1. A pattern recognition method applied to an information processing apparatus that executes a pattern recognition process by approximating distribution of a set of vectors and a class boundary in a vector space based on basis functions, the method comprising: inputting, to the information processing apparatus, a pattern as a string of pixel values of a digital image; defining directional basis functions between two basis vectors using vectors corresponding to the input pattern; performing the approximation using a linear combination of the directional basis functions; and displaying a result of the approximation on a display device associated with the information processing apparatus or storing the result of the approximation in a storage device associated with the information processing apparatus, wherein the approximation is performed using basis functions having peaks on the line segment connecting the two basis vectors and a profile formed by using the peaks as core, extending in directions perpendicular to it and having dimensions corresponding to the distance between them.