Experiment with different techniques to rotate an Actor around a pivot point with the Clutter toolkit.

clutter-pivot-rotate.c

#include <stdlib.h>
#include <clutter/clutter.h>

static gboolean
on_key_press (ClutterActor *stage G_GNUC_UNUSED,
              ClutterEvent *event,
              gpointer      data)
{
  guint key_symbol;
  ClutterActor *rect = CLUTTER_ACTOR (data);

  key_symbol = clutter_event_get_key_symbol (event);

  if (key_symbol == CLUTTER_KEY_Return)
    {
      clutter_actor_set_transform (rect, NULL);
      clutter_actor_set_pivot_point (rect, 0.5, 0.5);
      gfloat angle = clutter_actor_get_rotation_angle (rect, CLUTTER_Z_AXIS);
      clutter_actor_set_rotation_angle (rect, CLUTTER_Z_AXIS, angle + 30);
    }
  else if (key_symbol == CLUTTER_KEY_Tab)
    {
      /* This has been broken for a while due to a bug in ClutterActor.apply_trasform(),
       * see https://bugzilla.gnome.org/show_bug.cgi?id=690214 */
      static gfloat angle = 0;
      ClutterMatrix matrix;
      clutter_actor_set_pivot_point (rect, 0.5, 0.5);
      clutter_matrix_init_identity(&matrix);
      angle += 30;
      cogl_matrix_rotate (&matrix, angle, 0, 0, 1.0);
      clutter_actor_set_transform (rect, &matrix);
    }
  else if (key_symbol == CLUTTER_KEY_space)
    {
      gfloat px, py, pz;
      ClutterActorBox allocation;
      ClutterMatrix matrix;

      clutter_actor_set_pivot_point (rect, 0, 0);
      clutter_actor_get_allocation_box (rect, &allocation);
      px = 0.5;
      py = 0.5;
      px *= (allocation.x2 - allocation.x1);
      py *= (allocation.y2 - allocation.y1);
      pz = clutter_actor_get_pivot_point_z (rect);

      clutter_actor_get_transform (rect, &matrix);
      cogl_matrix_translate (&matrix, px, py, pz);
      cogl_matrix_rotate (&matrix, 30, 0, 0, 1.0);
      cogl_matrix_translate (&matrix, -px, -py, -pz);
      clutter_actor_set_transform (rect, &matrix);
    }

  return CLUTTER_EVENT_STOP;
}

int
main (int argc, char *argv[])
{
  ClutterActor *stage, *content, *rect, *info;

  if (argc < 1)
    {
      g_error ("Usage: %s", argv[0]);
      return EXIT_FAILURE;
    }

  if (clutter_init (&argc, &argv) != CLUTTER_INIT_SUCCESS)
    {
      g_error ("Failed to initialize clutter\n");
      return EXIT_FAILURE;
    }

  stage = clutter_stage_new ();
  clutter_actor_set_name (stage, "stage");

  content = clutter_actor_new ();
  clutter_actor_set_name (content, "content");
  clutter_actor_add_child (stage, content);

  info = clutter_text_new_with_text (NULL, "<tab>: set_pivot_point(0.5, 0.5)\n<return>: use set_rotation_angle()\n<space>: explicit translation");
  clutter_actor_add_child (stage, info);
  clutter_actor_set_position (info, 12, 12);

  rect = clutter_actor_new ();
  clutter_actor_set_name (rect, "rect");
  clutter_actor_set_position (rect, 0, 0);
  clutter_actor_set_size (rect, 60, 90);
  clutter_actor_set_background_color (rect, CLUTTER_COLOR_Blue);
  clutter_actor_add_child (content, rect);

  clutter_actor_show (stage);

  g_signal_connect (stage, "key-press-event", G_CALLBACK (on_key_press), rect);

  clutter_main ();

  return EXIT_SUCCESS;
}