Issue: Volume: 24 Issue: 6 (June 2001)

A Matter of Perspective

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 views are becoming obsolete-not because they're not valuable, but 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 professors Julie Dorsey and Leonard McMillan in the Computer Science Laboratory at MIT have developed a 2D projective drawing system and a 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."
An array of views of an outdoor plaza design is generated from the same digital sketch using a projective drawing system that combines the artistry of traditional perspective rendering with the wonders of CAD.

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 of directions, such as vanishing points, surface normals, and light sources. This type of representation enables re-projections of a view generated from an initial perspective sketch, so the user can easily create various renderings of a scene. By using the projective 2D representations for primitives in this way, rather than using a conventional 3D description, says Tolba, "the user has the impression of being immersed in the drawing or space."
Textured perspective rectangles composed against a panoramic backdrop help visualize a proposed architectural design in context.

To enhance the "3D-ness" of the application, the user interface provides a virtual camera, projective grids to guide in the construction of proportionate scenes, and the ability to underlay sketches with other drawings or photographic panoramas. These interactive capabilities preserve the 3D impression of perspective, says Tolba. "For example, the virtual camera gives an immersive experience, much like navigating a panorama in QuickTime VR." The user can also manipulate the primitives as if they were 3D objects, and the primitives appear 3D through the use of shading and shadows. In addition, certain primitives can use textures to enhance visual complexity.

The new system is somewhat limited due to the lack of relative depth information between the objects, which, according to Tolba, prohibits "true" 3D walkthroughs and general lighting operations, such as shading with a local light source.
An artistic rendering of the Court of Myrtes at Spain's Alhambra Palace uses shadows, shading, and textures that are re-calculated relative to even slight perspective changes.

To further extend the system's ability to support artistic expression, Tolba is hoping to add "expressiveness" to the system by emulating the strokes of traditional media, such as brushes, pencils, or pens. "Such strokes require a more sophisticated rendering procedure than we currently employ," he says.

Although the projective system is still on the R&D drawing board, Tolba believes the technology's appeal will be widespread. "The success of Apple's QuickTime VR suggests that an immersive drawing program has potential as a commercial product," he says, although there are no plans as of yet to market the technology.

In addition to the technology's use for creating traditional architectural perspectives, Tolba can imagine other applications. For example, he says, "Architects often generate hidden-line perspective drawings from 3D modeling systems and embellish them by hand. Our system provides a flexible means for drawing on 3D backdrop." Animators may also be interested in what the system has to offer, he notes. "In order to simulate camera panning, animators sometimes use multi-perspective backgrounds, which are difficult to create. Using a projective system would greatly simplify the process of drawing backdrops for cel animation."

Tolba's main goal, however, is to revive a dying art. "The techniques of perspective drawing are at great risk of being forgotten because of the emphasis on 3D computer graphics," he says. His hope is that this system will serve as a powerful reminder of why they shouldn't.

Diana Phillips Mahoney is chief technology editor of Computer Graphics World.
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