Check the source video before removing previously extracted frames

tests/process_data/test_video_input_validation.py

@@ -0,0 +1,24 @@
+from pathlib import Path
+from unittest import mock
+
+import pytest
+
+from nerfstudio.process_data.process_data_utils import convert_video_to_images
+
+
+@pytest.mark.parametrize("empty_video", [False, True])
+def test_invalid_video_preserves_existing_frames(tmp_path: Path, empty_video: bool):
+    image_dir = tmp_path / "images"
+    old_frames = [directory / "frame_00001.png" for directory in (image_dir, tmp_path / "images_2")]
+    for frame in old_frames:
+        frame.parent.mkdir()
+        frame.write_bytes(b"previous frame")
+    video = tmp_path / "capture.mp4"
+    if empty_video:
+        video.touch()
+
+    with mock.patch("nerfstudio.process_data.process_data_utils.get_num_frames_in_video", return_value=0):
+        with pytest.raises(SystemExit):
+            convert_video_to_images(video, image_dir, num_frames_target=3, num_downscales=1)
+
+    assert all(frame.read_bytes() == b"previous frame" for frame in old_frames)