Added proper tensor support for get_frames_at()

src/torchcodec/_core/SingleStreamDecoder.cpp

@@ -602,25 +602,34 @@ FrameOutput SingleStreamDecoder::getFrameAtIndexInternal(
 }
 
 FrameBatchOutput SingleStreamDecoder::getFramesAtIndices(
-    const std::vector<int64_t>& frameIndices) {
+    const torch::Tensor& frameIndices) {
   validateActiveStream(AVMEDIA_TYPE_VIDEO);
 
-  auto indicesAreSorted =
-      std::is_sorted(frameIndices.begin(), frameIndices.end());
+  auto frameIndicesAccessor = frameIndices.accessor<int64_t, 1>();
+
+  bool indicesAreSorted = true;
+  for (int64_t i = 1; i < frameIndices.numel(); ++i) {
+    if (frameIndicesAccessor[i] < frameIndicesAccessor[i - 1]) {
+      indicesAreSorted = false;
+      break;
+    }
+  }
 
   std::vector<size_t> argsort;
   if (!indicesAreSorted) {
     // if frameIndices is [13, 10, 12, 11]
     // when sorted, it's  [10, 11, 12, 13] <-- this is the sorted order we want
     //                                         to use to decode the frames
     // and argsort is     [ 1,  3,  2,  0]
-    argsort.resize(frameIndices.size());
+    argsort.resize(frameIndices.numel());
     for (size_t i = 0; i < argsort.size(); ++i) {
       argsort[i] = i;
     }
     std::sort(
-        argsort.begin(), argsort.end(), [&frameIndices](size_t a, size_t b) {
-          return frameIndices[a] < frameIndices[b];
+        argsort.begin(),
+        argsort.end(),
+        [&frameIndicesAccessor](size_t a, size_t b) {
+          return frameIndicesAccessor[a] < frameIndicesAccessor[b];
         });
   }
 
@@ -629,12 +638,12 @@ FrameBatchOutput SingleStreamDecoder::getFramesAtIndices(
   const auto& streamInfo = streamInfos_[activeStreamIndex_];
   const auto& videoStreamOptions = streamInfo.videoStreamOptions;
   FrameBatchOutput frameBatchOutput(
-      frameIndices.size(), videoStreamOptions, streamMetadata);
+      frameIndices.numel(), videoStreamOptions, streamMetadata);
 
   auto previousIndexInVideo = -1;
-  for (size_t f = 0; f < frameIndices.size(); ++f) {
+  for (int64_t f = 0; f < frameIndices.numel(); ++f) {
     auto indexInOutput = indicesAreSorted ? f : argsort[f];
-    auto indexInVideo = frameIndices[indexInOutput];
+    auto indexInVideo = frameIndicesAccessor[indexInOutput];
 
     if ((f > 0) && (indexInVideo == previousIndexInVideo)) {
       // Avoid decoding the same frame twice
@@ -776,7 +785,8 @@ FrameBatchOutput SingleStreamDecoder::getFramesPlayedAt(
     frameIndices[i] = secondsToIndexLowerBound(frameSeconds);
   }
 
-  return getFramesAtIndices(frameIndices);
+  // TODO: Support tensors natively instead of a vector to avoid a copy.
+  return getFramesAtIndices(torch::tensor(frameIndices));
 }
 
 FrameBatchOutput SingleStreamDecoder::getFramesPlayedInRange(

src/torchcodec/_core/SingleStreamDecoder.h

@@ -106,7 +106,7 @@ class SingleStreamDecoder {
 
   // Returns frames at the given indices for a given stream as a single stacked
   // Tensor.
-  FrameBatchOutput getFramesAtIndices(const std::vector<int64_t>& frameIndices);
+  FrameBatchOutput getFramesAtIndices(const torch::Tensor& frameIndices);
 
