I really admire graffiti artists. First of all, there's the effort they need to go through just to set up a proper session. Then you have to consider how the whole question of legality affects the artistic process. More often than not when painting a mural they're breaking the law, and have to think on their feet and work as quickly as they can.
This urgency seems to inherently neglect the pre-production process, and has always made me wonder how the graff guys plan their images - surely a notebook sketch doesn't translate effectively to a wall? It's not like there's time to set up a projector or something to provide guidelines to trace over first - it all has to happen in the moment.
