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Documentation for fold-image and fold-image/extra.

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Stephen Bloch 2012-04-28 18:00:30 -04:00
parent 2df9ca8789
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collects/picturing-programs/picturing-programs.scrbl
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@ -418,6 +418,48 @@ dithered are still dithered.
produces a @racket[my-picture]-shaped "window" on a color-gradient.
}
@defproc*[([(fold-image [f (-> color? any/c any/c)] [init any/c] [img image?]) any/c]
[(fold-image [f (-> natural-number/c natural-number/c color? any/c any/c)] [init any/c] [img image?]) any/c])]{
Summarizes information from all the pixels of an image.
The result is computed by applying f successively to each pixel, starting with @racket[init].
If @racket[f] accepts four parameters, it is called with the coordinates and color of each
pixel as well as the previously-accumulated result; if it accepts two parameters, it is
given just the color of each pixel and the previously-accumulated result.
You may not assume anything about the order in which the pixels are visited, only
that each pixel will be visited exactly once.
An example with a 2-parameter function:
@codeblock|{
; another-white : color number -> number
(define (another-white c old-total)
(+ old (if (color=? c "white") 1 0)))
; count-white-pixels : image -> number
(define (count-white-pixels pic)
(fold-image another-white 0 pic))}|
Note that the accumulator isn't restricted to be a number: it could be a structure or a list,
enabling you to compute the average color, or a histogram of colors, etc.
}
@defproc*[([(fold-image/extra [f (-> color? any/c any/c any/c)] [init any/c] [img image?] [extra any/c]) any/c]
[(fold-image/extra [f (-> natural-number/c natural-number/c color? any/c any/c any/c)] [init any/c] [img image?] [extra any/c]) any/c])]{
Like @racket[fold-image], but passes the @racket[extra] argument in as an additional argument in each call
to @racket[f]. This allows students who haven't learned closures yet to call @racket[fold-image] on an
operation that depends on a parameter to a containing function.
For example,
@codeblock|{
; another-of-color : color number color -> number
(define (another-of-color c old color-to-count)
(+ old (if (color=? c color-to-count) 1 0)))
; count-pixels-of-color : image color -> number
(define (count-pixels-of-color pic color-to-count)
(fold-image/extra count-pixels-of-color 0 pic))
}|
}
@defproc[(real->int [num real?])
integer?]{
Not specific to colors, but useful if you're building colors by arithmetic.