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

Thanks for your hints, Matthias,

I read the document, and I think I can get a solution from
frame% -> editor-canvas% -> image-snip% -> get-bitmap

I think I still need learn and practice more, to know better about the
canvas, editor, pasteboard, editor-canvas etc.

On 12/19/07, Matthias Felleisen <matthias at ccs.neu.edu> wrote:
>
> Now it's time to use the help desk and to search for terms like
> canvas, editor, pasteboard, editor-canvas and just chase the links.
> Everything is hyperlinked, so you can find your way around easily,
> not that this means everything is easy. -- Matthias
>
>
>
> On Dec 18, 2007, at 10:00 PM, Lian DanHui wrote:
>
> > 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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