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added section on alpha blending to 2htdp/image guide

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Robby Findler 2013-01-01 22:24:33 -06:00
parent 0d0503bda7
commit 836ce95343
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collects/teachpack/2htdp/scribblings/image-guide.scrbl
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@ -199,6 +199,54 @@ a little.
(rotary-dial 2)]
@section{Alpha Blending}
With shapes that have opaque colors like @racket["red"] and @racket["blue"],
overlaying one on top completely blots out the one one the bottom.
For example, the red rectangle here completely covers the blue one.
@image-interaction[(overlay
(rectangle 60 100 "solid" (color 127 255 127))
(rectangle 100 60 "solid" (color 127 127 255)))]
But @racketmodname[2htdp/image] also supports colors that are not
completely opaque, via the (optional) fourth argument to @racket[color].
@image-interaction[(overlay
(rectangle 60 100 "solid" (color 0 255 0 127))
(rectangle 100 60 "solid" (color 0 0 255 127)))]
In this example, the color @racket[(color 0 255 0 127)] looks just
like the color @racket[(color 127 255 127)] when the background
is white. Since white is @racket[(color 255 255 255)], we end up
getting @racket[1/2] of @racket[255] for the red and blue components
and @racket[255] for the green one.
We can also use alpha blending to make some interesting effects.
For example, the function @racket[spin-alot] takes an image argument
and repeatedly places it on top of itself, rotating it each time by
@racket[1] degree.
@image-interaction[(define (spin-alot t)
(local [(define (spin-more i θ)
(cond
[(= θ 360) i]
[else
(spin-more (overlay i (rotate θ t))
(+ θ 1))]))]
(spin-more t 0)))]
Here are some uses of @racket[spin-alot], first showing the original
shape and then the spun shape.
@image-interaction[(rectangle 12 120 "solid" (color 0 0 255))
(spin-alot (rectangle 12 120 "solid" (color 0 0 255 1)))
(triangle 120 "solid" (color 0 0 255))
(spin-alot (triangle 120 "solid" (color 0 0 255 1)))
(isosceles-triangle 120 30 "solid" (color 0 0 255))
(spin-alot (isosceles-triangle 120 30 "solid" (color 0 0 255 1)))]
@section{Recursive Image Functions}
It is also possible to make interesting looking shapes with little recursive functions.