1 files changed, 11 insertions, 1 deletions
diff --git a/sub/osd.h b/sub/osd.h
index 207d789094..17e3ae0282 100644
--- a/sub/osd.h
+++ b/sub/osd.h
@@ -73,10 +73,20 @@ struct sub_bitmaps {
     // box. (The origin of the box is at (0,0).)
     int packed_w, packed_h;
 
-    int change_id;  // Incremented on each change
+    int change_id;  // Incremented on each change (0 is never used)
 };
 
 struct sub_bitmap_list {
+    // Combined change_id - of any of the existing items change (even if they
+    // e.g. go away and are removed from items[]), this is incremented.
+    int64_t change_id;
+
+    // Bounding box for rendering. It's notable that SUBBITMAP_LIBASS images are
+    // always within these bounds, while SUBBITMAP_RGBA is not necessarily.
+    int w, h;
+
+    // Sorted by sub_bitmaps.render_index. Unused parts are not in the array,
+    // and you cannot index items[] with render_index.
     struct sub_bitmaps **items;
     int num_items;
 };