1. A computing system, comprising: a photometric stereo imaging system with a rectangular input aperture and four light sources arranged with one at each corner of the aperture to capture multiple images of a printing substrate; a storage to store paper classification information related to three-dimensional statistics of printing substrate types; and a processor to: determine a description of the three-dimensional characteristics of a printing substrate based on images of the printing substrate captured by the photometric stereo imaging system; determine statistical summary information related to the three-dimensional characteristics description; and classify the printing substrate based on a comparison of the statistical summary information to the stored printing substrate classification information.