[plt-scheme] Flicker-free redrawing of canvas

At Tue, 31 May 2005 05:44:19 -0700 (PDT), Noel Welsh wrote:
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> 
> Hi all,
> 
> I'm implementing Pacman using MrEd, so I can try out some
> AI ideas on top of it.  It works, but the screen flickers
> horribly.  Clearly this yields unacceptable working
> conditions for my AI.  I've tried using double buffering --
> drawing to an off screen bitmap% and then dumping that
> bitmap to the canvas's dc<%> using the draw-bitmap method. 
> It still flickers.  This code is thus:
> 
>      (define/override (on-paint)
>         (let ((dc (send this get-dc)))
>           ;; draw everything on the bitmap, then bulk copy
>           ;; the bitmap to the current display.  Basically
>           ;; double-buffering.
>           (send bitmap-dc clear)
>           (draw-map bitmap-dc)
>           (draw-pacman pacman bitmap-dc)
>           (send dc draw-bitmap bitmap 0 0 'solid
> background-colour #f)
>           ;(send dc clear)
>           ;(draw-pacman pacman dc)
>           ))
> 
> Is there any way I can I avoid this problem?  

Probably you also want to avoid creating the bitmap on each call to
on-paint and instead control when new bitmaps are created yourself
(since on-paint is called by system).

Robby