[plt-scheme] Question on Canvas object in GUI programming

Hello Matthias,

Really appreciate your answer provided! It is very clear, and I get it.

And I think of other scenarios:
One possible scene, if the picture is scaled up or down on the canvas?
One possible scene, if the user do some changes on the canvas, and then
click to get the pixel color?

I think for the first scene, I still can use the method you provided. But
for second case, I don't know how.
I wonder if I can get the bitmap-dc% from Canvas dc, or something similar
like that.
Can I get the pixels matrix of the current canvas operated?

thanks & regards,
Linda Lian

On 12/19/07, Matthias Felleisen <matthias at ccs.neu.edu> wrote:
>
>
> On Dec 18, 2007, at 5:08 AM, Lian DanHui wrote:
>
> > Hello,
> >
> > I want to implement something like this:
> >   1. Display a picture in a Canvas on a Frame;
> >   2. User can click on the picture;
> >   3. I want to get the color of the pixel that user clicked;
> >
> > For Canvas object,  Using what method or what strategy, that I can
> > get the pixel info?
> > Thanks a lot for the help!
>
> Here is a literal translation of your request into the most basic
> language. If you are a student and using world.ss, it is simpler. --
> Matthias
>
> #lang scheme/gui
>
> ;; obtain a bitmap and stick it into a bitmap dc
> (define plt
>   (make-object bitmap% (build-path (collection-path "icons")
> "plt.gif")))
> (define bm
>   (new bitmap-dc% [bitmap plt]))
>
> ;; --- set up a frame with a canvas in which to display this image
> (define frame
>   (new frame% [label "Test for Linda"]
>        [width (send plt get-width)] [height (send plt get-height)]))
>
> (define canvas (new
>                 ;; --- create a class on the fly that overrides the
> callback for
>                 ;; --- mouse events in a canvas; you could name it,
> if needed
>                 (class canvas%
>                   ;; Event -> Void
>                   ;; act on each mouse event
>                   (define/override (on-event e)
>                     (define kind (send e button-down?))
>                     (define x (send e get-x))
>                     (define y (send e get-y))
>                     (define c (new color%))
>                     (when kind
>                       (if (send bm get-pixel x y c)
>                           (printf "color is ~s ~s ~s\n"
>                                   (send c red) (send c green) (send
> c blue))
>                           (printf "no color"))))
>                   (super-new))
>                 ;;--- provide a frame and a paint method for
> displaying plt.gif
>                 [parent frame]
>                 [paint-callback (lambda (e dc) (send dc draw-bitmap
> plt 0 0))]))
>
> ;; --- run program run
> (send frame show #t)
>
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