ShadowScan(TM) – 3D Building Models from the Sun’s Shadows

We’ve all seen laser scanner’s that work by projecting laser lines (or grids) over the irregular surface of an object to capture 3D shape. But now some genius researchers at Washington University (which is in St. Louis, Mo BTW), have done the obvious…

Everyday the Sun ‘scans’ the entire planet, and using the sun as their moving “laser”, researchers realized they could analyze time-lapse footage, and develop software algorithms that would be able to extrapolate the shape and position of every building in the frame.

Fig 1-  Skyscrapers cast shadows over the city like a giant urban sundial. ShadowScan (TM) – a product name I made up for this software- uses building shadows to capture 3D shape of cities.

Yes, we’re talking about Heliometric Stereo here – using the sun as a moving light source to recover surface normals of objects in an outdoor scene. It works because the position of the sun is known very accurately, but can be challenging because of variations in lighting and weather.

Fig 2 – ShadowScan(TM)  works by calculating the geo-centric surface normals generated from heliometric stereo algorithm. Thus, it can recover 3D shapes of objects in the scene.

Fig 3 – Notice that the shape of the shell is captured nicely, even from extreme viewpoints(the figure on the right is taken as a side-on view)

In Short – In the near future, videos that have GPS coordinates have suddenly and inadvertently become ‘laser-scanners’ – grabbing the shape of the built environment by default.

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