Cache a full pick render if we have a static stage

The idea is that if we see multiple picks per frame then that implies
the visible scene has become static. In this case we can promote the
next pick render to be unclipped so we have valid pick values for the
entire stage. Now we can continue to read from this cached buffer until
the stage contents do visibly change.

Thanks to Luca Bruno on #clutter for this idea!

clutter/clutter-actor.c

@@ -1780,6 +1780,7 @@ clutter_actor_real_queue_redraw (ClutterActor *self,
                                  ClutterActor *origin)
 {
   ClutterActor *parent;
+  ClutterMainContext *context;
 
   CLUTTER_NOTE (PAINT, "Redraw queued on '%s' (from: '%s')",
                 get_actor_debug_name (self),
@@ -1797,6 +1798,17 @@ clutter_actor_real_queue_redraw (ClutterActor *self,
   if (!CLUTTER_ACTOR_IS_VISIBLE (self))
     return;
 
+  /* We have an optimization in _clutter_do_pick to detect when the scene is
+   * static so we can cache a full, un-clipped pick buffer to avoid continuous
+   * pick renders.
+   *
+   * Currently the assumption is that actors queue a redraw when some state
+   * changes that affects painting *or* picking so we can use this point
+   * to invalidate any currently cached pick buffer.
+   */
+  context = _clutter_context_get_default ();
+  context->have_complete_pick_buffer = FALSE;
+
   /* Although we could determine here that a full stage redraw
    * has already been queued and immediately bail out, we actually
    * guarantee that we will propagate a queue-redraw signal to our

clutter/clutter-main.c

@@ -301,6 +301,13 @@ _clutter_do_redraw (ClutterStage *stage)
 
   ctx = _clutter_context_get_default ();
 
+  /* We have an optimization to avoid pick renders while scene is static.
+   * This is implied by seeing multiple picks per frame so we have to
+   * reset a pick counter each frame and also invalidate any current pick
+   * buffer. */
+  ctx->picks_per_frame = 0;
+  ctx->have_complete_pick_buffer = FALSE;
+
   /* Before we can paint, we have to be sure we have the latest layout */
   _clutter_stage_maybe_relayout (CLUTTER_ACTOR (stage));
 
@@ -577,6 +584,7 @@ _clutter_do_pick (ClutterStage   *stage,
   guint32             id;
   GLboolean           dither_was_on;
   ClutterActor       *actor;
+  gboolean            is_clipped;
   CLUTTER_STATIC_COUNTER (do_pick_counter,
                           "_clutter_do_pick counter",
                           "Increments for each full pick run",
@@ -617,13 +625,52 @@ _clutter_do_pick (ClutterStage   *stage,
 
   context = _clutter_context_get_default ();
 
+  /* It's possible that we currently have a static scene and have renderered a
+   * full, unclipped pick buffer. If so we can simply continue to read from
+   * this cached buffer until the scene next changes. */
+  if (context->have_complete_pick_buffer)
+    {
+      CLUTTER_TIMER_START (_clutter_uprof_context, pick_read);
+      cogl_read_pixels (x, y, 1, 1,
+                        COGL_READ_PIXELS_COLOR_BUFFER,
+                        COGL_PIXEL_FORMAT_RGBA_8888_PRE,
+                        pixel);
+      CLUTTER_TIMER_STOP (_clutter_uprof_context, pick_read);
+
+      /* FIXME: This is a lazy copy and paste of the logic at the end of this
+       * function used when we actually do a pick render. It should be
+       * consolidated somehow.
+       */
+      if (pixel[0] == 0xff && pixel[1] == 0xff && pixel[2] == 0xff)
+        {
+          actor = CLUTTER_ACTOR (stage);
+          goto result;
+        }
+
+      id = _clutter_pixel_to_id (pixel);
+      actor = clutter_get_actor_by_gid (id);
+      goto result;
+    }
+
+  context->picks_per_frame++;
+
   _clutter_backend_ensure_context (context->backend, stage);
 
   /* needed for when a context switch happens */
   _clutter_stage_maybe_setup_viewport (stage);
 
-  if (G_LIKELY (!(clutter_pick_debug_flags & CLUTTER_DEBUG_DUMP_PICK_BUFFERS)))
-    cogl_clip_push_window_rectangle (x, y, 1, 1);
+  /* If we are seeing multiple picks per frame that means the scene is static
+   * so we promote to doing a non-scissored pick render so that all subsequent
+   * picks for the same static scene won't require additional renders */
+  if (context->picks_per_frame >= 2)
+    {
+      if (G_LIKELY (!(clutter_pick_debug_flags &
+                      CLUTTER_DEBUG_DUMP_PICK_BUFFERS)))
+        cogl_clip_push_window_rectangle (x, y, 1, 1);
+      is_clipped = TRUE;
+    }
+  else
+    is_clipped = FALSE;
 
   cogl_disable_fog ();
   cogl_color_set_from_4ub (&stage_pick_id, 255, 255, 255, 255);
@@ -647,8 +694,14 @@ _clutter_do_pick (ClutterStage   *stage,
   context->pick_mode = CLUTTER_PICK_NONE;
   CLUTTER_TIMER_STOP (_clutter_uprof_context, pick_paint);
 
-  if (G_LIKELY (!(clutter_pick_debug_flags & CLUTTER_DEBUG_DUMP_PICK_BUFFERS)))
-    cogl_clip_pop ();
+  if (is_clipped)
+    {
+      if (G_LIKELY (!(clutter_pick_debug_flags &
+                      CLUTTER_DEBUG_DUMP_PICK_BUFFERS)))
+        cogl_clip_pop ();
+    }
+  else
+    context->have_complete_pick_buffer = TRUE;
 
   /* Make sure Cogl flushes any batched geometry to the GPU driver */
   cogl_flush ();

clutter/clutter-private.h

@@ -188,6 +188,9 @@ struct _ClutterMainContext
   GList *repaint_funcs;
 
   ClutterSettings *settings;
+
+  gint picks_per_frame;
+  gboolean have_complete_pick_buffer;
 };
 
 #define CLUTTER_CONTEXT()	(_clutter_context_get_default ())