Interactive Design of Smooth Objects with Probabilistic Point Constraints

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Point displacement constraints constitute an attractive technique for interactive design of smooth curves, surfaces, and volumes. The user defines an arbitrary number of “control points” on the object and specifies their desired spatial location, while the system computes the object's degrees of freedom so that the constraints are satisfied. A constraint-based interface gives a feeling of direct manipulation of the object. In this article we introduce soft constraints, constraints which do not have to be met exactly. The softness of each constraint serves as a nonisotropic, local shape parameter enabling the user to explore the space of objects conforming to the constraints. Additionally, there is a global shape parameter which determines the amount of similarity of the designed object to a rest shape, or equivalently, the rigidity of the rest shape. We present an algorithm termed probabilistic point constraints 1994 for implementing soft constraints. The PPC algorithm views constraints as stochastic measurements of the state of a static system. The softness of a constraint is derived from the covariance of the “measurement.” The resulting system of probabilistic equations is solved using the Kalman filter, a powerful estimation tool in the theory of stochastic systems. We also describe a user interface using direct-manipulation devices for specifying and visualizing covariances in 2D and 3D. The algorithm is suitable for any object represented as a parametric blend of control points, including most spline representations. The covariance of a constraint provides a continuous transition from exact interpolation to controlled approximation of the constraint. The algorithm involves only linear operations and allows real-time interactive direct manipulation of curves and surfaces on current workstations.

Original languageAmerican English
Pages (from-to)156-176
Number of pages21
JournalACM Transactions on Graphics
Issue number2
StatePublished - 4 Jan 1994


  • Kalman filter
  • interpolation
  • probabilistic point constraints
  • relaxed design
  • shape parameter
  • smooth objects
  • soft constraints
  • splines


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