Abstract The interactive modification of free-form surfaces is a primary concern of CAD systems for the shape design of manufactured products. In this article, we present an interactive shape modification method for B-spline surfaces which maintains the continuity of surfaces and satisfies the boundary conditions with neighboring surfaces. Our method consists of two processes: a static modification, and an interactive modification. In the static modification process, a surface is modified so as to satisfy the slope continuity conditions at specified neighboring surfaces. Using the Oslo algorithm, we show that new control points can be calculated by a matrix multiplication when knots are inserted in a B-spline surface. In the interactive modification process, we introduce the concept of an operation surface which make it possible to perform the modification interactively.
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