   // Returns frames within a given range. The range is defined by [start, stop).
   // The values retrieved from the range are: [start, start+step,

src/torchcodec/_core/custom_ops.cpp

@@ -55,7 +55,7 @@ TORCH_LIBRARY(torchcodec_ns, m) {
   m.def(
       "get_frame_at_index(Tensor(a!) decoder, *, int frame_index) -> (Tensor, Tensor, Tensor)");
   m.def(
-      "get_frames_at_indices(Tensor(a!) decoder, *, int[] frame_indices) -> (Tensor, Tensor, Tensor)");
+      "get_frames_at_indices(Tensor(a!) decoder, *, Tensor frame_indices) -> (Tensor, Tensor, Tensor)");
   m.def(
       "get_frames_in_range(Tensor(a!) decoder, *, int start, int stop, int? step=None) -> (Tensor, Tensor, Tensor)");
   m.def(
@@ -378,11 +378,9 @@ OpsFrameOutput get_frame_at_index(at::Tensor& decoder, int64_t frame_index) {
 // Return the frames at given indices for a given stream
 OpsFrameBatchOutput get_frames_at_indices(
     at::Tensor& decoder,
-    at::IntArrayRef frame_indices) {
+    const at::Tensor& frame_indices) {
   auto videoDecoder = unwrapTensorToGetDecoder(decoder);
-  std::vector<int64_t> frameIndicesVec(
-      frame_indices.begin(), frame_indices.end());
-  auto result = videoDecoder->getFramesAtIndices(frameIndicesVec);
+  auto result = videoDecoder->getFramesAtIndices(frame_indices);
   return makeOpsFrameBatchOutput(result);
 }
 

src/torchcodec/_core/ops.py

@@ -114,7 +114,9 @@ def load_torchcodec_shared_libraries():
 get_next_frame = torch.ops.torchcodec_ns.get_next_frame.default
 get_frame_at_pts = torch.ops.torchcodec_ns.get_frame_at_pts.default
 get_frame_at_index = torch.ops.torchcodec_ns.get_frame_at_index.default
-get_frames_at_indices = torch.ops.torchcodec_ns.get_frames_at_indices.default
+_get_frames_at_indices_tensor_input = (
+    torch.ops.torchcodec_ns.get_frames_at_indices.default
+)
 get_frames_by_pts = torch.ops.torchcodec_ns.get_frames_by_pts.default
 get_frames_in_range = torch.ops.torchcodec_ns.get_frames_in_range.default
 get_frames_by_pts_in_range = torch.ops.torchcodec_ns.get_frames_by_pts_in_range.default
@@ -198,6 +200,18 @@ def encode_audio_to_file_like(
     )
 
 
+def get_frames_at_indices(
+    decoder: torch.Tensor, *, frame_indices: Union[torch.Tensor, list[int]]
+) -> Tuple[torch.Tensor, torch.Tensor, torch.Tensor]:
+    if isinstance(frame_indices, torch.Tensor):
+        # Ensure indices is the correct dtype (int64)
+        frame_indices = frame_indices.to(torch.int64)
+    else:
+        # Convert list to tensor for dispatch
+        frame_indices = torch.tensor(frame_indices)
+    return _get_frames_at_indices_tensor_input(decoder, frame_indices=frame_indices)
+
+
 # ==============================
 # Abstract impl for the operators. Needed by torch.compile.
 # ==============================
@@ -371,9 +385,7 @@ def get_frame_at_index_abstract(
 
 @register_fake("torchcodec_ns::get_frames_at_indices")
 def get_frames_at_indices_abstract(
-    decoder: torch.Tensor,
-    *,
-    frame_indices: List[int],
+    decoder: torch.Tensor, *, frame_indices: Union[torch.Tensor, List[int]]
 ) -> Tuple[torch.Tensor, torch.Tensor, torch.Tensor]:
     image_size = [get_ctx().new_dynamic_size() for _ in range(4)]
     return (

