## 4 Computing Devices  
With the proliferation of wearable devices, there are plenty of computing platforms suited for our purposes. Examples of such devices are the Odroid XU4 [[https://www.hardkernel.com/shop/odroid-xu4-special-price/](https://www.hardkernel.com/shop/odroid-xu4-special-price/)]([https://www.hardkernel.com/shop/odroid-xu4-special-price/](https://www.hardkernel.com/shop/odroid-xu4-special-price/)) (accessed on 28 July 2021), equipped with a Samsung Exynos5422 Cortex A15 2 Ghz and Cortex A7 Octa-core CPUs, and devices belonging to the Raspberry Pi family [[https://www.raspberrypi.org/](https://www.raspberrypi.org/)]([https://www.raspberrypi.org/](https://www.raspberrypi.org/)) (accessed on 28 July 2021). These two classes of devices, such as many others available on the market, rely on ARM architectures, and, in this very case, have a very similar price, around 50$. We decided to focus on the Raspberry Pi ecosystem since it provides a wide range of cheap and power-efficient devices that would perfectly fit our aims and has a very large and active community. Specifically, we considered in our evaluation the Raspberry Pi models 3B, 3B+, and 4 (model B), all available at a very competitive price. Moreover, they have a vast community of developers worldwide and many add-ons are readily available off-the-shelf for fast prototyping. Although potentially suited for our purposes, we did not consider less powerful devices belonging to the same ecosystem, such as the Raspberry Pi Zero. Their shallow energy consumption did not seem balanced by an adequate computing performance required in the envisioned application in our preliminary analysis. All the devices considered in our experiments are equipped with the Linux operating system (Raspbian Buster).  
The Raspberry Pi 3B contains Quad Core 1.2 GHz Broadcom BCM2837 64 bit CPU with 1 GB DDR2 RAM memory. It also features Ethernet, wireless, Bluetooth Low Energy (BLE), four USB 2.0 ports and has, as other devices introduced next, a 40-pin GPIO connector. As for other Raspberry devices discussed next, specific functionality can be disabled through software commands to save energy.  
Concerning our purposes, the Raspberry Pi model 3B+ shares very similar specifications compared to model 3B except for a higher CPU frequency (1.4 vs. 1.2 GHz).  
The Raspberry 4 model B represents a significant modification compared to the 3B/3B+ design to achieve a higher level of performance. Specifically, its design is built around the Broadcom BCM2711B0 quad-core A72 (ARMv8-A) 64-bit clocked at 1.5 GHz and the DDR4 RAM memory available may be 1, 2, or 4 GB according to the user choice. Among the similar yet improved networking specifications, model 4 features two USB 3.0 and two USB 2.0 ports.  
It is worth observing that all the Raspberry Pi included in our experiments come equipped with an integrated graphic unit (GPU), in different flavors, potentially suited for speeding up image processing operations. Nonetheless, we did not notice any improvement regarding execution time with 3B and 3B+ models. Finally, for the 4 series, the GPU is not supported at all yet by OpenCV.  
Figure 3: **(Top)** a video sequence acquired on the Teverer river, **(Bottom)** a sequence acquired on the Secchia river; **(Left)** Input frame of the video sequence, **(Middle)** FAST features extracted from the input frame, and **(Right)** trajectories inferred by the OTV algorithm.