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Researches on using virtual reality (VR) to support computer-aided design are rapidly increasing in recent years. This is because VR techniques are very helpful in improving the interactivity and 3D visualization function of CAD systems. In this paper, we present an approach to solid modeling in a semi-immersive virtual environment. The approach allows users to create, manipulate and visualize a precise solid model by direct 3D manipulations and voice commands in a virtual environment with a 3D input device. In the approach, to make the solid model created and modified by direct 3D manipulations precise, real-time constraint recognition, propagation and solution algorithms are developed and utilized. An extended solid model is also adopted to effectively support the solid modeling by direct 3D manipulations. The experiment shows that the approach enables users to conduct solid modeling intuitively …