• Conversion
  • Model Source Preparation
  • PyTorch to ONNX
  • TFLite to ONNX
  • Other TensorFlow representations
  • Obtaining Blob
  • Using BlobConverter
  • Conversion via BlobConverter Web Interface
  • Conversion via BlobConverter CLI
  • Local Conversion
  • Model Optimizer
  • Compile Tool
  • Advanced Settings
  • Model Optimizer Flags
  • Data Type
  • Mean and Scale Values
  • Model Layout
  • Color Order
  • Model Compiler Flags
  • Input Layer Precision
  • Shaves
  • Export Example
  • Exporting the Model to ONNX
  • Convert ONNX to .blob Using BlobConverter


To deploy your custom models on OAK devices, it's essential to convert them from their initial frameworks (such as PyTorch, TFLite, etc.) into a MyriadX blob format, which is compatible with DepthAI. In this guide, we'll cover how to do this using the BlobConverter tool and also demonstrate how to perform local conversion.The conversion process is illustrated on the following image:
  1. Model Source: Begin with a model developed in a framework such as ONNX, Caffe, or one of the TensorFlow formats.
  2. Model Optimizer: Use the Model Optimizer to convert the model into OpenVINO's Intermediate Representation (IR), resulting in .xml (configuration file) and .bin (weights file).
  3. Model Compiler: Take the .xml and .bin files and compile them using the Model Compiler to create a .blob file.
  4. Deployment: Deploy the .blob file onto the MYRIAD-X processor within an OAK device for inference.

Model Source Preparation

In the first step, you'll prepare your model by converting it from its original framework into a format suitable for further conversion, which may include ONNX or other formats depending on the model's origin.

PyTorch to ONNX

You can utilize the PyTorch ONNX API to convert and export your model:
1import torch
2# Load your PyTorch model
3your_model = Model()
4# Create a dummy input tensor matching the input shape of the model
5dummy_input = torch.randn(1, 3, 224, 224)
6# Convert and save as ONNX
7torch.onnx.export(your_model, dummy_input, 'output.onnx')

TFLite to ONNX

For models in TensorFlow Lite (.tflite) format, the recommended conversion tool is tflite2onnx. This tool converts TFLite models to the ONNX format:
  • Firstly, install the tflite2onnx package:
Command Line
1pip install tflite2onnx
  • Then the conversion can be done via the command line:
Command Line
1tflite2onnx your_model.tflite output.onnx
Or through Python:
1import tflite2onnx
2tflite2onnx.convert('your_model.tflite', 'output.onnx')

Other TensorFlow representations

For non-TFLite TensorFlow models (such as SavedModel or Frozen Graph), the conversion directly involves OpenVINO's Model Optimizer. See the OpenVINO documentation for detailed instructions. Following the OpenVINO conversion, you can then use either BlobConverter or OpenVINO's Compile Tool to obtain a .blob file.

Obtaining Blob

There are two ways to acquire a .blob file: the first and the easiest method is using BlobConverter, which is also recommended. Alternatively, you use the local method, which involves directly utilizing OpenVINO tools.

Using BlobConverter

BlobConverter offers a straightforward approach to obtain the .blob file. This tool is accessible through a web interface, an API, and a command-line interface (CLI). The following sections outline the steps for using each of these tools to convert your model into .blob.

Conversion via BlobConverter Web Interface

  • Go to the BlobConverter website.
  • Select the OpenVINO version you wish to utilize. We will be using the latest version supported by BlobConverter, which is currently 2022.1. For RAE and other devices with RVC3, you can simply pick RVC3. After choosing the version, indicate the model source. In our case, the ONNX Model, but it is also possible to upload your model in the IR format. Then click on Continue.
  • Upload the ONNX file by clicking on Choose file.
  • Additionally, before proceeding with the model conversion, you can customize conversion parameters by clicking on Advanced.
  • Finally, click on Convert and simply wait until the process is finished.
Alternatively, you can use the BlobConverter API, which is particularly useful for automated workflows. This can be done by making HTTP requests to the BlobConverter service with the necessary model and parameters. You can find more information of how to do this by clicking on the Use API button at the top right corner of the website.

