Ancient Rome was not built in a day. It took nearly a decade to build the Colosseum, and almost a century to construct the St Peter's Basilica. Now a new computer algorithm developed by an Indian-American uses thousands of tourist photos to automatically reconstruct an entire city in about a day.
"How to match these massive collections of images to each other was a challenge," said Sameer Agarwal, professor of computer science and engineering at the University of Washington (UW) and lead study author.
Until now, "even if we had all the hardware we could get our hands on, a reconstruction using this many photos would take forever," said Agarwal, who studied at the Indian Institute of Technology, Kanpur, India, where he did his M Sc (1995-2000) in maths and scientific computing.
Digital Rome was built from 150,000 tourist photos tagged with the word "Rome" or "Roma" that were downloaded from the popular photo-sharing website, Flickr.
The tool is the most recent in a series developed at the UW to harness the increasingly large digital photo collections available on photo-sharing websites.
Computers analysed each image and in 21 hours combined them to create a 3-D digital model. With this model a viewer can fly around Rome's landmarks, from the Trevi Fountain to the Pantheon to the inside of the Sistine Chapel.
Earlier versions of the UW photo-stitching technology are known as Photo Tourism. That technology was licensed in 2006 to Microsoft, 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 co-author Noah Snavely.
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.
Many historians see Venice as a candidate for digital preservation before water does more damage to the city, the researchers said.
Transitioning from landmarks to cities -- going from hundreds of photos to hundreds of thousands of photos -- is not trivial.
Previous versions of the Photo Tourism software 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.
For instance, 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 newly developed code works more than 100 times faster than the previous version. It first establishes likely matches and then concentrates on those parts.
These findings will be presented in October at the International Conference on Computer Vision in Kyoto.