# Automatic Design of Collective Behaviors for Robots that Can Display and Perceive Colors
## 2 Related Work  
In this section, we first introduce studies in which the robots of a swarm have the capabilities of displaying and perceiving colors. After, we revise related works on automatic design of robot swarms. Finally, we compare TuttiFruti with the other existing instances of AutoMoDe. Robots that can display or perceive colors have been largely used to demonstrate collective behaviors in swarm robotics. The literature on robot swarms that use visual information is extensive and it is not our intention to provide an exhaustive review. We exclude from our discussion any system in which robots only perceive visual information but do not display it--such as [18; 19; 20; 21; 22]. Instead, we focus on studies in which the robots both display and perceive colors to achieve collective behaviors [23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36; 37; 38; 39; 40; 41; 42; 43; 44; 45; 46; 47]. Designers of robot swarms commonly use color lights to represent specific information that robots in the swarm must identify, process and/or transmit--the nature of the information varies from one study to another and is used ad hoc to obtain a particular behavior. For example, [PERSON] et al. [35] designed a swarm that connects locations by establishing a chain of robots that act as waypoints for their peers. They conducted two experiments in which robots use colors differently: in the first experiment, robots repeat a pattern of 3 colors along the chain to indicate the sense in which the peers should move; in the second one, robots use colors to inform their peers about a location of interest. [PERSON] et al. [25] designed a swarm in which robots self-organize in mergeable structures. In their experiments, robots react to colored objects in their environment and display color signals that indicate their location. [PERSON] and [PERSON] [31] designed a robot swarm that autonomously identifies and perform sequences of tasks. In their experiments, robots emit color signals to coordinate their collective action and associate each task in a sequence with objects that display a particular color. In a more general sense, one can find a similar approach in swarms that exhibit self-assembly and morphogenesis [23; 24; 25; 26], collective fault detection [25; 27], collective exploration [28; 29; 30; 31], collective transport [28; 30], coordinated motion [25; 26; 32], human-swarm interaction [33; 34], chain formation [28; 31; 35], group size regulation [36], task allocation [31; 37; 38; 39; 40], object clustering [39; 41], and foraging [42]--according to the taxonomy proposed by [PERSON] et al. [4]. In these studies [23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36; 37; 38; 39; 40; 41; 42], designers manually established ad hoc relationships between the colors that a robot can perceive and the corresponding behavior that a robot must adopt when it perceives them. The research question we address in the present paper is whether automatic design methods [5; 17] can establish similar relationships. It is our contention that classes of missions that require the robots to communicate and react to color-based information are an appropriate benchmark to assess automatic design methods. First, the capability of displaying and perceiving colors is platform-independent and generalizes across different design methods--robot platforms used in swarm robotics often include LEDs and cameras [48]. Second, colors facilitate the conception and realization of complex missions--colored environments can be created in various manners [40; 41; 49; 50; 51]. Finally, colors simplify the visualization and monitoring of robot swarms [48]--a property relevant to the human understandability of collective behaviors, an open research area in swarm robotics [52]. Yet, no existing method for the automatic design of robot swarms targets classes of missions that require the robots to operate with color-based information. Specifically, we refer to methods that can be framed within the tenets of the automatic off-line design of robot swarms [17].