“Make your mark on your social network, not in the social network.”
For the Social Dev Camp 2010, together with Eelco Wagenaar and Mark Wubben, we made a simple and effective digital graffiti wall, on which you can write with a light: from your bike light or the screen of your mobile phone. Using a RFID card you post the result on your Facebook wall, like it on Facebook or clear the screen to begin fresh.

Multiple quick iterations during the camp brought us from a game using two cigaret buds and the smoke you exhale to control a boxing puppet and influence your opponents’ and other technically tricky games and toys to be applied in the smoking area of a venue (because the mandatory smoking areas in clubs and venues are often uninspiring non-places), to a simple digital graffiti wall. Having lots of tools and equipment available at Mediamatic BANK where we held the camp and having lots of bright minds at the same time, we were able to make a lot of small technical feasibility and play tests before deciding on the final project.

During these tests and demo setups, we found that the digital graffiti wall was actually really effective and also convincing as real graffiti. When on the third day —or night it was already— we tried the setup outside the Mediamatic BANK, using its glass facade and a mobile phone as the light source to draw out a huge digital graffiti piece, a police van immediately pulled over and stopped to see and inform what we were doing. On their first impression I must have looked like a genuine graffiti-“vandal” to be caught red-handed. Only after closer inspection the paint turned out to be projected light and my spray can an iPhone screen.

The Write on Wall (WOW) installation was made using a camera on one side of the frosted window. The camera receives the light from your mobile phone coming from the other side of the glass. The camera is connected to a computer which processes and analyses the camera feed. The processed and rendered image is then sent to the projector, which beams its light back onto the window. The illusion is created that you’re drawing on the glass wall.

As with a lot of computer vision projects the light conditions turned out to be crucial for successful operation of the work. The delicate interplay of minimal ambient light in front and behind the glass wall, a bright, but not too bright projector and a bright enough mobile light source, can make setting up and calibrating the installation arduous. Luckily the season helped: the fact that we started this project in wintertime, made it possible to test and run the installation outside —which it should be as graffiti— for much longer periods of the day, then in the summer.

The calibration process multiple times (for the multiple iterations) took a lot of effort each time, not only because of the changing light conditions and different threshold levels, but also to re-align and warp the image from the camera with the projector setup. So we developed special calibration software to automate this setup 90% of the way.
And to ease the hook up of different software components of this installation: the computer vision and graffiti drawing part, the RFID reading part, the server communicating with the network and with Facebook, and the visual on-screen feedback of your RFID swiping actions, we used a node.js server and hacked a special purpose browser. This Transparent web browser rendered the feedback as a simple HTML and CSS styled text and animations on top of the CPU and GPU intens OpenFrameworks C++ application responsible for the graffiti.

Adding the social network sharing functionality, so you can leave your digital graffiti on someone’s Facebook wall, made the work engaging and attractive for a much bigger crowd. An interesting aspect of a social digital graffiti wall like this, is that it flips around the the anonymous, yet public part of graffiti. Where traditional graffiti was to anonymously write your tag throughout the city to be visible in public, this work instead invites you to personally and publicly claim your piece using an RFID tag and share it privately, visible to only your social network.
Write On Wall was on display several month at Mediamatic BANK, during the Eurosonic Noorderslag festival in Groningen and during the 5DaysOff festival in Paradiso in 2011.