1. A method of providing a descriptor for at least one feature of an image, comprising: providing an image captured by a capturing device and extracting at least one feature from the image; and assigning a descriptor to the at least one feature, the descriptor depending on at least one parameter indicative of an orientation with respect to a direction of a gravitational force in 3D space, wherein the at least one parameter is determined from at least one tracking system, and wherein the at least one parameter is determined from depth information associated with the at least one feature.