Re: the right way to draw in a drawingarea

["Amadeus W. M." <amadeus84 cablespeed com>]

Thanks for the answer. What is this queue_redraw() function? I did a grep
-ri in /usr/include and I didn't find anything. Or is it a user defined
function which simply draws the items that need to be drawn. If so, when
do you call it? Do you not call it from within the mouse motion handler?


My mouse motion handler looks like this:

bool DisplayWindow::on_Darea_motion_notify_event(GdkEventMotion *ev)
{
    // int left,right,top,bot are DisplayWindow members.

    // draw whatever is on already, including pixbuf.
    update_display();

    // get coordinates of bottom right corner.
    right=ev->x; 
    if(left>right) swap(left,right); 
    bot=ev->y;
    if(top>bot) swap(top,bot);

    // (continuously) draw a rectangle.
    if(ev->state & Gdk::BUTTON1_MASK)
        Darea->get_window()->draw_rectangle(gc_, 0, left, top, 
						    right-left, bot-top);   
}

Is this what you had in mind? The problem stems from my update_display()
which renders the pixbuf into the Darea's window. It has to, because I
only want a single rectangle to be visible at the time. And it flickers.

Rendering only a smaller part of the pixbuf seems like a good idea, I hope
it's worth the trouble.





On Mon, 10 Oct 2005 20:06:46 -0500, Paul Davis wrote:

> 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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