Question

I’ve got a simple application that is supposed to rotate a decorated wheel so many degrees every x number of milliseconds using GTK+ and Cairo. I’ve got some code below that calls cairo_rotate() from a timer. However, the image doesn’t change. Do I have to invalidate the image to cause the expose-event to fire? I’m so new to Cairo that a simple example demonstrating how to rotate an image using Cairo in GTK+ would be highly appreciated.

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

cairo_surface_t *image;
cairo_t *cr;


gboolean rotate_cb( void )
{

    cairo_rotate (cr, 1);
    //cairo_paint(cr);
    printf("rotating\n");

    return( TRUE );
}

static gboolean
on_expose_event(GtkWidget *widget,
    GdkEventExpose *event,
    gpointer data)
{
  cr = gdk_cairo_create (widget->window);

  cairo_set_source_surface(cr, image, 0, 0);
  cairo_paint(cr);
  printf("Paint\n");

  //cairo_destroy(cr);

  return FALSE;
}


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

  image = cairo_image_surface_create_from_png("wheel.png");

  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), 500, 500); 
  gtk_widget_set_app_paintable(window, TRUE);

  gtk_widget_show_all(window);
  g_timeout_add(500, (GSourceFunc) rotate_cb, NULL);

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

  return 0;
}

Answer 1

You need to store the rotation in a variable and put the cairo_rotate(cr, rotation_amt); call into the on_expose_event method, before paint.

Also translate to the center of the window, rotate, and translate back, to make the wheel rotate around it’s center, if the image is centered.

cairo_translate(cr, width / 2.0, height / 2.0);
cairo_rotate(cr, rotation_amt);
cairo_translate(cr, - image_w / 2.0, - image_h / 2.0);
cairo_set_source_surface(cr, image, 0, 0);
cairo_paint(cr);

I hope that’s right.

And as ptomato said, you need to invalidate your drawing surface by calling gtk_widget_queue_draw from rotate_cb. And keeping a global variable for the Cairo context is redundant. The image doesn’t rotate because a newly created context is loaded with an identity matrix and all your previous transformations are reset.