ONNX format support in the OpenVINO™

Starting from the 2020.4 release, OpenVINO™ supports reading native ONNX models. Core::ReadNetwork() method provides a uniform way to read models from IR or ONNX format, it is a recommended approach to reading models. Example:

auto network = core.ReadNetwork("model.onnx");
This class represents Inference Engine Core entity.
Definition: ie_core.hpp:31

Reshape feature:

OpenVINO™ doesn't provide a mechanism to specify pre-processing (like mean values subtraction, reverse input channels) for the ONNX format. If an ONNX model contains dynamic shapes for input, please use the CNNNetwork::reshape method for shape specialization.

Weights saved in external files:

OpenVINO™ supports ONNX models that store weights in external files. It is especially useful for models larger than 2GB because of protobuf limitations. To read such models, use the ReadNetwork overload which takes modelPath as input parameter (both std::string and std::wstring). Note that the binPath argument of ReadNetwork should be empty in this case, because paths to external weights are saved directly in an ONNX model. Otherwise, a runtime exception is thrown. Reading models with external weights is not supported by the ReadNetwork(const std::string& model, const Blob::CPtr& weights) overload.

Paths to external weight files are saved in an ONNX model; these paths are relative to the model's directory path. It means that if a model is located at: home/user/workspace/models/model.onnx and a file that contains external weights: home/user/workspace/models/data/weights.bin the path saved in model should be: data/weights.bin.


  • A single model can use many external weights files.
  • Data of many tensors can be stored in a single external weights file (it is processed using offset and length values, which can be also saved in a model).

The described mechanism is the only possibility to read weights from external files. The following input parameters of the ReadNetwork function overloads are NOT supported for ONNX models and should be passed as empty:

  • const std::wstring& binPath
  • const std::string& binPath
  • const Blob::CPtr& weights

You can find more details about the external data mechanism in ONNX documentation. To convert a model to use the external data feature, you can use ONNX helper functions.

Unsupported types of tensors:

  • string,
  • complex64,
  • complex128.