Support direct AVFrame conversion to tensor in CPU device interface

Signed-off-by: Dmitry Rogozhkin <dmitry.v.rogozhkin@intel.com>

Author: dvrogozh

src/torchcodec/_core/CpuDeviceInterface.cpp

@@ -83,6 +83,24 @@ void CpuDeviceInterface::convertAVFrameToFrameOutput(
   enum AVPixelFormat frameFormat =
       static_cast<enum AVPixelFormat>(avFrame->format);
 
+  // This is an early-return optimization: if the format is already what we
+  // need, and the dimensions are also what we need, we don't need to call
+  // swscale or filtergraph. We can just convert the AVFrame to a tensor.
+  if (frameFormat == AV_PIX_FMT_RGB24 &&
+      avFrame->width == expectedOutputWidth &&
+      avFrame->height == expectedOutputHeight) {
+    outputTensor = toTensor(avFrame);
+    if (preAllocatedOutputTensor.has_value()) {
+      // We have already validated that preAllocatedOutputTensor and
+      // outputTensor have the same shape.
+      preAllocatedOutputTensor.value().copy_(outputTensor);
+      frameOutput.data = preAllocatedOutputTensor.value();
+    } else {
+      frameOutput.data = outputTensor;
+    }
+    return;
+  }
+
   // By default, we want to use swscale for color conversion because it is
   // faster. However, it has width requirements, so we may need to fall back
   // to filtergraph. We also need to respect what was requested from the
@@ -159,7 +177,7 @@ void CpuDeviceInterface::convertAVFrameToFrameOutput(
           std::make_unique<FilterGraph>(filtersContext, videoStreamOptions);
       prevFiltersContext_ = std::move(filtersContext);
     }
-    outputTensor = convertAVFrameToTensorUsingFilterGraph(avFrame);
+    outputTensor = toTensor(filterGraphContext_->convert(avFrame));
 
     // Similarly to above, if this check fails it means the frame wasn't
     // reshaped to its expected dimensions by filtergraph.
@@ -208,23 +226,20 @@ int CpuDeviceInterface::convertAVFrameToTensorUsingSwsScale(
   return resultHeight;
 }
 
-torch::Tensor CpuDeviceInterface::convertAVFrameToTensorUsingFilterGraph(
-    const UniqueAVFrame& avFrame) {
-  UniqueAVFrame filteredAVFrame = filterGraphContext_->convert(avFrame);
-
-  TORCH_CHECK_EQ(filteredAVFrame->format, AV_PIX_FMT_RGB24);
+torch::Tensor CpuDeviceInterface::toTensor(const UniqueAVFrame& avFrame) {
+  TORCH_CHECK_EQ(avFrame->format, AV_PIX_FMT_RGB24);
 
-  auto frameDims = getHeightAndWidthFromResizedAVFrame(*filteredAVFrame.get());
+  auto frameDims = getHeightAndWidthFromResizedAVFrame(*avFrame.get());
   int height = frameDims.height;
   int width = frameDims.width;
   std::vector<int64_t> shape = {height, width, 3};
-  std::vector<int64_t> strides = {filteredAVFrame->linesize[0], 3, 1};
-  AVFrame* filteredAVFramePtr = filteredAVFrame.release();
-  auto deleter = [filteredAVFramePtr](void*) {
-    UniqueAVFrame avFrameToDelete(filteredAVFramePtr);
+  std::vector<int64_t> strides = {avFrame->linesize[0], 3, 1};
+  AVFrame* avFrameClone = av_frame_clone(avFrame.get());
+  auto deleter = [avFrameClone](void*) {
+    UniqueAVFrame avFrameToDelete(avFrameClone);
   };
   return torch::from_blob(
-      filteredAVFramePtr->data[0], shape, strides, deleter, {torch::kUInt8});
+      avFrameClone->data[0], shape, strides, deleter, {torch::kUInt8});
 }
 
 void CpuDeviceInterface::createSwsContext(

src/torchcodec/_core/CpuDeviceInterface.h

@@ -39,8 +39,7 @@ class CpuDeviceInterface : public DeviceInterface {
       const UniqueAVFrame& avFrame,
       torch::Tensor& outputTensor);
 
-  torch::Tensor convertAVFrameToTensorUsingFilterGraph(
-      const UniqueAVFrame& avFrame);
+  torch::Tensor toTensor(const UniqueAVFrame& avFrame);
 
   struct SwsFrameContext {
     int inputWidth = 0;