1. An image processing apparatus comprising: an input unit, configured to input image data representing a characteristic of a surface of paper, the image data being obtained by reading the surface of the paper using an image input device; a determination unit, configured to determine a type of finish of the paper by analyzing the image data; a filtering processor unit, configured to perform filtering processing corresponding to the type of finish; and a map generating unit, configured to generate a normal map from the image data on which the filtering processing is performed.