Test samplers against reference (#297)

Author: NicolasHug

src/torchcodec/samplers/_time_based.py

@@ -71,7 +71,7 @@ def _validate_sampling_range_time_based(
     if sampling_range_start is None:
         sampling_range_start = begin_stream_seconds
     else:
-        if sampling_range_start <= begin_stream_seconds:
+        if sampling_range_start < begin_stream_seconds:
             raise ValueError(
                 f"sampling_range_start ({sampling_range_start}) must be at least {begin_stream_seconds}"
             )

test/samplers/test_samplers.py

@@ -148,6 +148,51 @@ def test_time_based_sampler(sampler, seconds_between_frames):
         )
 
 
+@pytest.mark.parametrize(
+    "sampler",
+    (
+        partial(
+            clips_at_regular_indices,
+            num_clips=1,
+            sampling_range_start=0,
+            sampling_range_end=1,
+        ),
+        partial(
+            clips_at_random_indices,
+            num_clips=1,
+            sampling_range_start=0,
+            sampling_range_end=1,
+        ),
+        partial(
+            clips_at_random_timestamps,
+            num_clips=1,
+            sampling_range_start=0,
+            sampling_range_end=0.01,
+        ),
+        partial(
+            clips_at_regular_timestamps,
+            seconds_between_clip_starts=1,
+            seconds_between_frames=0.0335,  # forces consecutive frames
+            sampling_range_start=0,
+            sampling_range_end=0.01,
+        ),
+    ),
+)
+def test_against_ref(sampler):
+    # Force the sampler to sample a clip containing the first 5 frames of the
+    # video. We can then assert the exact frame values against our existing test
+    # resource reference.
+    decoder = VideoDecoder(NASA_VIDEO.path)
+
+    num_frames_per_clip = 5
+    expected_clip_data = NASA_VIDEO.get_frame_data_by_range(
+        start=0, stop=num_frames_per_clip
+    )
+
+    clip = sampler(decoder, num_frames_per_clip=num_frames_per_clip)[0]
+    assert_tensor_equal(clip.data, expected_clip_data)
+
+
 @pytest.mark.parametrize(
     "sampler, sampling_range_start, sampling_range_end, assert_all_equal",
     (