Estimating head orientation based upon sky-ground representation

The sense of balance is the basic skill of biological system. It is combined and unified with other motion skills of animals. By sensing the direction of the gravity from accelerometers, we can acquire a universal vertical reference which makes it much easier to estimation head orientation from visual sensors. This paper proposes a new method of estimating head orientation of a humanoid-like mobile robot by mounting a fisheye camera on its head. The camera rotation is estimated based upon a sky-ground representation, which is a spherical image combined with a vertical reference determined by sensing the direction of gravity. The spherical image can then be divided into two parts, sky part and ground part, along the horizon. Since the direction of the gravity is acquired from accelerometers, what we require to estimate about camera rotation is only the yaw component. Since the yaw rotation does not give rise to any content change for the sky part (the upper part of a spherical image), but a shift along the azimuth angle in a polar spherical coordinate system, the yaw component can be estimated by a phase correlation method which is referred to as the Fourier shift property. By combining the gravity with visual cues, we can estimate camera rotation without accumulated errors for a local scene. In our method no complicated computation is required and robust results are acquired for a dynamic and cluttered outdoor environment even if there is some illumination change.