A Matter of Perspective.(2D projective drawing system)(Technology Information)

A projective drawing system borrows from traditional illustration techniques to give architects more creative freedom

What goes around comes around. For years, vendors of 3D modeling software have striven to develop tools that enable users to approach photorealism in their computer designs. The result has been a decade full of "close-to-real" images tinged with a decidedly computer-generated appearance. Unfortunately, for some applications, the digital revolution has forced users to abandon some tried and true manual techniques for achieving artistic expression in favor of the quicker, more flexible CAD model.

One such example can be seen in architecture, with the diminishing presence of perspective drawing. Once an important technique for exploring and presenting designs, particularly in the early stages as a way to flesh out concepts, traditional perspective view are becoming obsolete--not because they're not suited to existing CAD systems. In an effort to resurrect the perspective scene among the CAD generation, researcher Osama Tolba and professor Julie Dorsey and Leonard McMillan in the Computer Science Laboratory at MIT have developed a 2D projective drawing system and range of user-interface and interaction techniques that combine a freehand-like creation of perspective scenes with the benefits of 3D CAD.

Perspective is the central projection of a 3D scene onto an image plane. "Since the Renaissance, artists have been able to generate perspective views that convey 3D scenes on 2D surfaces using line construction techniques," says Tolba. "But because perspective is constructed on a flat surface--paper or canvas--it lends itself to creative expression and fluid freehand strokes. This is hard to achieve with 3D models." While available 3D modeling systems can be used to generate perspective scenes, he notes, "they tend to convey rigid geometry and can be cumbersome." He further attributes the "difficulty" with 3D models to the fact that they are constructed using 2D computer interfaces "that are one dimension lower than the models."

Today's 2D drawing systems are equally ill-suited to creating the appearance of 3D perspective because of their reliance on the Euclidean representation of points, whereby primitives are specified through a collection of 2D points described by two coordinates lying on a plane. "The process of constructing a perspective drawing with these systems is nearly as tedious as with traditional media," says Tolba. In addition, he notes, the resulting static views reduce the 3D impression.

The system Tolba has developed relies on an alternative geometric approach that uses a projective representation of points--basically projecting through the 2D plane to create a 3D-like view. In this model, all the projective points lie on a unit sphere centered about the viewer. "You can think of these points as representing vision rays," says Tolba. In addition, the projective points are used to store various kinds ...