FreeDrawer - A Free-Form Sketching System on the Responsive Workbench (Wesche 2001)

Summary:
FreeDrawer is a collection of 3D curve drawing and deformation tools targeted towards product design. The Responsive Workbench program models objects using Catmull-Clark surfaces and B-spline curves as opposed to Constructive Solid Geometry or voxel-based virtual clay. Deformations allowed by B-spline curves and Catmull-Clark surfaces are beneficial for product design, in which original sketches need to be modified to produce the final shape.
The first curve of an object can be freely drawn anywhere in space, but subsequent curves are woven into the existing curve network - being altered if they do not intersect an existing curve. When a curve is modified, the rest of the model can be altered in two ways. In local mode, the modified curve and curves adjacent to it are altered. In global mode, all curves are altered recursively. Surfaces can be filled in with Catmull-Clark or Kuriyama surfaces when a closed loop of curves is selected.
The non-dominant hand is responsible for translating and rotating the model, while the dominant hand is responsible for editing and deforming the model. An original 3D widget with multiple pointers spread out in a fan shape support multiple editing tools - each pointer is mapped to a different tool. A sketch of a chair seat took an experienced user approximately 15 minutes to create.

Discussion:
The idea of the designer drawing the full sweep of a curve across both halves of a mirrored plane and the program averaging each to match symmetrically is new to me. Previously, I had only seen modeling programs where the shape being drawn is mirrored as it is being drawn, and the modeler is only required to draw half of the shape.
The subdivision level for surfaces are chosen so that each curve piece has the same number of segments. This makes creating the subdivided surfaces easier to compute, but is not the best option for models containing areas of small detail and large areas without much detail. In this case, the large areas of less detail are most likely over defined.
Neighboring surfaces are connected with only G1 continuity. For several real-world modeling applications, higher continuity connection is required. Usually, G2 continuity is the minimum smoothness demanded for surfaces which reflect or which need to have good air flow characteristics. If this system were to be used for real-world design, more work would have to be done so that higher continuity can be achieved. Unfortunately, the option of higher continuity may make the system slightly less easy to use.
While the fan-shaped, multi-function widget is novel and may reduce tool selection time for expert users, I think it would produce confusion and tool selection errors in novice users.