## 2 System Architecture
### The Wearable Camera Architecture  
For the HMT gesture recognition in this study, we designed a new wearable wrist-worn camera. The hardware structure is shown in Figure 2. We used SolidWorks to design the lightweight and foldable structures firstly, as shown in Figure 2a. The device is designed according to the requirements of lightweight, unobtrusiveness, and portability, as a wearable device for the daily use of the elderly [26]. The base, shell, and camera hood are all manufactured by the 3D printing of nylon material (the thickness of printing shell is one mm), which has the advantage of being lightweight (the final prototype is 114 g in weight). The foldable structure of the camera partially endows the device with a compact structure. The base and an elastic fabric wristband are bonded by melt adhesive (we use a dispensing gun to heat the melt adhesive and apply the base to the fabric). The mentioned foldable structure guarantees the unobtrusiveness of the device for users. The drawer type structure between the shell and the base is adopted, which can be easily dismantled and installed, and satisfies the user's usage requirements of portability. The selected image-based sensor is the Raspberry Pi Camera Module, which is a CMOS-type (Complementary Metal Oxide Semiconductor) 175-degree wide-angle camera that is especially compatible with Raspberry Pi with a resolution of five million pixels (2952 \(\times\) 1944 pixels). As the control unit, Raspberry Pi Zero W integrates a 1-Ghz single-core central processing unit (CPU) and 512 MB RAM with additional support for 802.11 b/g/n wireless LAN connectivity. The module is suitable for prototype development and the verification of smart infrastructure under the Internet of Things technology due to its small size (65 mm \(\times\) 30 mm \(\times\) 5 mm) and wireless transmission compatibility. In addition, compared with other controller modules such as Arduino, it has a higher clock frequency, which is more suitable for fast image processing and acquisition. In order to ensure the small size of the integrated design, the camera module and the Raspberry Pi are connected by a flexible flat cable (FFC). According to the power supply requirement and the size limitation of the integrated design, two rechargeable lithium batteries with a rated voltage of 3.7 V are selected for parallel output with a  
Figure 2: (a) Concept design of the wearable wrist-worn camera with an emphasis on wearability and user’s comfort; (b) Integrated implementation of the wearable wrist-worn camera. total capacity of 2000 mAh. Meanwhile, in order to meet the demand of Raspberry Pi and the camera module's power supply, the boost converter is used to get 5 V of voltage output. The integrated implementation of the wearable wrist-worn camera described above and the prototype are shown in Figure 2b. In the process of collecting the original video data, the Raspberry Pi runs the python script on the Raspbian system (a system based on Debian GNU/Linux for Raspberry Pi hardware development) to establish the TCP (Transmission Control Protocol) server firstly, and then connects with the TCP client on the computer. After the successful connection, the Raspberry Pi collects the video data from the camera module, and then transmits the video data to the computer with the configuration of 320 \(\times\) 240 resolution and a frame rate of 12 frames per second (FPS). The above parameters based on experimental optimization can reduce the packet loss and satisfy the data processing requirements of subsequent algorithms. The algorithmic details of data processing will be introduced in detail in Section 3. The exact size of the wearable camera can be found in Figure 2a. The camera hood is equipped with a camera module, which can rotate 0-150 degrees between the camera hood and the shell through a rotating shaft. There are irregular grooves designed at the bottom of the base for uniform melt adhesive.