Fix change detection in a rare case

If an empty track is passed to ass_render_frame, always set the
change detection value to 2 (content and positions changed). This is
acceptable in both the case that the previous call resulted in images
(obviously) and in the case that the previous call resulted in no
images (in that case the change detection result doesn't actually
matter).

1 files changed, 5 insertions, 1 deletions

diff --git a/libass/ass_render.c b/libass/ass_render.c
index b11c681..980dac3 100644
--- a/libass/ass_render.c
+++ b/libass/ass_render.c
@@ -2525,8 +2525,12 @@ ASS_Image *ass_render_frame(ASS_Renderer *priv, ASS_Track *track,
 
     // init frame
     rc = ass_start_frame(priv, track, now);
-    if (rc != 0)
+    if (rc != 0) {
+        if (detect_change) {
+            *detect_change = 2;
+        }
         return 0;
+    }
 
     // render events separately
     cnt = 0;