[cogl_clip_push_window_rect] fix Cogl -> GL coordinate conversion

Firstly this now uses the draw buffer height not the viewport height
when we need to perform a y = height - y conversion, since (as the
name suggests) we are dealing with window coordinates not viewport
coordinates.

Secondly this skips any conversion when the current draw buffer is an
offscreen draw buffer since offscreen rendering is always forced to be
upside down and in this case Cogl window coordinates == GL window
coordinates.

1 files changed, 26 insertions, 6 deletions

diff --git a/cogl/cogl-clip-stack.c b/cogl/cogl-clip-stack.c
index cac156fa..93900338 100644
--- a/cogl/cogl-clip-stack.c
+++ b/cogl/cogl-clip-stack.c
@@ -114,6 +114,13 @@ struct _CoglClipStackEntryPath
   CoglPathNode           path[1];
 };
 
+/* FIXME: deprecate and replace with:
+ * void
+ * cogl_clip_push_window_rectangle (int x_offset,
+ *                                  int y_offset,
+ *                                  int width,
+ *                                  int height);
+ */
 void
 cogl_clip_push_window_rect (float x_offset,
 	                    float y_offset,
@@ -123,8 +130,8 @@ cogl_clip_push_window_rect (float x_offset,
   CoglHandle draw_buffer;
   CoglClipStackState *clip_state;
   CoglClipStack *stack;
+  int draw_buffer_height;
   CoglClipStackEntryWindowRect *entry;
-  float viewport_height;
 
   _COGL_GET_CONTEXT (ctx, NO_RETVAL);
 
@@ -137,17 +144,30 @@ cogl_clip_push_window_rect (float x_offset,
 
   stack = clip_state->stacks->data;
 
-  viewport_height = _cogl_draw_buffer_get_viewport_height (draw_buffer);
+  draw_buffer_height = _cogl_draw_buffer_get_height (draw_buffer);
 
   entry = g_slice_new (CoglClipStackEntryWindowRect);
 
-  /* We convert from coords with (0,0) at top left to coords
-   * with (0,0) at bottom left. */
+  /* We store the entry coordinates in OpenGL window coordinate space and so
+   * because Cogl defines the window origin to be top left but OpenGL defines
+   * it as bottom left we may need to convert the incoming coordinates.
+   *
+   * NB: Cogl forces all offscreen rendering to be done upside down so in this
+   * case no conversion is needed.
+   */
   entry->type = COGL_CLIP_STACK_WINDOW_RECT;
   entry->x0 = x_offset;
-  entry->y0 = viewport_height - y_offset - height;
   entry->x1 = x_offset + width;
-  entry->y1 = viewport_height - y_offset;
+  if (cogl_is_offscreen (draw_buffer))
+    {
+      entry->y0 = y_offset;
+      entry->y1 = y_offset + height;
+    }
+  else
+    {
+      entry->y0 = draw_buffer_height - y_offset - height;
+      entry->y1 = draw_buffer_height - y_offset;
+    }
 
   /* Store it in the stack */
   stack->stack_top = g_list_prepend (stack->stack_top, entry);