Click the image to redraw it in different colours.
The GML file format is quite new and is a markup language for drawing virtual graffiti. Therefore it's not about defacing public buildings, but rather a digital artform that's easily shared over the internet. GML (graffiti markup language) stores not only the XYZ positions of pen strokes, but also the timing of the strokes. That means GML data can be drawn more expressively than can be represented in a simple picture, as the above demo shows. If you aren't interested in virtual graffiti, GML could have many other uses, including maybe adding a feeling of time to GPS data (you could even add a map image to the background of the processing sketch).
If you decide to follow the source code link in the sidebar, you'll find that the GML data is compressed into arrays to place in the processing.js sketch. That was done using a simple prototype Processing sketch on a desktop PC. You can find that sketch (gml2array) in the sidebar for use with your own GML files. To find out more about GML why not take a trip over to http://graffitianalysis.com to see what they're up to. Also, the tag we've used here was skillfully made by Antonio, so why not visit him at @hellocatfood on Twitter!