one behind duplicated, and the correct

cht/stream/ffmpeg.py

@@ -98,7 +98,15 @@ def receive_record_relay_and_detect(stream_url, output_path, relay_url,
 
     # Scene detection: CUDA decode (GPU) → select filter (CPU, lightweight)
     # → showinfo → MJPEG piped to stdout
-    select_expr = f"gt(scene,{scene_threshold})"
+    #
+    # Flush trick: select the scene-change frame AND the next 2 frames.
+    # The pipeline has 2 levels of buffering (encoder + muxer), so we
+    # need 2 flush frames to push the real scene-change frame out.
+    # mod(selected_n,3) prevents chaining: after 2 flushes, selected_n
+    # hits a multiple of 3 and the chain stops.
+    scene_expr = f"gt(scene,{scene_threshold})"
+    flush_expr = "eq(n,prev_selected_n+1)*mod(selected_n,3)"
+    select_expr = f"{scene_expr}+{flush_expr}"
     scene_stream = stream.filter("select", select_expr).filter("showinfo")
     scene_out = ffmpeg.output(
         scene_stream, "pipe:1",