What Do the Sun and the Sky Tell Us
About the Camera?


Camera parameters automatically estimated from the sky appearance. (webcam sequences from the AMOS database)

People

Abstract

Sky/cloud segmentation

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 in an image depends on the sun position, weather conditions, photometric and geometric parameters of the camera, and the location of capture. In this paper, we analyze two sources of information available within the visible portion of the sky region: the sun position, and the sky appearance. By fitting a model of the predicted sun position to an image sequence, we show how to extract camera parameters such as the focal length, and the zenith and azimuth angles. Similarly, we show how we can extract the same parameters by fitting a physically-based sky model to the sky appearance. In short, the sun and the sky serve as geometric calibration targets, which can be used to annotate a large database of image sequences. We use our methods to calibrate 22 real, low-quality webcam sequences scattered throughout the continental US, and show deviations below 4% for focal length, and 3 degrees for the zenith and azimuth angles. Once the camera parameters are recovered, we use them to define a camera-invariant sky appearance model, which we exploit 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.

Citation - IJCV 2010

Jean-François Lalonde, Srinivasa G. Narasimhan and Alexei A. Efros. What Do the Sun and the Sky Tell Us About the Camera?, International Journal on Computer Vision, 88(1):24-51, May 2010. [PDF pre-print] [BibTeX]
(original publication is available at www.springerlink.com)

Citation - ECCV 2008

Jean-François Lalonde, Srinivasa G. Narasimhan and Alexei A. Efros. What Does the Sky Tell Us About the Camera?, European Conference on Computer Vision, 2008. [PDF] [BibTeX]

Poster - ECCV 2008

Download the poster that was presented at ECCV 2008 here: [PDF, 11.6MB]

Animations

Original sequences (1 day):

Corresponding camera parameters and sun position:

All movies are in MPEG-4 format.

Additional results

Camera parameters and corresponding sun positions.

Code

Download the code for this paper in a ZIP file, or get it from github.

Funding

This research is supported by:

ONR N00014-08-1-0330
NSF IIS-0643628
NSF CCF-0541307
NSF CCF-0541230

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