diff --git a/src/hb-face.cc b/src/hb-face.cc
index 2c0087370..2f6f29121 100644
--- a/src/hb-face.cc
+++ b/src/hb-face.cc
@@ -143,7 +143,7 @@ hb_face_create_for_tables (hb_reference_table_func_t reference_table_func,
typedef struct hb_face_for_data_closure_t {
hb_blob_t *blob;
- unsigned int index;
+ uint16_t index;
} hb_face_for_data_closure_t;
static hb_face_for_data_closure_t *
@@ -156,7 +156,7 @@ _hb_face_for_data_closure_create (hb_blob_t *blob, unsigned int index)
return nullptr;
closure->blob = blob;
- closure->index = index;
+ closure->index = (uint16_t) (index & 0xFFFFu);
return closure;
}
@@ -195,9 +195,15 @@ _hb_face_for_data_reference_table (hb_face_t *face HB_UNUSED, hb_tag_t tag, void
* @index: The index of the face within @blob
*
* Constructs a new face object from the specified blob and
- * a face index into that blob. This is used for blobs of
- * file formats such as Dfont and TTC that can contain more
- * than one face.
+ * a face index into that blob.
+ *
+ * The face index is used for blobs of file formats such as TTC and
+ * and DFont that can contain more than one face. Face indices within
+ * such collections are zero-based.
+ *
+ * Note: If the blob font format is not a collection, @index
+ * is ignored. Otherwise, only the lower 16-bits of @index are used.
+ * The unmodified @index can be accessed via hb_face_get_index().
*
* Return value: (transfer full): The new face object
*
@@ -420,7 +426,8 @@ hb_face_reference_blob (hb_face_t *face)
* Assigns the specified face-index to @face. Fails if the
* face is immutable.
*
- * Note: face indices within a collection are zero-based.
+ * Note: changing the index has no effect on the face itself
+ * This only changes the value returned by hb_face_get_index().
*
* Since: 0.9.2
**/