255 255 0)))
(place-image/align the-hero (- d dx) (- d dy) "left" "top" blank))]
and now the rotating hero looks reasonable:
@image-interaction[(overlay (rotate 0 (hero-on-blank 255))
(rotate 10 (hero-on-blank 125))
(rotate 20 (hero-on-blank 100))
(rotate 30 (hero-on-blank 75))
(rotate 40 (hero-on-blank 50))
(rotate 50 (hero-on-blank 25)))]
@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

6
pkgs/htdp/teachpack/2htdp/scribblings/image.scrbl
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@ -1144,7 +1144,7 @@ the parts that fit onto @racket[scene].
@defproc[(rotate [angle angle?] [image image?]) image?]{
Rotates @racket[image] by @racket[angle] degrees in a counter-clockwise direction.
@image-examples[(rotate 45 (ellipse 60 20 "solid" "olivedrab"))
(rotate 5 (rectangle 50 50 "outline" "black"))
(rotate 45
@ -1153,11 +1153,13 @@ the parts that fit onto @racket[scene].
(rectangle 40 20 "solid" "darkseagreen")
(rectangle 20 100 "solid" "darkseagreen")))]
See also @seclink["rotate-center"].
}
@defproc[(scale [factor (and/c real? positive?)] [image image?]) image?]{
Scales @racket[image] by @racket[factor].
Scales @racket[image] by @racket[factor].
The pen sizes are also scaled and thus draw thicker (or thinner)
lines than the original image, unless the pen was size