Re: the right way to draw in a drawingarea

[Paul Davis <pjdavis engineering uiowa edu>]

Amadeus W. M. wrote:

> On Tue, 11 Oct 2005 03:16:54 -0500, Paul Davis wrote:
>
>  
>
> > Not at all.
> >
> > My bad about that method, its actually queue_draw()
> > http://www.gtkmm.org/docs/gtkmm-2.4/docs/reference/html/classGtk_1_1Widget.html#a153
> >
> > I can't really explain this too well cause I haven't really read all the 
> > specifics on how gtk does its drawing of widgets and the such.  
> > Basically, you want to let gtk decide when it needs an update.  
> > queue_draw just says, please update now.  When you get into repainting 
> > the entire screen you'll get the flashes you menitoned.
> >
> > Oh and just to make sure, you aren't doing anything silly like clearing 
> > the widget with something like
> > DrawRectangle( background, 0, 0, WIDTH, HEIGHT )  (obviously not a real 
> > function call, just illustrating the point) cause that would
> > also introduce flickering.
> >
> > The way to do this is to make your on_expose_event draw the box.
> >
> > ie:
> >
> > void on_expose_event( GdkEventExpose ev )
> > {
> >    //Draw background pixbuf.
> >    update_display() ;
> >
> >    //draw rectangle
> >    if( left > 0 && top > 0 && right > 0 && bot > 0 )
> >         Darea->get_window()->draw_rectangle(gc_, 0, left, top,
> >                right-left, bot-top);
> > }
> >
> > Then add ( and connect this to the button press signal obviously. )
> > on_Darea_button_press_event( GdkEventButton *ev )
> > {
> >       if( ev->state & Gdk::BUTTON1_MASK )
> >       {
> >          left = ev->x ;
> >          top = ev->y ;
> >
> >          right = ev->x ;
> >          bottom = ev->y ;
> >       }
> >       else
> >       {
> >             //Erase rectangle.
> >             left = -1 ;
> >             top = -1 ;
> >
> >             right = -1 ;
> >             bottom = -1 ;
> >       }
> >
> >       Darea->queue_draw() ;
> > }
> >
> > And then in your
> > on_Darea_motion_notify_event( GdkEventButton* ev )
> > {
> >    right=ev->x;
> >    if(left>right) swap(left,right);
> >    bot=ev->y;
> >    if(top>bot) swap(top,bot);
> >
> >    Darea->queue_draw() ;
> > } 
> >    
>
> Darea->queue_draw() works in mysterious ways, but it definitely works
> better. By not calling queue_draw() rather than my custom update_display()
> (which renders the entire pixbuf into the background) from the
> motion_notify_event() handler, stops the flickering: neither the
> background image nor the already drawn shapes flicker anymore. 
>
> And the rectangle or the ellipse being drawn by dragging with the mouse
> does update dynamically, as I want it to. This was what I didn't
> understand - how it was possible to draw a changing rectangle without
> repainting the pixbuf into the background from within
> motion_notify_event(). I still don't understand how queue_draw() does it,
> but I'm glad it works.
>
> Indeed, not hard to fix, if you know about the right function calls. Or if
> you ask the right people. Thank you all for the help.
>
>
>
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>  
The complexities abound.  Insert apropriate quote here.  I can't explain 
the gtk drawing system because I don't entirely understand it.  I'm glad 
hings are working.

Cheers,
Paul