18th century Painters give Photographers new Perspective

Wide-angle lenses are great for taking dramatic photographs with a big scenic sweep, but they’ve got a big weakness too – they distort objects towards the edge of the frame. Now software can make wide-angled digital photos with perfect perspective, thanks to a secret of 18th-century painters. The so-called Panini software can even turn shots taken with a fish-eye lens into natural-looking images.

Wide-angle views are notoriously difficult to display on a flat surface because of perspective problems. In the case of photography, most cameras use rectilinear lenses, which are designed to keep straight-edged features straight in the captured image. That’s fine for many shots, but lenses that capture views wider than 120 degrees skew objects towards the edge of the frame, says Thomas Sharpless, a retired software engineer, based in Philadelphia, Pennsylvania, who created the Panini software.

He and team member Bruno Postle, an architect in London, knew that the vedutisti painters who worked in 18th-century Venice – now in Italy – had excelled at painting wide-angled views of the city that appeared to preserve perspective perfectly. They knew that the painters had not used rectilinear projections to achieve this, so set about reverse-engineering the trick.

Sharpless picked 14 vedutisti paintings of the interiors of buildings for which ground plans were available. In each painting, he identified about 20 points and located them on the plan. Comparing the coordinates from the painting and the plan, he arrived at mathematical projection functions to map transformations in images, and called them the Panini projection.

The team then used this as the basis of a software panorama viewer – the Panini – that can make a panoramic image from several photos stitched together to create a single realistic view. It can even transform a shot taken with a fish-eye lens, making it appear “normal”. It has already been included in open-source panoramic image-stitching programmes such as Hugin and PTGui.

Straight up

In Panini projection, vertical straight lines in the real world remain straight and vertical in the image. Similarly, radial lines are not distorted by the process. However, horizontal lines become curved, but users can minimise such apparent distortion.

There is no way to make flat wide-angle images completely distortion-free, says Sharpless. “It will always be a matter of choosing the distortion you find least objectionable.”

The Panini projection is particularly useful for architectural vistas, deep views and scenes where features in the centre are more important than those near the edges, where other projection methods fail, says Helmut Dersch an imaging researcher at Furtwangen University in Germany.

Daniel German, a computer scientist at the University of Victoria in British Columbia, Canada, who was part of the Panini projection team, presented the work at the Computational Aesthetics 2010 conference in London this week.