1. A unified framework based on extensible styles for 3D non-photorealistic rendering, the unified framework comprising: 3D model data processing means for generating a scene graph by converting a 3D model input into 3D data and organizing the scene graph using vertexes, faces, and edges; face painting means for selecting a brusher to paint faces (interiors) of the 3D model using the scene graph; line drawing means for extracting line information from the 3D model using the scene graph and managing the extracted line information; style expressing means for generating a rendering style for the 3D model and storing the rendering style as a stroke, the rendering style being equally applied to a face-painting method and a line-drawing method; and rendering means for combining the stroke and the selected brusher to render the 3D model using both the face-painting method and the line-drawing method.