Abstract: Non-photorealistic rendering (NPR) as a field of research is generally described by what it is not and as a result it is often hard to embrace the strengths of non-photorealistic rendering as a discipline beyond digitizing or replicating traditional artistic techniques. Towards generating more discussion within the discipline, this paper provides a simple theory of NPR as a way of mapping perceived changes in a scene to perceived changes in a display. One can think of a photorealistic image as one that preserves a one-to-one mapping, such that parameters such as color, intensity, texture, edges, etc. in a scene are mapped to the same parameters in the display. NPR mappings are not one-to-one. For example edges in a scene may be mapped to black lines to generate a cartoon effect. Within this framework of mappings, a partial listing of previous techniques within the discipline is provided. The aim of this paper is to provide a type of road map to stimulate the future growth of the area of non-photorealistic rendering.