Reverse graffitti is starting to be more prevalent in San Francisco in two forms: advertising and art. It is created by washing away the dirt and grime of city life to leave an image in the negative, or reverse. I first noticed ads stenciled onto the sidewalks downtown. Watch this video from Channel 7 to learn more about video ads that are sweeping [sic] San Francisco. (please click on the link for the Channel 7 video)
I have mixed views about reverse graffiti advertising. The city should be clean enough that advertisers can't possibly even create these ads. On the other hand, living in a city can be a dirty, grimy experience, and these reverse ads aren't harming anything, perhaps just the ocassional ad that is visually unpleasing. A good solution might be to allow companies to create the ads with the promise of cleaning the area around the ads after a certain time frame. Currently, the city has no laws that prohibit reverse graffiti.
While exploring this, I learned about the work of a local artist, Moose (aka Paul Curtis) whose reverse graffiti is beautiful. Check out an installation in SF's Broadway Tunnel, where he restores the images of indigenous plants that were likely in the area a few hundred years ago before we brought our cities with our congestion and pollution.