68e005fb2c
Also, use keywords for `make-pen' and `make-brush'. Adding `make-pen' and `make-color' creates many conflicts among teaching libraries, such as `2htdp/image'. These are easy to fix up in the tree, but adding such obvious names to `racket/draw' may create other compatibility problems, so we might have to reconsider the names. In consultation with Asumu.
128 lines
5.6 KiB
Racket
128 lines
5.6 KiB
Racket
#lang scribble/doc
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@(require (for-label 2htdp/image
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(except-in lang/htdp-beginner posn make-posn posn? posn-x posn-y image?)
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lang/posn
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(except-in racket/gui/base make-color make-pen))
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"shared.rkt"
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"img-eval.rkt"
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scribble/decode
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scribble/manual
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scribble/eval)
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@(define guide-eval (make-img-eval))
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@(define-syntax-rule
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(image-examples exp ...)
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(examples #:eval guide-eval exp ...))
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@(define-syntax-rule
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(image-interaction exp ...)
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(interaction #:eval guide-eval exp ...))
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@(define-syntax-rule
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(image-interaction/margin num exp)
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(begin
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(racketinput exp)
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(guide-eval '(extra-margin num))
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(interaction-eval-show #:eval guide-eval exp)
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(guide-eval '(extra-margin 0))))
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@title[#:tag "image-guide"]{Image Guide}
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This section introduces the @racketmodname[2htdp/image] library
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through a series of increasingly complex image constructions.
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@section[#:tag "nitty-gritty"]{The nitty gritty of pixels, pens, and lines}
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The image library treats coordinates as if they are in the upper-left corner
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of each pixel, and infinitesimally small (unlike pixels which have some area).
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Thus, when drawing a solid @racket[square] of whose side-length is 10, the image library
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colors in all of the pixels enclosed by the @racket[square] starting at the upper
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left corner of (0,0) and going down to the upper left corner of (10,10),
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so the pixel whose upper left at (9,9) is colored in, but the pixel
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at (10,10) is not. All told, 100 pixels get colored in, just as expected for
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a @racket[square] with a side length of 10.
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When drawing lines, however, things get a bit more complex. Specifically,
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imagine drawing the outline of that rectangle. Since the border is
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between the pixels, there really isn't a natural pixel to draw to indicate
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the border. Accordingly, when drawing an outline @racket[square] (without a
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@racket[pen] specification, but just a color as the last argument),
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the image library uses a pen whose width is 1 pixel, but draws a line
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centered at the point (0.5,0.5) that goes down and around to the point (10.5,10.5).
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This means that the outline slightly exceeds the bounding box of the shape.
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Specifically, the upper and left-hand lines around the square are within
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the bounding box, but the lower and right-hand lines are just outside.
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This kind of rectangle is useful when putting rectangles next to each other
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and avoiding extra thick lines on the interior. For example, consider
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building a grid like this:
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@image-interaction[(let* ([s (rectangle 20 20 "outline" "black")]
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[r (beside s s s s s s)])
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(above r r r r r r))]
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The reason interior lines in this grid are the same thickness as the lines around the edge
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is because the rectangles overlap with each other.
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That is, the upper-left rectangle's right edge is right on top of the
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next rectangle's left edge.
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The special case of adding 0.5 to each coordinate when drawing the square
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applies to all outline polygon-based shapes that just pass color,
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but does not apply when a @racket[pen]
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is passed as the last argument to create the shape.
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For example, if using a pen of thickness 2 to draw a rectangle, we get a
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shape that has a border drawing the row of pixels just inside and just outside
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the shape. One might imagine that a pen of thickness 1 would draw an outline around the shape with
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a 1 pixel thick line, but this would require 1/2 of each pixel to be illuminated, something
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that is not possible. Instead, the same pixels are lit up as with the 2 pixel wide pen, but
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with only 1/2 of the intensity of the color. So a 1 pixel wide black @racket[pen] object draws
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a 2 pixel wide outline, but in gray.
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@image-interaction/margin[2
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(rectangle
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20 20 "outline"
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(make-pen "black" 1 "solid" "round" "round"))]
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When combining pens and cropping, we can make a rectangle that has a line that is one pixel
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wide, but where the line is drawn entirely within the rectangle. This rectangle has a two-pixel wide
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black pen, but we can crop out the outer portion of the pen.
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@image-interaction[(crop
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0 0 20 20
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(rectangle
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20 20 "outline"
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(make-pen "black" 2 "solid" "round" "round")))]
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Using that we can build a grid now too, but this grid has doubled lines on the
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interior.
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@image-interaction[(let* ([s (crop
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0 0 20 20
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(rectangle
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20 20 "outline"
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(make-pen "black" 2 "solid" "round" "round")))]
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[r (beside s s s s s s)])
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(above r r r r r r))]
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While this kind of rectangle is not useful for building grids, it
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is important to be able to build rectangles whose drawing does not
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exceed its bounding box. Specifically, this kind of drawing is used
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by @racket[frame] and @racket[empty-scene] so that the extra drawn pixels
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are not lost if the image is later clipped to its bounding box.
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When using @racket[image->color-list] with outline shapes, the results
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can be surprising for the same reasons. For example, a
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2x2 black, outline rectangle consists of nine black pixels, as discussed above,
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but since @racket[image->color-list] only returns the pixels that are
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within the bounding box, we see only three black pixels and one white one.
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@image-interaction[(image->color-list
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(rectangle 2 2 "outline" "black"))]
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The black pixels are (most of) the upper and left edge of the outline shape,
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and the one white pixel is the pixel in the middle of the shape.
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