cogl-path: Optimise paths that are just a rectangle

Drawing and clipping to paths is generally quite expensive because the
geometry has to be tessellated into triangles in a single VBO which
breaks up the journal batching. If we can detect when the path
contains just a single rectangle then we can instead divert to calling
cogl_rectangle which will take advantage of the journal, or by pushing
a rectangle clip which usually ends up just using the scissor.

This patch adds a boolean to each path to mark when it is a
rectangle. It gets cleared whenever a node is added or gets set to
TRUE whenever cogl2_path_rectangle is called. This doesn't try to
catch cases where a rectangle is composed by cogl_path_line_to and
cogl_path_move_to commands.

cogl/cogl-clip-stack.c

@@ -445,9 +445,22 @@ _cogl_clip_stack_push_from_path (CoglClipStack *stack,
                                  CoglPath *path,
                                  const CoglMatrix *modelview_matrix)
 {
-  CoglClipStackPath *entry;
   float x_1, y_1, x_2, y_2;
 
+  _cogl_path_get_bounds (path, &x_1, &y_1, &x_2, &y_2);
+
+  /* If the path is a simple rectangle then we can divert to pushing a
+     rectangle clip instead which usually won't involve the stencil
+     buffer */
+  if (_cogl_path_is_rectangle (path))
+    return _cogl_clip_stack_push_rectangle (stack,
+                                            x_1, y_1,
+                                            x_2, y_2,
+                                            modelview_matrix);
+  else
+    {
+      CoglClipStackPath *entry;
+
       entry = _cogl_clip_stack_push_entry (stack,
                                            sizeof (CoglClipStackPath),
                                            COGL_CLIP_STACK_PATH);
@@ -456,13 +469,12 @@ _cogl_clip_stack_push_from_path (CoglClipStack *stack,
 
       entry->matrix = *modelview_matrix;
 
-  _cogl_path_get_bounds (path, &x_1, &y_1, &x_2, &y_2);
-
       _cogl_clip_stack_entry_set_bounds ((CoglClipStack *) entry,
                                          x_1, y_1, x_2, y_2, modelview_matrix);
 
       return (CoglClipStack *) entry;
     }
+}
 
 CoglClipStack *
 _cogl_clip_stack_ref (CoglClipStack *entry)

cogl/cogl-path-private.h

@@ -87,6 +87,14 @@ struct _CoglPathData
   CoglVertexArray     *stroke_vbo;
   CoglAttribute      **stroke_vbo_attributes;
   unsigned int         stroke_vbo_n_attributes;
+
+  /* This is used as an optimisation for when the path contains a
+     single contour specified using cogl2_path_rectangle. Cogl is more
+     optimised to handle rectangles than paths so we can detect this
+     case and divert to the journal or a rectangle clip. If it is TRUE
+     then the entire path can be described by calling
+     _cogl_path_get_bounds */
+  gboolean             is_rectangle;
 };
 
 void
@@ -101,4 +109,7 @@ _cogl_path_get_bounds (CoglPath *path,
                        float *max_x,
                        float *max_y);
 
+gboolean
+_cogl_path_is_rectangle (CoglPath *path);
+
 #endif /* __COGL_PATH_PRIVATE_H */

cogl/cogl2-path.c

@@ -190,6 +190,11 @@ _cogl_path_add_node (CoglPath *path,
       if (y > data->path_nodes_max.y)
         data->path_nodes_max.y = y;
     }
+
+  /* Once the path nodes have been modified then we'll assume it's no
+     longer a rectangle. cogl2_path_rectangle will set this back to
+     TRUE if this has been called from there */
+  data->is_rectangle = FALSE;
 }
 
 static void
@@ -466,6 +471,19 @@ cogl2_path_fill (CoglPath *path)
   if (path->data->path_nodes->len == 0)
     return;
 
+  /* If the path is a simple rectangle then we can divert to using
+     cogl_rectangle which should be faster because it can go through
+     the journal instead of uploading the geometry just for two
+     triangles */
+  if (path->data->is_rectangle)
+    {
+      float x_1, y_1, x_2, y_2;
+
+      _cogl_path_get_bounds (path, &x_1, &y_1, &x_2, &y_2);
+      cogl_rectangle (x_1, y_1, x_2, y_2);
+    }
+  else
+    {
       framebuffer = _cogl_get_draw_buffer ();
 
       _cogl_framebuffer_flush_journal (framebuffer);
@@ -479,6 +497,7 @@ cogl2_path_fill (CoglPath *path)
 
       _cogl_path_fill_nodes (path);
     }
+}
 
 void
 cogl2_path_stroke (CoglPath *path)
@@ -612,11 +631,28 @@ cogl2_path_rectangle (CoglPath *path,
                       float x_2,
                       float y_2)
 {
+  gboolean is_rectangle;
+
+  /* If the path was previously empty and the rectangle isn't mirrored
+     then we'll record that this is a simple rectangle path so that we
+     can optimise it */
+  is_rectangle = (path->data->path_nodes->len == 0 &&
+                  x_2 >= x_1 &&
+                  y_2 >= y_1);
+
   cogl2_path_move_to (path, x_1, y_1);
   cogl2_path_line_to (path, x_2, y_1);
   cogl2_path_line_to (path, x_2, y_2);
   cogl2_path_line_to (path, x_1, y_2);
   cogl2_path_close (path);
+
+  path->data->is_rectangle = is_rectangle;
+}
+
+gboolean
+_cogl_path_is_rectangle (CoglPath *path)
+{
+  return path->data->is_rectangle;
 }
 
 static void
@@ -975,6 +1011,7 @@ cogl2_path_new (void)
   data->last_path = 0;
   data->fill_vbo = COGL_INVALID_HANDLE;
   data->stroke_vbo = NULL;
+  data->is_rectangle = FALSE;
 
   return _cogl_path_object_new (path);
 }