GTK3 and cairo too slow

Question

I've made a very simple animation with GTK3 and cairo and it's too slow for this simple graphics. I don't understand why it's so slow. I tried to use gtk_widget_queue_draw_area, but result is the same, nothing changes. Who can explain me, why it's so slow and how can i fix it?

Here's the program:

#include <gtk/gtk.h>
#include <cairo.h>

void draw(GtkWidget* widget, cairo_t* cr)
{
    static int width, height,
               posX = 0,
               vX = 1;

    GtkWidget* window = gtk_widget_get_toplevel(widget);
    gtk_window_get_size(GTK_WINDOW(window), &width, &height);

    cairo_set_source_rgb(cr, 0, 0, 0);
    cairo_set_line_width(cr, 1);

    cairo_rectangle(cr, posX, height/2, 1, 1);
    cairo_stroke(cr);

    if(posX + vX >= width || posX + vX == 0)
        vX = -vX;
    posX += vX;

    gtk_widget_queue_draw(window);
}

int main(int argc, char** argv)
{
    GtkWidget* window;
    GtkWidget* darea;

    gtk_init(&argc, &argv);

    window = gtk_window_new(GTK_WINDOW_TOPLEVEL);
    darea = gtk_drawing_area_new();

    gtk_container_add(GTK_CONTAINER(window), darea);

    gtk_window_set_default_size(GTK_WINDOW(window), 500, 400);

    g_signal_connect(G_OBJECT(window), "destroy", G_CALLBACK(gtk_main_quit), NULL);
    g_signal_connect(G_OBJECT(darea), "draw", G_CALLBACK(draw), NULL);

    gtk_widget_show_all(window);
    gtk_main();
}

Answer 1

You should use timers, e.g. g_timeout_add. You could register with g_timeout_add your new function redraw called every 0.05 seconds (e.g. every 50 milliseconds), and that redraw should not only draw things but also return TRUE to be restarted 50 milliseconds later. You'll find out that such a 50 millisecond delay is probably too small, and you surely want to increase it.

(notice that your program takes only 3% of CPU time, measured with time(1)... on Linux/Debian/Sid/x86-64 on a Intel i5-4690S, so it is not too slow; the computer spends 97% of time waiting! remember also that the human eyes don't see a lot faster than 30 to 60 Hz most of the time)

There is no animation code in your program (because any animation should be periodically run). The event loop (in gtk_main) is called only when needed. You could want to repaint your window only partially.

Your approach is wrong. You don't want to repeat the drawing with gtk_widget_queue_draw, you want it to happen again periodically (e.g. call that gtk_widget_queue_draw from a new redraw routine registered with g_timeout_add). You'll tune that period experimentally.

You could also use gtk_widget_add_tick_callback (as commented by c-smile). I don't think you need it (because I guess your animation might then run too fast).

Study the source code of some GTK examples (notably the clock example). See also this and that. Look into the custom drawing example, and into the source of aclock.

^{(your program is not too slow, but your draw is probably called not frequently enough; there is no code to repeat it; you should use the gdb debugger and compile your code with -Wall -g passed to gcc - in addition of other flags provided by pkg-config)}