Designers working in public spaces may use the physical movement of the people in that space as the means of interaction [1,2,3,4,5]. Often, this requires designing systems of interaction that can track movement or gesture. Motion and feature tracking computer vision algorithms can be used to achieve this on a scale of granularity from simple detection of moving pixels, to more robust tracking of features, faces or gait [6,7,8,9]. Other forms of movement tracking rely on 3D sensing of the space using personal electromagnetic signatures such as Bluetooth, mobile phone signals and WiFi connection, beacons like RFID, or other human characteristics such as temperature or sound [10,11,12]. Such systems of interaction depend on the people in the space becoming visible to the tracking system. Depending on the type of system, this visibility can be affected by degree of movement, lighting conditions, proximity and orientation to the sensing system and available digital devices.
Personal visibility in public space
Being able to manage our visibility status is an important part of our personal use of public space. Some people may desire to become invisible, hiding their presence from systems that can accurately pinpoint location via, for example, mobile phone networks, WiFi proximity, CCTV and RFID travel cards . Others, such as players of locative games who rely on constant connectivity to tracking systems for seamless game play, may seek to improve their visibility status . Similarly, disempowered people may wish to raise their profile in public space and so legitimise their presence . Some people may inadvertently become visible when traces of their activity are discovered via practices such as war chalking and wide band radio scanners  and the ability to manage one's own visibility status can be removed from certain groups, for example, sex offenders in California , for reasons of security or control.
Willis describes how our desired degree of visibility to potential tracking technologies affects patterns of inhabitation as laptop users cluster around the café WiFi router and mobile phone users wander in search for the optimum signal strength . Meanwhile, Fussey argues that for those engaged in covert activities invisibility to CCTV cameras is of primary concern  and Maccoll describes how locative game players exploit the seams between technological systems to gain advantage .
We suggest that these examples highlight how desire for visibility or anonymity in public space affects the places we choose to linger and the routes we take. In addition, interactive systems are increasingly becoming entwined with architecture and physical spaces to form what de Souza e Silva calls 'hybrid spaces' that are able to detect the presence of individuals and to collate this presence with electronically archived personal information such as name, shopping habits, music taste and travel routine . As a result, we argue that the personal control of visibility should be a factor considered in designing interactive public space.
A number of strategies are available to people who want to alter their visibility status in public space; RFID-based travel cards can remain unregistered; mobile phones can be turned off and you can opt to follow the path of least surveillance .
Figure 1. Zebra by Desiree Palmen.
To increase physical invisibility people have co-opted strategies from nature, for example, army camouflage. This approach has been adapted for urban conditions with nature-based patterns replaced by designs based on branding and strategies for disguise including street features such as road crossings  (figure 1) and vending machines  (figure 2).
Figure 2. Urban camouflage by Aya Tsukioka.
There may also be alternative strategies for invisibility in modern living that are about hiding specific features such as face, identity or intent. Wearing clothing that hides facial features, for example hooded tops or niqabs, has the side effect of increasing invisibility from recognition systems.
Figure 3. Designs for CV Dazzle by Adam Harvey.
Other strategies may not attempt to achieve invisibility but instead play with the constraints of a system to inhibit its ability to accurately detect location, trajectory or identity. For example, CV Dazzle is described as open-source camouflage from computer vision and uses hair and make up to render faces unrecognisable by face recognition software . Parallels can be drawn here to the dazzle paint schemes adopted in WWI that were bright and eye-catching, but disrupted the ability of the viewer to predict the direction, size and identity of the ships .
Figure 4. HMS Furious painted in Dazzle Camouflage during World War I
Designing systems to recognise and respond to people in a space can be informed by considering the various failure modes of different technical approaches to tracking and recognition. By explicitly engineering modes of failure, we can learn how visibility and invisibility can be managed, while also considering potential improvements in robust tracking for interaction design. This workshop will provide the opportunity for participants to engage hands-on with a computer vision tracking system by inventing, constructing and testing speculative strategies for managing personal visibility.
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