What Does the Sky Tell Us about the Camera?

As the main observed illuminant outdoors, the sky is a rich source of information about the scene. However, it is yet to be fully explored in computer vision because its appearance depends on the sun position, weather conditions, photometric and geometric parameters of the camera, and the location of capture. In this paper, we propose the use of a physically-based sky model to analyze the information available within the visible portion of the sky, observed over time. By fitting this model to an image sequence, we show how to extract camera parameters such as the focal length, and the zenith and azimuth angles. In short, the sky serves as a geometric calibration target. Once the camera parameters are recovered, we show how to use the same model in two applications: 1) segmentation of the sky and cloud layers, and 2) data-driven sky matching across different image sequences based on a novel similarity measure defined on sky parameters. This measure, combined with a rich appearance database, allows us to model a wide range of sky conditions.