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added a para about how the pinhole is drawn

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Robby Findler 2010-09-06 06:44:45 -05:00
parent e3b51e8cf4
commit 4b14e1a855
1 changed files with 9 additions and 1 deletions
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collects/teachpack/2htdp/scribblings/image.scrbl
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@ -1278,8 +1278,16 @@ Two images are equal if they draw exactly the same way, at their current size
A pinhole is an optional property of an image that identifies a point somewhere
in the image. The pinhole can then be used to facilitate overlaying images by
lining them up on the their pinholes. When an image has a pinhole, the pinhole
lining them up on the their pinholes.
When an image has a pinhole, the pinhole
is drawn with crosshairs drawn across the image.
The crosshairs are drawn with a two one pixel wide black lines and two one pixel wide white lines,
where the black lines is drawn .5 pixels to the left and above the pinhole, and the
white lines are drawn .5 pixels to the right and below the pinhole.
Accordingly, when the pixel is on an integral coordinate, then black and white lines all
take up a single pixel and in the center of their intersections is the actual pinholes.
See @secref["nitty-gritty"] for more details about pixels.
@defproc[(center-pinhole [image image?]) image?]{
Creates a pinhole in @racket[image] at its center.