Using Cloud Shadows to Infer Scene Structure and Camera Calibration

We explore the use of clouds as a form of structured lighting
to capture the 3D structure of outdoor scenes observed
over time from a static camera. We derive two cues that relate
3D distances to changes in pixel intensity due to clouds
shadows. The first cue is primarily spatial, works with low
frame-rate time lapses, and supports estimating focal length
and scene structure, up to a scale ambiguity. The second
cue depends on cloud motion and has a more complex, but
still linear, ambiguity. We describe a method that uses the
spatial cue to estimate a depth map and a method that combines
both cues. Results on time lapses of several outdoor
scenes show that these cues enable estimating scene geometry
and camera focal length.