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 convening a 3D model into 3D data and organizing the scene graph using vertexes, faces, and edges; style expressing means for generating a rendering style for the 3D model based on the 3D data including said vertexes, faces, and edges, and for storing the generated rendering style as a stroke, the rendering style being commonly applied to both a face-painting method and a line-drawing method; face painting means for selecting a face-painting brusher, in accordance with the generated rendering style, to paint interior faces of the 3D model using the scene graph; line drawing means for selecting a line-drawing brusher, in accordance with the generated rendering style, to draw lines of the 3D model using the scene graph; and rendering means for combining the stroke and the selected brushers to render the 3D model using both the face-painting method and the line-drawing method.