I just saw the coolest thing ever: a Wall Printer!
It's really nothing more than a 4-color inkjet printer on its side, pointed at a wall instead of a piece of paper. The head goes up and down, and because moving walls is hard, the beam holding the head moves slowly along the horizontal track set up to hold it.
It's clearly modular so you can set up as much track as you like by joining sections. The drive motor for moving the beam is mounted in the self-contained vertical unit: it turns a gear engaged with a toothed belt that is stretched the whole length of the horizontal track. Loops of extra belt at the end indicate that this is not the maximum length. Really, there shouldn't be any practical upper limit to how long a wall you could paint with this thing.
A laser cross-hairs lets you position it just right:
Apparently I wasn't the only person who thought this was a pretty rad invention:
After the spectacle we had fire shrimp:
And after the fire shrimp we went back to see the finished wall:
The guy said it can print about three square meters per hour, so apparently he'd been at this for a while:
The resolution is not super high, but for a billboard-sized image, it's perfectly servicable.
It did show some bleeding or over-spray in some places but not others.
All in all I think it did a great job under difficult circumstances. It appears to run autonomously from a built-in battery (no cords), and the image is presumably on the memory stick you can see inserted on the front of the print head. So it should be possible to set it up anywhere in the middle of nothing and paint a wall. Imagine the possibilities for slow-motion urban graffiti!
According to this TaoBao (Chinese eBay/Amazon-like market) link, this lovely, lovely device costs about $2700, which seems like a very reasonable price to me.