## 6 Design of the Monitoring System  
In this section, we design and present a system for monitoring elderly subjects, based on the proposed monitoring approach, set of actions, collected datasets, and trained action recognition model, described in the previous Sections. We will start by defining the high-level goals of such system, in order to gradually refine them in terms of features of a client-server application.  
Figure 7: Dependence of classification accuracy on the apparent size of the subject in the image frame.  
The typical end-user, called a \"guardian\" in the following, is a person in charge of monitoring a subject who can take care for him/her-self, but who is at high risk of domestic accident when living alone. The main goal of our application is to give the guardian an effective exploration of the events regarding the monitored subject. Specifically, the application should satisfy the following set of requirements:  
1. A quick and intuitive way to reach the desired information. If the guardian has a precise idea of what he/she wants (a specific timestamp for example).
2. A synthetic yet exhaustive abstraction of the detected events. If the guardian is broadly exploring a given time range.
3. Timely notifications for situations of interest. The application should actively reach the guardian when specific conditions are met.  
These requirements are fulfilled by the designed client-server application, whose architecture is shown in Figure 8. In the envisioned scenario, the monitored subject is recorded through a camera installed in the chosen environment. The recorded video is sent to the server, where it is processed for action recognition. The guardian accesses a client web application, and uses it to request to the server the desired analysis. The web application renders the video stream and the corresponding analysis, both received by the server (requirements 1 and 2). If the server-side processing triggers a condition of interest, a notification is sent to the guardian (requirement 3).  
The high-level requirements are concretely implemented with the following set of tools:  
* Coarse-to-fine temporal navigation (time range and timeline view)
* Instant-level description of the subject behavior (snapshot)
* Automatic events partitioning and description (storyboard view)
* Global statistics for the selected time range (global view)
* User-customizable filters and notifications  
These tools are shown in Figure 9 in context of the designed web application, and are further described in the following.  
Figure 8: Architecture of our client-server application for assisted living monitoring.  
Since the guardian can watch the subject's videos, an important matter that must be kept in consideration for this kind of applications is the privacy of the subject. The proposed system was designed to be used in a private setting, keeping in mind that there are different laws for different countries which impose different rules. Assuming the guardian can see the monitored subject, the problem of guest privacy can be instead addressed with the use of people identification approaches, anonymizing all the individuals in the scene that are not the monitored subject. An investigation into the most suitable and performant methods will be carried out in future works.