Building custom processing blocks
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Extracting meaningful features from your data is crucial to building small and reliable machine learning models, and in Edge Impulse this is done through processing blocks. We ship a number of processing blocks for common sensor data (such as vibration and audio), but they might not be suitable for all applications. Perhaps you have a very specific sensor, want to apply custom filters, or are implementing the latest research in digital signal processing. In this tutorial you'll learn how to support these use cases by adding custom processing blocks to the studio.
There is also a complete video covering how to implement your custom DSP block:
This creates a copy of the example project locally. Then, you can run the example either through Docker or locally via:
Docker
Locally
Install the ngrok binary for your platform.
Get a URL to access the processing block from the outside world via:
This yields a public URL for your block under Forwarding. Note down the address that includes https://.
Now that the custom processing block was created, and you've made it accessible to the outside world, you can add this block to Edge Impulse. In a project, go to Create Impulse, click Add a processing block, choose Add custom block (in the bottom left corner of the modal), and paste in the public URL of the block:
After you click Add block the block will show like any other processing block.
Add a learning bloc, then click Save impulse to store the impulse.
Processing blocks have configuration options which are rendered on the block parameter page. These could be filter configurations, scaling options, or control which visualizations are loaded. These options are defined in the parameters.json file. Let's add an option to smooth raw data. Open example-custom-processing-block-python/parameters.json and add a new section under parameters:
Then, open example-custom-processing-block-python/dsp.py and replace its contents with:
Restart the Python script, and then click Custom block in the studio (in the navigation bar). You now have a new option 'Smooth'. Every time an option changes we'll re-run the block, but as we have not written any code to respond to these changes nothing will happen.
To show the user what is happening we can also draw visuals in the processing block. Right now we support graphs (linear and logarithmic) and arbitrary images. By showing a graph of the smoothed sample we can quickly identify what effect the smooth option has on the raw signal. Open dsp.py and replace the content with the following script. It contains a very basic smoothing algorithm and draws a graph:
Restart the script, and click the Smooth toggle to observe the difference. Congratulations! You have just created your first custom processing block.
If you extract set features from the signal, like the mean, that you that return, you can also label these features. These labels will be used in the feature explorer. To do so, add a labels array that contains strings that map back to the features you return (labels and features should have the same length).
In the previous step we drew a linear graph, but you can also draw logarithmic graphs or even full images. This is done through the type parameter:
This draws a graph with a logarithmic scale:
To show an image you should return the base64 encoded image and its MIME type. Here's how you draw a small PNG image:
If you output high-dimensional data (like a spectrogram or an image) you can enable dimensionality reduction for the feature explorer. This will run UMAP over the data to compress the features into three dimensions. To do so, set:
On the info object in parameters.json.
Your custom block behaves exactly the same as any of the built-in blocks. You can process all your data, train neural networks or anomaly blocks, and validate that your model works.
However, we cannot automatically generate optimized native code for the block, like we do for built-in processing blocks, but we try to help you write this code as much as possible.
Export as a C++ Library:
In the Edge Impulse platform, export your project as a C++ library.
Choose the model type that suits your target device (quantized vs. float32).
Forward Declaration:
You don't need to add this part, it is automatically generated!
In the model-parameters/model_variables.h file of the exported C++ library, you can see a forward declaration for the custom DSP block you created.
For example:
The name of that function comes from the cppType field in your custom DSP parameter.json. It takes your {cppType} and generates the following extract_{cppType}_features function.
Implement the Custom DSP Block:
In the main.cpp file of the C++ library, implement the extract_my_preprocessing_features block. This block should:
Call into the Edge Impulse SDK to generate features.
Execute the rest of the DSP block, including neural network inference.
Also, please have a look at the video on the top of this page (around minute 25) where Jan explains how to implement your custom DSP block with your C++ library.
Compile and Run the App
Copy a test sample's raw features into the features[] array in source/main.cpp
Enter make -j in this directory to compile the project. If you encounter any OOM memory error try make -j4 (replace 4 with the number of cores available)
Enter ./build/app to run the application
Compare the output predictions to the predictions of the test sample in the Edge Impulse Studio.
With good feature extraction you can make your machine learning models smaller and more reliable, which are both very important when you want to deploy your model on embedded devices. With custom processing blocks you can now develop new feature extraction pipelines straight from Edge Impulse. Whether you're following the latest research, want to implement proprietary algorithms, or are just exploring data.
This is the format for the parameters.json file:
\
Make sure you follow the tutorial, and have a trained impulse.
This tutorial shows you the development flow of building custom processing blocks, and requires you to run the processing block on your own machine or server. Enterprise customers can share processing blocks within their organization, and run these on our infrastructure. See for more details.
Processing blocks take data and configuration parameters in, and return features and visualizations like graphs or images. To communicate to custom processing blocks, Edge Impulse studio will make HTTP calls to the block, and then use the response both in the UI, while generating features, or when training a machine learning model. Thus, to load a custom processing block we'll need to run a small server that responds to these HTTP calls. You can write this in any language, but we have created in Python. To load this example, open a terminal and run:
Then go to and you should be shown some information about the block.
As this block is running locally the studio cannot reach the block. To resolve this we can use which can make a local port accessible from a public URL. After you've finished development you can move the processing block to a server with a publicly accessible address (or run it on our infrastructure through your enterprise account). To set up a tunnel:
Sign up for .
For the full documentation on customizing parameters, and a list of all configuration options; see .
For all options that you can return in a graph, see the return types in the API documentation.
For examples, have a look at our official DSP blocks implementations in our
Blog post:
For inspiration we have published all our own blocks here: . If you've made an interesting block that you think is valuable for the community, please let us know on the or by opening a pull request. We'd be happy to help write efficient native code for the block, and then publish it as a standard block!
