The 3D Tractus: A Three-Dimensional Drawing Board (Lapides 2006)

Summary:
The 3D Tractus is a system for creating three dimensional sketches whose interface consists of a 2D drawing surface that slides up and down to incorporate the third dimension. A user sitting at the Tractus draws on the tabletop surface with one hand and slides the tabletop up and down with the other. The goal of directly mapping virtual and physical space with the tabletop height is achieved with a string potentiometer-based sensor. A Toshiba Portege M200 Tablet PC was used as the interactive top of the 3D Tractus table. A counterweight system was used to ease the act of adjusting table height.
The software interface for the Tractus incorporates a 2D drawing area and a 3D overview window. The drawing area accepts pen based input and the 3D view can be rotated by dragging the pen in the desired direction. Multiple attempts at expressing depth on the 2D drawing surface were attempted. Changing color and color intensity in response to line depth did not provide intuitive feedback with enough contrast to communicate distance accurately. The current implementation uses varying line width and a perspective projection to convey depth. Lines above the drawing plane are not rendered.
Three students with art backgrounds evaluated the system for 30 minutes and asked for feedback. The most requested feature was the ability to tilt the drawing surface to allow work to be done from any angle, similar to how the 3D view can be rotated.

Discussion:
One of the programs discussed in the related work section allows a user to select a slice of their 3D object, pull it out of the stack, and edit it. This approach would be unmanageably tedious if working in th direction normal to the surface of the slices. Even if the Tractus doesn't completely remove the problem of working in a direction along the normal of slice surfaces, it greatly reduces the time required to do so by removing the steps of selecting to remove, and replacing, slices in the stack.
Directly mapping physical and virtual space limits the detail of sketches that could be more easily accomplished if zoom functionality was available. The ability to scale the mapping would allow the user to work at multiple levels of detail. Another improvement could be the use of stereovision to give the impression of depth on the 2D drawing area. Also, the addition of a button to toggle the rendering of lines above the drawing plane would aid sketch construction in some cases.