Question

I wrote an app that uses Cairo to draw things on screen (on a Gtk::DrawingArea, to be exact). It needs to redraw everything frequently. It turns out, that despite the draphics drawn are very simple, the X server uses LOTS of CPU when redrawing, and the applications works terribly slow. Is there any way to speed this up? Or maybe I shouldn’t use DrawingArea and some other widget?

What I draw is set of rectangles, which user can move around by dragging them with mouse. The whole drawing is done withing on_expose_event, but as the mouse pointer moves around (with button pressed), I call queue_draw() to refresh drawing.

Answer 1

I finally forced to use maximally 25 fps, by using a lock flag.

bool lock = 0;
bool needs_redraw = 0;

void Redraw(){
    if(lock){
        needs_redraw = 1;
        return;
    }

    //draw image to a surface

    needs_redraw = 0;
    lock = 1;
    Glib::signal_timeout().connect(Unlock, 20);

    queue_draw();
}

bool Unlock(){
    lock = 0;
    if(needs_redraw) Redraw();
    return false;
}

void on_expose_event(something){
    //copy image from surface to widget's context
}

This is a sample code, but that’s the idea. It will disallow redraw to be done more often then once per 20 ms.