Geometric Sketch Constraint Solving with User Feedback

Modern CAD systems rely on two-dimensional sketches as the basis for the creation of most three-dimensional geometry. These sketches consist of qualitative shape information as well as a series of constraints to quantify the exact size and shape of the various segments, arcs, and splines that comprise the sketch. Example constraints include overall lengths, symmetries, parallel and/or perpendicular segments, and arc radii. For a given sketch, there are many sets of constraints that unambiguously define the shape. Unfortunately, sometimes a user prescribes a set of constraints that do not allow the sketch to be solved. Providing feedback to the user as to how to change the constraint definitions to remedy the problem is still a largely unsolved problem. Herein is described a series of techniques for helping a user diagnose sketch problems and provide to the user a series of options that will cause the sketch to be properly solved. The techniques are incorporated in the Open CSM solid modeling system. Several sample configurations that use these techniques are shown, with a description of the user interaction that the system provides.