cairo_rotate.c

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

cairo_surface_t *image;
cairo_t *cr;
gdouble rotation = 0;
GtkWidget *window;

gint image_w, image_h;


gboolean rotate_cb( void )
{
    // Any rotation applied to cr here will be lost, as we create
    // a new cairo context on every expose event
    //cairo_rotate (cr, 4);
    rotation += 0.1;
    //cairo_paint(cr);
//  printf("rotating\n");
    // Tell our window that it should repaint itself (ie. emit an expose event)
    gtk_widget_queue_draw(window);

    return( TRUE );
}

static gboolean on_expose_event(GtkWidget *widget, GdkEventExpose *event,gpointer data)
{
  // Make sure our window wasn't destroyed yet
  // (to silence a warning)
  g_return_if_fail(GTK_IS_WIDGET(widget));

  cr = gdk_cairo_create (widget->window);
  // We need to apply transformation before setting the source surface
  // We translate (0, 0) to the center of the screen,
  // so we can rotate the image around its center point,
  // not its upper left corner
  cairo_translate(cr, image_w/2, image_h/2);
  cairo_rotate(cr, rotation);
  cairo_set_source_surface(cr, image, -image_w/2, -image_h/2);
  // We need to clip around the image, or cairo will paint garbage data
  cairo_rectangle(cr, -image_w/2, -image_h/2, image_w, image_h);
  cairo_clip(cr);

  cairo_paint(cr);
//  printf("Paint\n");
  cairo_destroy(cr);
  return FALSE;
}


int main(int argc, char *argv[])
{
  image = cairo_image_surface_create_from_png("wheel.png");
  image_w = cairo_image_surface_get_width(image);
  image_h = cairo_image_surface_get_height(image);

  gtk_init(&argc, &argv);

  window = gtk_window_new(GTK_WINDOW_TOPLEVEL);

  g_signal_connect(window, "expose-event",
      G_CALLBACK (on_expose_event), NULL);
  g_signal_connect(window, "destroy",
      G_CALLBACK (gtk_main_quit), NULL);

  gtk_window_set_position(GTK_WINDOW(window), GTK_WIN_POS_CENTER);
  gtk_window_set_default_size(GTK_WINDOW(window), image_w, image_h);
  gtk_widget_set_app_paintable(window, TRUE);

  gtk_widget_show_all(window);
  g_timeout_add(40, (GSourceFunc) rotate_cb, NULL);

  gtk_main();
//  cairo_destroy(cr);
  cairo_surface_destroy(image);

  return 0;
}