src/torchcodec/_samplers/video_clip_sampler.py

@@ -227,6 +227,8 @@ def _get_clips_for_index_based_sampling(
                 clip_start_idx + i * index_based_sampler_args.video_frame_dilation
                 for i in range(index_based_sampler_args.frames_per_clip)
             ]
+            # Need torch.stack to convert List[Tensor[int]] into 1D Tensor[int]
+            batch_indexes = torch.stack(batch_indexes)
             frames, *_ = get_frames_at_indices(
                 video_decoder,
                 frame_indices=batch_indexes,

src/torchcodec/decoders/_video_decoder.py

@@ -240,24 +240,20 @@ def get_frame_at(self, index: int) -> Frame:
             duration_seconds=duration_seconds.item(),
         )
 
-    def get_frames_at(self, indices: list[int]) -> FrameBatch:
+    def get_frames_at(self, indices: Union[torch.Tensor, list[int]]) -> FrameBatch:
         """Return frames at the given indices.
 
         Args:
-            indices (list of int): The indices of the frames to retrieve.
+            indices (torch.Tensor or list of int): The indices of the frames to retrieve.
 
         Returns:
             FrameBatch: The frames at the given indices.
         """
-        if isinstance(indices, torch.Tensor):
-            # TODO we should avoid converting tensors to lists and just let the
-            # core ops and C++ code natively accept tensors.  See
-            # https://github.com/pytorch/torchcodec/issues/879
-            indices = indices.to(torch.int).tolist()
 
         data, pts_seconds, duration_seconds = core.get_frames_at_indices(
             self._decoder, frame_indices=indices
         )
+
         return FrameBatch(
             data=data,
             pts_seconds=pts_seconds,

test/VideoDecoderTest.cpp

@@ -222,7 +222,8 @@ TEST_P(SingleStreamDecoderTest, DecodesFramesInABatchInNCHW) {
       *ourDecoder->getContainerMetadata().bestVideoStreamIndex;
   ourDecoder->addVideoStream(bestVideoStreamIndex);
   // Frame with index 180 corresponds to timestamp 6.006.
-  auto output = ourDecoder->getFramesAtIndices({0, 180});
+  auto frameIndices = torch::tensor({0, 180});
+  auto output = ourDecoder->getFramesAtIndices(frameIndices);
   auto tensor = output.data;
   EXPECT_EQ(tensor.sizes(), std::vector<long>({2, 3, 270, 480}));
 
@@ -246,7 +247,8 @@ TEST_P(SingleStreamDecoderTest, DecodesFramesInABatchInNHWC) {
   videoStreamOptions.dimensionOrder = "NHWC";
   ourDecoder->addVideoStream(bestVideoStreamIndex, videoStreamOptions);
   // Frame with index 180 corresponds to timestamp 6.006.
-  auto output = ourDecoder->getFramesAtIndices({0, 180});
+  auto frameIndices = torch::tensor({0, 180});
+  auto output = ourDecoder->getFramesAtIndices(frameIndices);
   auto tensor = output.data;
   EXPECT_EQ(tensor.sizes(), std::vector<long>({2, 270, 480, 3}));
 

test/test_decoders.py

@@ -569,7 +569,9 @@ def test_get_frames_at_fails(self, device, seek_mode):
         with pytest.raises(IndexError, match="Invalid frame index=390"):
             decoder.get_frames_at([390])
 
-        with pytest.raises(RuntimeError, match="Expected a value of type"):
+        with pytest.raises(
+            RuntimeError, match="expected scalar type Long but found Float"
+        ):
             decoder.get_frames_at([0.3])
 
     @pytest.mark.parametrize("device", all_supported_devices())

test/test_ops.py

@@ -1209,7 +1209,7 @@ def seek(self, offset: int, whence: int) -> int:
         torch.manual_seed(0)
         indices = torch.randint(
             0, len(NASA_VIDEO.frames[NASA_VIDEO.default_stream_index]), size=(50,)
-        ).tolist()
+        )
 
         frames_file_like, *_ = get_frames_at_indices(
             decoder_file_like, frame_indices=indices