\section{Contributions}
+With these research questions in mind, I will describe now some of my contributions leveraging the users' motor abilities for the design of interactive systems and interaction techniques.
+Instead of presenting the research below the way it was described in research papers, I will focus on hindsights and how these contributions influenced my research.
+
+
\subsection{Input latency measurement}
\label{sec:lagmeter}
+The first contribution I will present is not an interaction technique or an interactive system.
+It is a methodology and study about the latency of touch-based interactive systems in the general sense.
+Every interactive system has a delay between the moment users perform an action and when the system produces a response.
+This delay is refered to as \defword{end-to-end latency}.
+This latency is known to cause performance and usability issues~\cite{deber15,jota13,teather09,waltemate16}.
+Therefore, there are research studies about strategies to mitigate these effects or reduce latency~\cite{nancel18}.
+
+In order to reduce latency or its effects, it is importantto measure it, and understand the contribution of each part of the system to it.
+There are many possible sources of latency: the input and output, the software and hardware, the device and the host.
+Typical latency measurement methods used high-speed cameras~\cite{ng12,steed08,teather09}.
+The method is simple, but it is difficult to make many measurements and it is impossible to slice the latency between the several steps of the process.
+
+%Any interactive system exhibits some delay between a user’s action and the corresponding system response, known as the end-to-end latency. This latency is caused by many factors pertaining to the input device, the operating system (e.g. de- vice drivers, control and communication policies), the soft- ware toolkits and application used, as well as the output de- vice. Depending on its amount and variation over time (jit- ter), the end-to-end latency can degrade the action-perception loop, impact user performance and make the system feel less responsive and interactive.
+%Latency has long been known to affect performance and user perception in indirect pointing tasks, Virtual Reality and touch interactions [1, 8–10, 12, 13, 16, 21]. While today’s touchscreens show latencies ranging from 50 to 200 ms [14], it is known that users can perceive touchscreen latency as low as 2 ms [14] and that performance for dragging tasks degrades above 25 ms [10]. In indirect interaction, latencies above 50 ms are noticed and affect performance [8, 9].
+%Considering the importance of latency in interactive systems, it is essential to measure and report it, to assess how it af- fects users or the results of an experiment. Until recently, its measure has been a tedious process based on the use of external cameras [11, 14, 15, 18–21] but methods have been proposed to ease the process through interactive tools [2, 6] or repeated measures [3, 7]. These tools can be used to try to reduce latency but it can be a trial and error process without the understanding of the influence of each part of a system. In addition, none of the measurement tools allows to measure end-to-end latency while a user interacts with the system, and existing tools are limited to specific technologies.
+
Latency \cite{casiez17}
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