The University of Washington has developed a photo-stitching algorithm that produced a 3D model of Rome in under a day, from 150,000 tourist photos.
The tool is the most recent in a series developed at the university, and in this used photos tagged either Rome or Roma from Flickr.
Computers took 21 hours to combine the images to create a virtual city that can be 'flown' through. It includes the Trevi Fountain, the Pantheon and the inside of the Sistine Chapel.
"How to match these massive collections of images to each other was a challenge," said University of Washington researcher Sameer Agarwal. Until now "even if we had all the hardware we could get our hands on and then some, a reconstruction using this many photos would take forever."
Earlier versions of the university's photo-stitching technology were dubbed Photo Tourism and licensed to Microsoft in 2006, which now offers it as a free tool called Photosynth.
"With Photosynth and Photo Tourism, we basically reconstruct individual landmarks. Here we're trying to reconstruct entire cities," said fellow scientist Dr Noah Snavely, who developed Photo Tourism.
In addition to Rome, the team recreated the Croatian coastal city of Dubrovnik, processing 60,000 images in less than 23 hours using a cluster of 350 computers, and Venice, processing 250,000 images in 65 hours using a cluster of 500 computers.
Previous versions of the Photo Tourism matched each photo to every other photo in the set. "But as the number of photos increases the number of matches explodes, increasing with the square of the number of photos," said the University. "A set of 250,000 images would take at least a year for 500 computers to process, Agarwal said. A million photos would take more than a decade."
The new algorithm works more than a hundred times faster than the previous version by first establishing likely matches and concentrating on those.
It also uses parallel processing, running simultaneously on many computers.