ffmpeg fix

cht/stream/ffmpeg.py

@@ -72,27 +72,23 @@ def extract_scene_frames(input_path, output_dir, scene_threshold=0.10,
     meaningfully vs the previous frame. No periodic fallback so static content
     produces no spurious frames.
 
-    start_time/duration: applied via the select filter expression (NOT as -ss/-t
-    input options, which break h264 scene detection on MKV).
+    Uses -ss input seeking for O(1) startup regardless of file size.
+    pts_time in showinfo output is relative to the seek point.
     Returns (stdout, stderr) as decoded strings for timestamp parsing.
     """
     scene_expr = f"gt(scene,{scene_threshold})"
 
-    # Add time range filter if specified (incremental processing)
-    time_conditions = []
-    if start_time > 0:
-        time_conditions.append(f"gte(t,{start_time})")
+    # With -ss input seeking, t starts at 0 from the seek point.
+    # Only need end boundary (duration), start is handled by -ss.
     if duration is not None:
-        time_conditions.append(f"lte(t,{start_time + duration})")
+        scene_expr = f"({scene_expr})*lte(t,{duration})"
 
-    if time_conditions:
-        time_filter = "*".join(time_conditions)
-        select_expr = f"({scene_expr})*{time_filter}"
-    else:
-        select_expr = scene_expr
+    input_kwargs = {}
+    if start_time > 0:
+        input_kwargs["ss"] = start_time
 
-    stream = ffmpeg.input(str(input_path))
-    stream = stream.filter("select", select_expr).filter("showinfo")
+    stream = ffmpeg.input(str(input_path), **input_kwargs)
+    stream = stream.filter("select", scene_expr).filter("showinfo")
 
     output = (
         ffmpeg.output(