flush configurable

cht/config.py

@@ -16,6 +16,7 @@ RELAY_PORT = 4445  # UDP loopback relay for live display
 
 # Frame extraction — scene-only, no interval fallback
 SCENE_THRESHOLD = 0.10  # 0-1, lower = more sensitive; 0.1 catches slide/window changes
+SCENE_FLUSH_FRAMES = 2  # extra frames after scene change to flush encoder/muxer buffer (0 to disable)
 
 # Segment recording
 SEGMENT_DURATION = 60  # seconds per .ts segment

cht/stream/ffmpeg.py

@@ -64,7 +64,7 @@ def receive_record_and_relay(stream_url, output_path, relay_url):
 
 
 def receive_record_relay_and_detect(stream_url, output_path, relay_url,
-                                    scene_threshold=0.10):
+                                    scene_threshold=0.10, flush_frames=2):
     """Single process: receive TCP → record fMP4 + relay UDP + scene detect.
 
     One ffmpeg process, three output branches from the same TCP input:
@@ -99,14 +99,16 @@ 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
     #
-    # 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)"
+    if flush_frames > 0:
-    select_expr = f"{scene_expr}+{flush_expr}"
+        # Flush trick: select extra frames after each scene change to push
+        # the real frame through the encoder+muxer buffer pipeline.
+        # mod(selected_n, 1+flush_frames) prevents chaining.
+        mod_val = 1 + flush_frames
+        flush_expr = f"eq(n,prev_selected_n+1)*mod(selected_n,{mod_val})"
+        select_expr = f"{scene_expr}+{flush_expr}"
+    else:
+        select_expr = scene_expr
     scene_stream = stream.filter("select", select_expr).filter("showinfo")
     scene_out = ffmpeg.output(
         scene_stream, "pipe:1",

cht/stream/manager.py

@@ -19,6 +19,7 @@ from cht.config import (
     STREAM_PORT,
     RELAY_PORT,
     SCENE_THRESHOLD,
+    SCENE_FLUSH_FRAMES,
     SESSIONS_DIR,
     AUDIO_EXTRACT_INTERVAL,
     AUDIO_SAFETY_MARGIN,
@@ -197,6 +198,7 @@ class StreamManager:
         node = ff.receive_record_relay_and_detect(
             self.stream_url, self.recording_path, self.relay_url,
             scene_threshold=self.scene_threshold,
+            flush_frames=SCENE_FLUSH_FRAMES,
         )
         proc = ff.run_async(node, pipe_stdout=True, pipe_stderr=True)
         self._procs["recorder"] = proc