Support negative index in SimpleVideoDecoder (#743)

Author: Dan-Flores

src/torchcodec/decoders/_video_decoder.py

@@ -195,6 +195,8 @@ def get_frame_at(self, index: int) -> Frame:
         Returns:
             Frame: The frame at the given index.
         """
+        if index < 0:
+            index += self._num_frames
 
         if not 0 <= index < self._num_frames:
             raise IndexError(
@@ -218,6 +220,9 @@ def get_frames_at(self, indices: list[int]) -> FrameBatch:
         Returns:
             FrameBatch: The frames at the given indices.
         """
+        indices = [
+            index if index >= 0 else index + self._num_frames for index in indices
+        ]
 
         data, pts_seconds, duration_seconds = core.get_frames_at_indices(
             self._decoder, frame_indices=indices

test/test_decoders.py

@@ -328,6 +328,19 @@ def test_getitem_slice(self, device, seek_mode):
         )
         assert_frames_equal(ref386_389, slice386_389)
 
+        # slices with upper bound greater than len(decoder) are supported
+        slice387_389 = decoder[-3:10000].to(device)
+        assert slice387_389.shape == torch.Size(
+            [
+                3,
+                NASA_VIDEO.num_color_channels,
+                NASA_VIDEO.height,
+                NASA_VIDEO.width,
+            ]
+        )
+        ref387_389 = NASA_VIDEO.get_frame_data_by_range(387, 390).to(device)
+        assert_frames_equal(ref387_389, slice387_389)
+
         # an empty range is valid!
         empty_frame = decoder[5:5]
         assert_frames_equal(empty_frame, NASA_VIDEO.empty_chw_tensor.to(device))
@@ -437,6 +450,11 @@ def test_get_frame_at(self, device, seek_mode):
             expected_frame_info.duration_seconds, rel=1e-3
         )
 
+        # test negative frame index
+        frame_minus1 = decoder.get_frame_at(-1)
+        ref_frame_minus1 = NASA_VIDEO.get_frame_data_by_index(389).to(device)
+        assert_frames_equal(ref_frame_minus1, frame_minus1.data)
+
         # test numpy.int64
         frame9 = decoder.get_frame_at(numpy.int64(9))
         assert_frames_equal(ref_frame9, frame9.data)
@@ -470,7 +488,7 @@ def test_get_frame_at_fails(self, device, seek_mode):
         decoder = VideoDecoder(NASA_VIDEO.path, device=device, seek_mode=seek_mode)
 
         with pytest.raises(IndexError, match="out of bounds"):
-            frame = decoder.get_frame_at(-1)  # noqa
+            frame = decoder.get_frame_at(-10000)  # noqa
 
         with pytest.raises(IndexError, match="out of bounds"):
             frame = decoder.get_frame_at(10000)  # noqa
@@ -480,7 +498,8 @@ def test_get_frame_at_fails(self, device, seek_mode):
     def test_get_frames_at(self, device, seek_mode):
         decoder = VideoDecoder(NASA_VIDEO.path, device=device, seek_mode=seek_mode)
 
-        frames = decoder.get_frames_at([35, 25])
+        # test positive and negative frame index
+        frames = decoder.get_frames_at([35, 25, -1, -2])
 
         assert isinstance(frames, FrameBatch)
 
@@ -490,12 +509,20 @@ def test_get_frames_at(self, device, seek_mode):
         assert_frames_equal(
             frames[1].data, NASA_VIDEO.get_frame_data_by_index(25).to(device)
         )
+        assert_frames_equal(
+            frames[2].data, NASA_VIDEO.get_frame_data_by_index(389).to(device)
+        )
+        assert_frames_equal(
+            frames[3].data, NASA_VIDEO.get_frame_data_by_index(388).to(device)
+        )
 
         assert frames.pts_seconds.device.type == "cpu"
         expected_pts_seconds = torch.tensor(
             [
                 NASA_VIDEO.get_frame_info(35).pts_seconds,
                 NASA_VIDEO.get_frame_info(25).pts_seconds,
+                NASA_VIDEO.get_frame_info(389).pts_seconds,
+                NASA_VIDEO.get_frame_info(388).pts_seconds,
             ],
             dtype=torch.float64,
         )
@@ -508,6 +535,8 @@ def test_get_frames_at(self, device, seek_mode):
             [
                 NASA_VIDEO.get_frame_info(35).duration_seconds,
                 NASA_VIDEO.get_frame_info(25).duration_seconds,
+                NASA_VIDEO.get_frame_info(389).duration_seconds,
+                NASA_VIDEO.get_frame_info(388).duration_seconds,
             ],
             dtype=torch.float64,
         )
@@ -520,8 +549,11 @@ def test_get_frames_at(self, device, seek_mode):
     def test_get_frames_at_fails(self, device, seek_mode):
         decoder = VideoDecoder(NASA_VIDEO.path, device=device, seek_mode=seek_mode)
 
-        with pytest.raises(RuntimeError, match="Invalid frame index=-1"):
-            decoder.get_frames_at([-1])
+        expected_converted_index = -10000 + len(decoder)
+        with pytest.raises(
+            RuntimeError, match=f"Invalid frame index={expected_converted_index}"
+        ):
+            decoder.get_frames_at([-10000])
 
         with pytest.raises(RuntimeError, match="Invalid frame index=390"):
             decoder.get_frames_at([390])