feat: add stream-copy exports

core/ffmpeg.py

@@ -82,11 +82,12 @@ def build_ffmpeg_command(
     target_fps: float | None = None,
     snap32: bool = False,
     frames: int | None = None,
+    stream_copy: bool = False,
 ) -> list[str]:
-    # -ss before -i: fast input-seeking. Safe here because we always re-encode,
-    # so there is no keyframe-alignment issue from pre-input seek.
+    # Re-encoded output is not constrained to source keyframes. Stream-copy
+    # output remains keyframe-limited even though it uses the same fast seek.
     # Image sequences always use libwebp, so skip HW encoder setup.
-    use_hw_vaapi = (encoder == "h264_vaapi" and not image_sequence
+    use_hw_vaapi = (not stream_copy and encoder == "h264_vaapi" and not image_sequence
                     and sys.platform == "linux")
     cmd = [_bin("ffmpeg"), "-y"]
 
@@ -96,6 +97,31 @@ def build_ffmpeg_command(
         cmd += ["-hwaccel", "vaapi", "-hwaccel_output_format", "vaapi",
                 "-vaapi_device", vaapi_dev]
 
+    if stream_copy:
+        incompatible = (
+            image_sequence or short_side is not None or portrait_ratio is not None
+            or target_fps is not None or snap32 or frames is not None
+        )
+        if incompatible:
+            raise ValueError("Stream copy cannot be combined with image or video transforms")
+        # Matroska/WebM input seeking can retain a whole cluster of keyframe
+        # pre-roll and then count -t from its shifted timestamps. Keeping source
+        # timestamps avoids that extension. MOV/MP4 needs normal timestamp
+        # rebasing instead, so this workaround is deliberately container-only.
+        ext = os.path.splitext(output_path)[1].lower()
+        timestamp_args = (
+            ["-copyts", "-start_at_zero"] if ext in (".mkv", ".webm") else []
+        )
+        return cmd + [
+            "-threads", "0",
+            "-ss", str(start),
+            "-i", input_path,
+            "-t", str(duration),
+            *timestamp_args,
+            "-c", "copy",
+            output_path,
+        ]
+
     cmd += [
         "-threads", "0",
         "-ss", str(start),