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The Inventors have developed a passive technique to measure air flow using video sequences. Light rays bend as they travel through the air of differencing densities. Such deflections are exploited in various air measurement techniques. As the air moves, small changes in the refractive properties appear as small visual deformation, called “refraction wiggles,” of the background texture, similar to the shimmering effect experienced when viewing objects across hot asphalt or through exhaust gases. These motions can be tracked using regular video cameras to infer information about the velocity and depth of a refractive fluid layer.

The Inventors observe that while intensity features result from a background layer and their location is not directly related to the fluid layer, the wiggles correspond to 3D positions and motion of points on the transparent fluid surface. The movement of those wiggles between multiple frames indicates the motion of the transparent fluid, and its disparity between viewpoints is a good cue for the depth of the fluid surface. The algorithms to track motion and recover position of points on the fluid surface are based on the constant motion of a refraction field. This distortion is measured by computing the wiggle features in an input video, and then using those features to estimate the motion and depth of the fluid, by matching them across frames and viewpoints.