This paper presents a practical technique to automatically compute approximations of polygonal representations of 3D objects. It is based on a previously developed model simplification technique which applies vertex-clustering. Major advantages of the vertex-clustering technique are its low computational cost and high data reduction rate, and thus suitable for use in interactive applications. This paper advances the technique with careful consideration of approximation quality and smoothness in transitions between different levels of simplification, while maintaining its efficiency and effectiveness. Its major contributions include: accuracy in grading vertices for indication of their visual importance, robustness in clustering for better preservation of important features and consistencies between levels of simplification, thick-lines with dynamic normals to maximize visual fidelity, and exploitation of object and image space relationship for levels-of-simplification determination. C...