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In a smooth point of a surface the Gauss Curvature is the product K=K1 * K2 of the two principal curvatures. In elliptic points this is positive (blue in the model), in saddle-like points, it is negative (red in the model). Areas of positive and negative curvature are seperated by the so-called parabolic curve which is indicated as a white curve between the two colored parts in the model. Feel free to contact as if you are interested in other objects related to curvature; not all of them are already on shapeways.