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Support negative index in SimpleVideoDecoder #743

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Merged
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Jun 26, 2025
Merged

Support negative index in SimpleVideoDecoder #743

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1b75a95
Support negative index in get_frame(s)_at
Dan-Flores Jun 25, 2025
3204722
Add tests for out of range negative index
Dan-Flores Jun 25, 2025
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5 changes: 5 additions & 0 deletions src/torchcodec/decoders/_video_decoder.py
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Original file line number Diff line number Diff line change
Expand Up @@ -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(
Expand All @@ -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
Expand Down
40 changes: 36 additions & 4 deletions test/test_decoders.py
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Original file line number Diff line number Diff line change
Expand Up @@ -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))
Expand Down Expand Up @@ -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)
Expand Down Expand Up @@ -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
Expand All @@ -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)

Expand All @@ -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,
)
Expand All @@ -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,
)
Expand All @@ -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)
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It is not necessary to get the expected index here, but I added it to match the other tests. Alternatively, the test can simply match the text: "Invalid frame index".

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It's fine to do, but I usually default to just matching the text that doesn't change.

with pytest.raises(
RuntimeError, match=f"Invalid frame index={expected_converted_index}"
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This error is from SingleStreamDecoder.cpp. Should we also be checking valid indices at the Python level, in _video_decoder.py?

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Good observation - at the Python level, we currently don't check the input when we get a list of indices (get_frames_at()) or a list of timestamps (get_frames_played_at()). The weak rationale was that we'd have to walk the list to do the check, and that seemed wasteful. Now that we're walking the list in get_frames_at() to deal with negative indices, we might as well do error checking at the same time.

The question, then, is what to do in get_frames_played_at()? For completeness, I think it does make sense to do error checking at the Python level there, but let's do that in a separate PR. We can also do some refactoring there, and make _getitem_slice() call VideoDecoder.get_frames_in_range() in order to get that error checking at the Python level as well.

):
decoder.get_frames_at([-10000])

with pytest.raises(RuntimeError, match="Invalid frame index=390"):
decoder.get_frames_at([390])
Expand Down
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