Object detection on the Raspberry Pi 4 with the Coral USB accelerator

SSD object detection with the Coral USB accelerator had been running on a Raspberry Pi 3 but the performance was disappointing and I was curious to see what would happen on the Raspberry Pi 4.

This is the test rt-ai design. The UVCCam and MediaView SPEs are running on an Ubuntu desktop, the CoralSSD SPE is running on the Raspberry Pi 4. It is getting a respectable 12fps with 1280 x 720 frames (an earlier version of this post had reported much worse performance but that was due to some silly image loading code). The utilization of one CPU core is around 93% which is fair enough for a Python SPE. I am sure that a C++ version of this SPE would be considerably faster again.

Getting this running at all was interesting as the Pi 4 requires Raspbian Buster and that comes with Python 3.7 which is not supported by the edgetpu_api toolkit at this point in time.

After writing the original blog post I discovered that in fact it is trivial to convert the edgetpu_api installation to work with Python 3.7. Without doing any virtualenv and Python 3.5 stuff, just run install.sh (modified as described below to recognize the Pi 4 and fix the sudo bug) and enter these commands:

cd /usr/local/lib/python3.7/dist-packages/edgetpu/swig
sudo cp _edgetpu_cpp_wrapper.cpython-35m-arm-linux-gnueabihf.so _edgetpu_cpp_wrapper.cpython-37m-arm-linux-gnueabihf.so

Turns out all it needed was a correctly named .so file to match the Python version. Anyway, if you want to go the Python 3.5 route…

The ARM version of the Python library is only compiled for Python 3.5. So, Python 3.5 needs to be installed alongside Python 3.7. To do this, download the GZipped source from here and expand and build with:

tar xzf Python-3.5.7.tgz
cd Python-3.5.7
sudo apt-get install libssl-dev
./configure --enable-optimizations
sudo make -j4 altinstall
virtualenv --python=python3.5 venv
source venv/bin/activate

The result of all of this should be Python 3.5 available in a virtual environment. Any specific packages that need to be installed should be installed using pip3.5 as required. Regarding numpy, I found that the install didn’t work for some reason (there were missing dependencies when imported) and I had to use this command (as described here):

pip3.5 install numpy --upgrade --no-binary :all:

Now it is time to install the edgetpu_api which is basically a case of following the instructions here. However, install.sh has a small bug and also will not recognize the Pi 4.

Modify install.sh to recognize the Pi 4 by adding this after line 59:

  elif [[ "${MODEL}" == "Raspberry Pi 4 Model B Rev"* ]]; then
    info "Recognized as Raspberry Pi 4 B."

Once that is added, go to line 128 and replace it with:

sudo udevadm control --reload-rules && sudo udevadm trigger

The original is missing the second sudo. Once that is done, the Coral USB accelerator should be able to run the bird classifier example.