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A two-handed 3D styling system for fl'ee-form surfaces in a table-like Virtual Environment, the Responsive Workbench (I~WB)TM , is described. Intuitive curve and surface deformation tools based on variational modeling and interaction techniques adapted to 3D V R modeling applications are proposed. The user draws curves (cubic B-splines) directly in the Virtual Environment using a stylus as an inpue device, The era'yes are connected automatically, such that a curve network develops. A combination of automatic and usercontrolled topology extraction modules crea~es the connectivity information. The underlying surface model is based on B-spline surfaces, or, alternatively, uses multisided patches [20] bounded by closed loops of curve pieces. 1, STYLING IN VIRTUAL ENVIRONMENTSMany designers would like sketching free4brm shapes quickly in the conceptual design phase without using a complex CAD system [6]. Projection-based Virtual Environments like the Responsive Workbench (RYVB)TM t [19] offer the following advantages for sketching applications over traditinal desktop interfaces:3.The three-dimensionality of perception in combination with 3D intera.ction gives an immediate understanding of the shape.Regions can be identified, localized, and selected directly in space.The large, high-resolution projection plane enables the representation of objects on a scale which corresponds ~o the working region of both hands.

This paper describes a novel 3D interaction technique called the "SkeweR", dedicated to the 2-user collaborative manipulation of objects in virtual environments. This technique enables two users to move simultaneously the same virtual object in 3D. For this aim, each user manipulates the object by one crushing point, like handling the extremity of a skewer. The SkeweR uses only translation information from the users' motions to change both the position and orientation of the manipulated object. By using more crushing points, this technique could easily be extended to 3 or more users. Thus, the SkeweR technique could be used to improve the collaborative manipulation of objects in numerous applications of Virtual Reality, such as: virtual prototyping, maintenance and training simulations, architectural mock-up reviews, etc.

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