Street art in the concrete jungle of Philadelphia is a treat for the eyes

A girl with ponytails looks out of a window into a multicoloured fantasy world.

- 

Yellow cones of light shine from her eyes. A few walls away is the "Tree of Knowledge", a mural that depicts a tree with books, saws and other objects hidden among the leaves. And just around the corner, not from Chinatown, finely drawn origami birds flutter up the facade of a building, merging into a red-crowned crane.

Take a walk around Philadelphia and you'll discover thousands of colourful murals. There are over 4,500 paintings throughout "Philly", the second most-populous city on the East Coast of the United States after New York. Although it hasn't broken any world records, Philadelphia defines itself through its large-scale public art programme, celebrating itself as the "mural capital". The murals tell the story of the city, and are a major social project.

Part of this story is the illegal graffiti that Philadelphia, like many other big cities, has to grapple with, says Jane Golden from the Mural Arts Philadelphia organisation. Gangs often mark their territory with tags, which make the affected neighbourhood seem untidy and unsafe. A kind of artist signature, the ambitious graffiti artists spray their pseudonym in aesthetically pleasing letters. Although illegal, tags often illustrate artistic potential.

Redirecting this creative energy and inspiring communities and artists to revitalise public spaces was the idea behind the "Mural Arts" campaign, which is still popular more than 40 years after it was launched as a measure against graffiti vandalism. Every year, more than 25,000 Philadelphians contribute to up to one hundred new murals, with even criminally prosecuted graffiti artists getting involved.

Artists make designs and suggest locations, residents give the thumbs up or thumbs down, then everyone gets to work and paints together.