The notable amount and variation of current techniques in non-photorealistic rendering (NPR) indicates a level of maturity whereby the categorization of algorithms has become possible. We present a conceptual model for NPR, on which we base a modular system, OPENNPAR, which integrates NPR algorithms into distinct classes. Components in OPENNPAR are modularized and consequently reintegrated for various rendering purposes, allowing many kinds of NPR algorithms to be reproduced, including the integration of 2D and 3D methods. Additionally, the system provides support for a range of users (developers, programmers, designers) according to their respective levels of abstraction, thus being available in multiple contexts. Ultimately, OPENNPAR holds great potential as a tool in the development, augmentation, and creation of NPR effects.