Estimation of Sunlight Direction Using 3D Object Models

The direction of sunlight is an important informative cue in a number of applications in image processing, such as augmented reality and object recognition. In general, existing methods to estimate the direction of the sunlight rely on different image features (e.g., sky, texture, shadows, and shading). These features can be considered as weak informative cues as no single feature can reliably estimate the sunlight direction. Moreover, existing methods may require that the camera parameters are known limiting their applicability. In this paper, we present a new method to estimate the sunlight direction from a single (outdoor) image by inferring casts shadows through object modeling and recognition. First, objects (e.g., cars or persons) are first (automatically) recognized in images by exemplar-SVMs. Instead of training the Support Vector Machine (SVMs) using natural images (limited variation in viewpoints), we propose to train on 2D object samples generated from 3D object models. Then, the recognized objects are used as sundial cues (probes) to estimate the sunlight direction by inferring the corresponding shadows generated by 3D object models considering different illumination directions. We demonstrate the effectiveness of our approach on synthetic and real images. Experiments show that our method estimates the azimuth angle accurately within a quadrant (smaller than 45°) and compute the zenith angle with mean angular error of 23°.