Conversion via BlobConverter CLI

Command Line
1python3 -m pip install blobconverter
  • Using the package, you can convert your model both from the command line and from the Python script directly:
Command Line
1python3 -m blobconverter --onnx-model /path/to/model.onnx --shaves 6
1import blobconverter
3blob_path = blobconverter.from_onnx(
4    model="/path/to/model.onnx",
5    data_type="FP16",
6    shaves=6,

Local Conversion

Local conversion is ideal for offline use, allowing you to obtain the .blob file using your own system. It's particularly useful in settings with limited internet access or for integrating the conversion into your workflow. The upcoming steps will guide you through this process using tools like OpenVINO's Model Optimizer and Compile Tool.

Model Optimizer

OpenVINO's Model Optimizer converts the model from its original framework format into OpenVINO's Intermediate Representation (IR) standard format (.bin and .xml). This standardized model format can be deployed on various Intel devices, including VPU. Moreover, you can customize the conversion process by specifying various flags, which we will explain in the upcoming sections.To perform the conversion, ensure you have OpenVINO-dev installed. Please keep in mind that this method supports OpenVINO version 2022.1 and does not include support for later versions:
Command Line
1pip install openvino-dev==2022.1
Then run the command as follows:
Command Line
1mo --input_model path/to/model.onnx --data_type=FP16 --mean_values=[0,0,0] --scale_values=[255,255,255]

Compile Tool

Once the model has been transformed into OpenVINO's IR format, the next step is to utilize OpenVINO's Compile Tool. This tool is employed to compile the model in IR format into a .blob file, which is then ready for deployment on the device.
  • The Compile Tool is part of the OpenVINO toolkit. Its location will depend on your installation path. Typically, it's found in the .../tools/compile_tool directory of your OpenVINO installation:
Command Line
1cd .../tools/compile_tool
  • Use the following command format to compile your IR model into a .blob file:
Command Line
1./compile_tool -m path_to_model/model_name.xml -d MYRIAD

Advanced Settings

Model Optimizer Flags

Data Type

Because we are converting for VPU (which supports FP16), it's necessary to use the parameter --data_type=FP16. For OpenVINO version 2022.3 and later, the parameter --compress_to_fp16 should be utilized instead. You can find additional details here.

Mean and Scale Values

The normalization of input images for the model is achieved through the --mean_values and --scale_values. By default, frames from ColorCamera/MonoCamera are in U8 data type, ranging from [0,255].However, models are typically trained with normalized frames within the range of [-1,1] or [0,1]. To ensure accurate inference results, frames need to be normalized beforehand.Although creating a custom model that normalizes frames before inference is an option (example here), it is more efficient to include this normalization directly within the model itself using the flags during model optimizer step.Here are some common normalization options (assuming that the initial input is in the range of [0,255]):
  • For required input with values between 0 and 1, use mean=0 and scale=255, computed as ([0,255] - 0) / 255 = [0,1].
  • For required input with values between -1 and 1, use mean=127.5 and scale=127.5, computed as ([0,255] - 127.5) / 127.5 = [-1,1].
  • For required input with values between -0.5 and 0.5, use mean=127.5 and scale=255, computed as ([0,255] - 127.5) / 255 = [-0.5,0.5].
For more information, refer to OpenVINO's documentation.

Model Layout

The model layout can be defined using the --layout parameter. For example:
Command Line
1--layout NCHW
In following configuration:
  • N - batch size
  • C - channels
  • H - height
  • W - width
If the image layout does not match the model layout, DepthAI will display a corresponding error message: [NeuralNetwork(0)] [warning] Input image (416x416) does not match NN (3x416)It's important to note that the ColorCamera node typically outputs preview frames in the Interleaved / HWC layout by default, which is native to OpenCV. However, you have the option to switch it to the Planar / CHW layout through the API:
1import depthai as dai
2pipeline = dai.Pipeline()
3colorCam = pipeline.createColorCamera()
4colorCam.setInterleaved(False) # False = Planar layout
You can find further details in OpenVINO's documentation.

