Decoder-native resize public implementation

src/torchcodec/_core/Transform.cpp

@@ -42,7 +42,7 @@ int toSwsInterpolation(ResizeTransform::InterpolationMode mode) {
 std::string ResizeTransform::getFilterGraphCpu() const {
   return "scale=" + std::to_string(outputDims_.width) + ":" +
       std::to_string(outputDims_.height) +
-      ":sws_flags=" + toFilterGraphInterpolation(interpolationMode_);
+      ":flags=" + toFilterGraphInterpolation(interpolationMode_);
 }
 
 std::optional<FrameDims> ResizeTransform::getOutputFrameDims() const {

src/torchcodec/decoders/_video_decoder.py

@@ -8,7 +8,7 @@
 import json
 import numbers
 from pathlib import Path
-from typing import Literal, Optional, Tuple, Union
+from typing import Any, List, Literal, Optional, Tuple, Union
 
 import torch
 from torch import device as torch_device, Tensor
@@ -103,6 +103,7 @@ def __init__(
         dimension_order: Literal["NCHW", "NHWC"] = "NCHW",
         num_ffmpeg_threads: int = 1,
         device: Optional[Union[str, torch_device]] = "cpu",
+        transforms: List[Any] = [],  # TRANSFORMS TODO: what is the user-facing type?
         seek_mode: Literal["exact", "approximate"] = "exact",
         custom_frame_mappings: Optional[
             Union[str, bytes, io.RawIOBase, io.BufferedReader]
@@ -148,13 +149,16 @@ def __init__(
 
         device_variant = _get_cuda_backend()
 
+        transform_specs = make_transform_specs(transforms)
+
         core.add_video_stream(
             self._decoder,
             stream_index=stream_index,
             dimension_order=dimension_order,
             num_threads=num_ffmpeg_threads,
             device=device,
             device_variant=device_variant,
+            transform_specs=transform_specs,
             custom_frame_mappings=custom_frame_mappings_data,
         )
 
@@ -432,6 +436,22 @@ def _get_and_validate_stream_metadata(
     )
 
 
+def make_transform_specs(transforms: List[Any]) -> str:
+    from torchvision.transforms import v2
+
+    transform_specs = []
+    for transform in transforms:
+        if isinstance(transform, v2.Resize):
+            if len(transform.size) != 2:
+                raise ValueError(
+                    f"Resize transform must have a (height, width) pair for the size, got {transform.size}."
+                )
+            transform_specs.append(f"resize, {transform.size[0]}, {transform.size[1]}")
+        else:
+            raise ValueError(f"Unsupported transform {transform}.")
+    return ";".join(transform_specs)
+
+
 def _read_custom_frame_mappings(
     custom_frame_mappings: Union[str, bytes, io.RawIOBase, io.BufferedReader]
 ) -> tuple[Tensor, Tensor, Tensor]:

test/generate_reference_resources.py

@@ -123,6 +123,15 @@ def generate_nasa_13013_references():
             NASA_VIDEO, frame_index=frame, stream_index=3, filters=crop_filter
         )
 
+    frames = [17, 230, 389]
+    # Note that the resize algorithm passed to flags is exposed to users,
+    # but bilinear is the default we use.
+    resize_filter = "scale=240:135:flags=bilinear"
+    for frame in frames:
+        generate_frame_by_index(
+            NASA_VIDEO, frame_index=frame, stream_index=3, filters=resize_filter
+        )
+
 
 def generate_h265_video_references():
     # This video was generated by running the following:

test/test_transform_ops.py

@@ -22,15 +22,75 @@
     get_json_metadata,
     get_next_frame,
 )
+from torchcodec.decoders import VideoDecoder
 
 from torchvision.transforms import v2
 
-from .utils import assert_frames_equal, NASA_VIDEO, needs_cuda
+from .utils import assert_frames_equal, NASA_VIDEO, needs_cuda, psnr
 
 torch._dynamo.config.capture_dynamic_output_shape_ops = True
 
 
-class TestVideoDecoderTransformOps:
+class TestPublicVideoDecoderTransformOps:
+    @pytest.mark.parametrize(
+        "height_scaling_factor, width_scaling_factor",
+        ((1.5, 1.31), (0.5, 0.71), (0.7, 1.31), (1.5, 0.71), (1.0, 1.0)),
+    )
+    def test_resize_torchvision(self, height_scaling_factor, width_scaling_factor):
+        height = int(NASA_VIDEO.get_height() * height_scaling_factor)
+        width = int(NASA_VIDEO.get_width() * width_scaling_factor)
+
+        decoder_resize = VideoDecoder(
+            NASA_VIDEO.path, transforms=[v2.Resize(size=(height, width))]
+        )
+        decoder_full = VideoDecoder(NASA_VIDEO.path)
+        for frame_index in [0, 10, 17, 100, 230, 389]:
+            expected_shape = (NASA_VIDEO.get_num_color_channels(), height, width)
+            frame_resize = decoder_resize[frame_index]
+
+            frame_full = decoder_full[frame_index]
+            frame_tv = v2.functional.resize(frame_full, size=(height, width))
+
+            assert frame_resize.shape == expected_shape
+            assert frame_tv.shape == expected_shape
+
+            # Copied from PR #992; not sure if it's the best way to check
+            assert psnr(frame_resize, frame_tv) > 25
+
+    def test_resize_ffmpeg(self):
+        height = 135
+        width = 240
+        expected_shape = (NASA_VIDEO.get_num_color_channels(), height, width)
+        resize_filtergraph = f"scale={width}:{height}:flags=bilinear"
+        decoder_resize = VideoDecoder(
+            NASA_VIDEO.path, transforms=[v2.Resize(size=(height, width))]
+        )
+        for frame_index in [17, 230, 389]:
+            frame_resize = decoder_resize[frame_index]
+            frame_ref = NASA_VIDEO.get_frame_data_by_index(
+                frame_index, filters=resize_filtergraph
+            )
+
+            assert frame_resize.shape == expected_shape
+            assert frame_ref.shape == expected_shape
+            assert_frames_equal(frame_resize, frame_ref)
+
+    def test_resize_fails(self):
+        with pytest.raises(
+            ValueError,
+            match=r"must have a \(height, width\) pair for the size",
+        ):
+            VideoDecoder(NASA_VIDEO.path, transforms=[v2.Resize(size=(100))])
+
+    def test_transform_fails(self):
+        with pytest.raises(
+            ValueError,
+            match="Unsupported transform",
+        ):
+            VideoDecoder(NASA_VIDEO.path, transforms=[v2.RandomHorizontalFlip(p=1.0)])
+
+
+class TestCoreVideoDecoderTransformOps:
     # We choose arbitrary values for width and height scaling to get better
     # test coverage. Some pairs upscale the image while others downscale it.
     @pytest.mark.parametrize(

test/utils.py

@@ -430,7 +430,7 @@ def empty_chw_tensor(self) -> torch.Tensor:
             [0, self.num_color_channels, self.height, self.width], dtype=torch.uint8
         )
 
-    def get_width(self, *, stream_index: Optional[int]) -> int:
+    def get_width(self, *, stream_index: Optional[int] = None) -> int:
         if stream_index is None:
             stream_index = self.default_stream_index