They are not actually painting anything onto the walls, not even white or green paint.
I think what i'm seeing here is, they put some kind of a wireless mouse on the roller, which is uncovering layers in a paint program. The stop-mo serves two purposes: Long exposure time gets a brighter, more saturated image out of even a relatively weak projector, and it also covers small positioning errors in the animation and mouse-uncoering action.
You could theoretically skip the projector as well, if you use xyz positioning date from the roller to drive an AE 3-d comp in post.
I think it's a little more complicated than that. I'd be more apt to think it's some custom software/hardware.
Why? There is a constant line of sight with the paint roller. It makes me think that there is a sensor when the roller is being rolled, and that it's also being motion tracked (IR light source/camera maybe). You're then tracking a 2D plane; the wall basically.
With my limited knowledge, street art like this usually comes from people with programming and microcontroller experience. Doesn't seem like an unusual stretch.
It says in the YouTube post that "it works through custom software that they wrote that tracks the position of the paint rollers and projects video wherever they choose to paint, allowing them to explore the relationship between video, mark making and architecture and create live video collages in real time."
But what hardware? We're thinking in our office that the roller itself is pressure-sensitive and has an built-in tilt sensor, and motion-tracking camera systems set off to the sides track the location of the roller.
These elements, coupled with some custom code, most likely made this happen.
Of course, the only way to know for sure is if they tell us. But that ain't gonna happen.
Essex Television Group