Color Order

Neural network models are commonly trained using images in RGB color order. The ColorCamera node, by default, outputs frames in BGR format. Mismatching the color order between input frames and the trained model can lead to inaccurate predictions. To address this, the --reverse_input_channels flag is utilized.Moreover, there is an option to switch the camera output to RGB via the API, eliminating the need for the flag:
1import depthai as dai
2pipeline = dai.Pipeline()
3colorCam = pipeline.createColorCamera()
4colorCam.setColorOrder(dai.ColorCameraProperties.ColorOrder.RGB) # RGB color order, BGR by default
You can find further details in OpenVINO's documentation.

Model Compiler Flags

Input Layer Precision

Using -ip U8 will incorporate a conversion layer U8->FP16 on all input layers of the model, which is typically the desired configuration. However, in specific scenarios, such as when working with data other than frames, using FP16 precision directly is necessary. In such cases, you can opt for -ip FP16, as demonstrated in the Cosine distance model example.


Increasing the number of SHAVEs during compilation can enhance the model's speed, although the relationship between SHAVE cores and performance is not linear. The firmware will provide a warning suggesting an optimal number of SHAVE cores, which is typically half of the available cores.

Export Example

This guide will walk you through the process of exporting ResNet18, a widely-used deep neural network for image classification, to a .blob file for deployment on OAK devices. We will use torchvision for accessing the pre-trained version of the model.

Exporting the Model to ONNX

First, we will export the ResNet18 model from PyTorch to the ONNX format.
1import torch
2import torchvision.models as models
4# Load the pretrained ResNet18 model from torchvision
5resnet18 = models.resnet18(pretrained=True)
7# Set the model to evaluation mode
10# Create a dummy input tensor matching the input shape of the model
11dummy_input = torch.randn(1, 3, 224, 224)
13# Export the model to an ONNX file
15    resnet18,
16    dummy_input,
17    'resnet18.onnx',
18    export_params=True,
19    opset_version=11,
20    input_names=['input'],
21    output_names=['output']
Parameters Explanation:
  • export_params: This flag ensures that the trained parameters are exported along with the model structure.
  • opset_version: Specifies the ONNX version to use. While we typically use version 11 to ensure compatibility with ResNet18's requirements, higher versions could also be applicable.
  • input_names and output_names: We use these flags to name the model's input and output nodes for clarity. In our example, the input node is named "input" and the output node "output".
  • After exporting, you'll get a file named "resnet18.onnx" as defined in the third argument.

Convert ONNX to .blob Using BlobConverter

Instead of manually converting the ONNX file to OpenVINO IR and then compiling it, we'll use BlobConverter to handle both steps.
  • Go to the BlobConverter website.
  • Choose the appropriate OpenVINO version, which for this example, is 2022.1.
  • Upload the .onnx file and enter any necessary Model Optimizer parameters in the 'Advanced' settings.
  • --data_type: Set to 'FP16' for compatible precision with the VPU processor.
  • --mean_values: Set to [123.675, 116.28, 103.53]. These values correspond to the average of the red, green, and blue channels across all images in the ImageNet dataset (on which ResNet18 was trained).
  • --scale_values: Set to [58.395, 57.12, 57.375] which are the standard deviations of each channel. This scaling ensures that the range of pixel values in the input image matches the range in the training data, which is important for the model to perform correctly.
  • --reverse_input_channels: Use this flag to switch from BGR to RGB, since the ColorCamera node outputs frames in the BGR format, and the model requires RGB images.
  • So at the end, the flags should look like this:
Command Line
1--data_type=FP16 --mean_values=[123.675,116.28,103.53] --scale_values=[58.395,57.12,57.375] --reverse_input_channels
  • Click Convert to start the conversion and then download the .blob file once the process is completed.
After following these instructions, you will get a resnet18.blob file that is ready for inference on OAK devices. The converted model will expect images in the BGR format with pixel values ranging from 0 to 255. Then these will be scaled to a range of 0 to 1 and normalized using the flags we set.