feat: add build_mask_output_dir utility

main.py

@@ -63,6 +63,12 @@ def build_ffmpeg_command(
     return cmd
 
 
+def build_mask_output_dir(video_path: str) -> str:
+    """Return path of mask output directory: <stem>_masks/ next to the video."""
+    p = Path(video_path)
+    return str(p.parent / f"{p.stem}_masks")
+
+
 _RATIOS: dict[str, tuple[int, int]] = {
     "9:16": (9, 16),
     "4:5":  (4, 5),

tests/test_utils.py

@@ -1,5 +1,5 @@
 import tempfile, os
-from main import build_export_path, format_time, build_ffmpeg_command
+from main import build_export_path, format_time, build_ffmpeg_command, build_mask_output_dir
 from main import _normalize_filename, ProcessedDB
 
 
@@ -173,3 +173,12 @@ def test_ffmpeg_command_portrait_and_resize():
 def test_ffmpeg_command_portrait_off():
     cmd = build_ffmpeg_command("/in/video.mp4", 0.0, "/out/clip.mp4")
     assert "-vf" not in cmd
+
+def test_mask_output_dir_basic():
+    assert build_mask_output_dir("/out/clip_001.mp4") == "/out/clip_001_masks"
+
+def test_mask_output_dir_mkv():
+    assert build_mask_output_dir("/out/my_clip.mkv") == "/out/my_clip_masks"
+
+def test_mask_output_dir_nested():
+    assert build_mask_output_dir("/a/b/c/shot_042.mp4") == "/a/b/c/shot_042_masks"