Open source input before frame pipe

docs/module-internals.md

@@ -158,6 +158,10 @@ tail when diagnostics are unusually large. Aborts release it even when ffmpeg
 has already exited. A real subprocess regression writes diagnostics beyond pipe
 capacity while streaming frames, checks bounded failure reporting, and the Linux
 full-clip CI job guards the complete path.
+The finite source file is opened before the frame pipe, so ffmpeg can initialize
+the copied audio stream before producer backpressure is possible. Stream and
+metadata mappings are source-indexed accordingly; regressions assert the input
+order and both map targets.
 `probe_video_encode_profile` reads the first source video stream with ffprobe
 and preserves the supported properties that survive the 8-bit BGR boundary:
 `yuv420p`/`yuv422p`/`yuv444p` chroma sampling, recognized color tags, encoder

src/remove_ai_watermarks/video_encoding.py

@@ -358,21 +358,21 @@ def raw_video_command(
         "-loglevel",
         "error",
         *(["-copyts"] if copy_input_timestamps else []),
-        *frame_input,
         "-i",
         str(source),
+        *frame_input,
         "-map",
-        "0:v:0",
+        "1:v:0",
         "-map",
-        "1:a?",
+        "0:a?",
         *_video_codec_args(output.suffix.lower(), crf=crf, profile=profile),
         *_profile_args(profile),
         "-c:a",
         "copy",
         "-map_metadata",
-        "-1" if strip_metadata else "1",
+        "-1" if strip_metadata else "0",
         "-map_chapters",
-        "-1" if strip_metadata else "1",
+        "-1" if strip_metadata else "0",
     ]
     if timestamped_input:
         command.extend(["-fps_mode", "passthrough"])

tests/test_video_invisible.py

@@ -128,6 +128,10 @@ def test_encoder_command_discards_metadata_and_copies_audio(
         "colorprim=bt709:transfer=bt709:colormatrix=bt709:range=limited"
     )
     assert "pipe:0" in command
+    assert command.index(str(source)) < command.index("pipe:0")
+    assert command[command.index("-map") + 1] == "1:v:0"
+    second_map = command.index("-map", command.index("-map") + 1)
+    assert command[second_map + 1] == "0:a?"
     assert "-shortest" not in command