1. A method using a computer of determining congruence between physical objects, the physical objects each having a surface described by a model, comprising: determining, for a model of a first physical object, a first plurality of tangency points; determining, for the model of the first physical object, a first plurality of lines coupling tangency points that do not intersect a surface defined by the model of the first physical object in a vicinity of the tangency points coupled by the first plurality of lines; determining, for a model of a second physical object, a second plurality of tangency points; determining, for the model of the second physical object, a second plurality of lines coupling tangency points that do not intersect a surface defined by the model of the second physical object in a vicinity of the tangency points coupled by the second plurality of lines; registering the model of the first physical object and the model of the second physical object to a common coordinate system using the first plurality of lines and the second plurality of lines; determining a measure of discrepancy between the model of the first physical object and the second physical object; and declaring that the first physical object is the same as the second physical object if the measure of discrepancy is less than a predefined magnitude.