Re: the right way to draw in a drawingarea

[Paul Davis <pjdavis engineering uiowa edu>]

I've done something similar to this.  It was just basic drawing with the 
mouse.

I did the standard inheriting from a Gtk::DrawingArea and overriding the 
on_expose_event.
In the mouse even signals I just added the notified the drawing area 
with the necessary information( simple x,y coordinates if I recall )
then just call queue_redraw()

You could probably make this a bit more efficient by only queuing 
(queueing? ) the redraw in a rectangle of some small area around the 
mouse event or something.  But it worked well without flickering.

If that doesn't make any sense I can try and dig up that code 
somewhere.  But the main idea is to just queue redraws.

Cheers,
Paul

Amadeus W. M. wrote:

> I need to draw stuff (shapes rectangles, ellipses, splines, points,
> etc), overlayed on top of an image, and I want to update the display in
> real time, as the mouse moves. This is similar to doing a selection in
> gimp. The way I do this right now is as follows:
>
> The background image is in a Gdk::Pixbuf. I also have a Gtk::DrawingArea
> in which I draw my shapes. The display needs to be repainted upon each
> expose event, but upon mouse movement as well. So upon each mouse movement
> event I render the pixbuf and the shapes into the Darea->get_window(). 
> This works ok, but when the user draws something by dragging the mouse,
> the display flickers, even when drawing a simple square.
>
> Now this does not happen in gimp: it does not happen e.g. when a selection
> is made, nor when I move the nodes of a path, even though the display must
> be re-drawn as the mouse moves, etc. The window is steady and the
> drawing smooth. 
>
> So I guess there's a better way of overlaying shapes on top of an image,
> than the obvious Pixbuf + Darea method. Does anybody know anything about
> this? 
>
> Thanks!
>
>
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