1. A method comprising: using one or more processing devices to determine coordinates for an image point in a multi-dimensional eigenspace, the image point in the multi-dimensional eigenspace characterizing an image of an object, and the multi-dimensional eigenspace having been defined based on a set of training images of one or more fingers; using the one or more processing devices to access an equation that describes a geometric model in the multi-dimensional eigenspace, the geometric model being characteristic of the set of training images of the one or more other fingers, and the equation that describes the geometric model including an equation that describes a cone in the multi-dimensional eigenspace; using the one or more processing devices to apply the coordinates determined for the image point in the multi-dimensional eigenspace to the equation that describes the geometric model in the multi-dimensional eigenspace to determine a distance between the image point and the geometric model in the multi-dimensional eigenspace; and using the one or more processing devices to determine whether the object in the image includes a finger based on the determined distance between the image point and the geometric model in the multi-dimensional eigenspace.