[ltk-user] Cannot access mouse coords relative to CANVAS widget

[Peter Herth]

Hi Jim,

to grab the mouse position in a canvas widget, I don't call the
screen-mouse functions, but rather bind the motion and button press
events. The callback gets passed the event structure which contains
the slots event-x and event-y which are canvas coordinates. Not only
are you getting the right values directly this way, but also it is
more efficient, as you don't have to poll the mouse position - you get
updates automatically when it changes. In your case you could either
choose to update the barrier on  mouse-move or alternatively, just
store the mouse coordinates in a variable you read inside your update
loop.

Peter

On Sat, Jul 7, 2012 at 6:05 PM, Jim W <james.robert.watson at gmail.com> wrote:
> I am working with SBCL for Linux on an AMD64 machine.
> Function ANIMTEST instantiates an LTK window with a CANVAS widget. Two items,
> BARRIER and FOLLOWER, live in the canvas. Both spin continuously, with BARRIER
> at the center of the canvas and FOLLOWER intended to follow the mouse, which is
> not working as intended. My first attempt (see comment) resulted in absolute
> screen coordinates of the mouse being interpreted as relative coordinates within
> the canvas with no account of the offset between the two. After searching
> through ltk.lisp and docs, I found SCREEN-MOUSE-X/Y (Second Attempt, see
> comment). I feel like I am using SCREEN-MOUSE-X & -Y according to the
> documentation, but why is it not working? = Note = The file that contains
> ANIMTEST and the packages that support it load and run with no errors. Functions
> I have defined (UCTK-BEAM, etc.) are tested and run fine.
>
> (defun animtest ()
>       "Test a spinning figure in LTK"
>       (with-ltk ()
>         (let* ((cnvs (make-instance 'canvas :width 400 :height 400))
>            (barrier (uctk-beam 200 200 40 20))
>            (follower (uctk-beam 0 40 40 20))
>            (slp-time 50) ; in ms
>            (bar-theta 0)
>            (fol-theta 0))
>           (labels ((update ()
>              (draw barrier nil)
>              (draw follower nil)
>              (incf bar-theta (/ pi 15))
>              (incf fol-theta (/ pi 15))
>              (geo:set-theta barrier bar-theta)
>              (geo:set-theta follower fol-theta)
>              (geo:set-center follower
>                      ;== FIRST ATTEMPT ==
>                      (cons (screen-mouse-x cnvs)
>                        (screen-mouse-y cnvs)))
>                      ; == SECOND ATTEMPT ==
>                      ;(cons (canvasx cnvs (screen-mouse-x cnvs))
>                     ;   (canvasy cnvs (screen-mouse-y cnvs))))
>              (after slp-time #'update)))
>         (pack cnvs :fill :both :expand 1)
>         (update)))))
> Thanks in advance!
>
>
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