"Reverse graffiti," a trick used to draw art or advertising into filthy city surfaces, is a trend we've been watching for a few years now. And while some like the idea of advertising that actually makes the world a bit cleaner, San Francisco regulators appear to be cracking down on what they see as unlawful marketing in a public right-of-way. In the news clip below, reverse graffiti pioneers GreenGraffiti make the case that they're providing a service, one that cities could theoretically monetize to help support schools and other government programs. Most cities don't technically have laws against cleaning sidewalks and walls, but San Francisco Public Works spokeswoman Gloria Chan says the city is looking to close the loophole and officially prohibit such advertising on government property: "We're going to be investigating and taking a look at what enforcement code this falls under."