Design Integration Laboratory

Three-Dimensional Sketching

MArch Master's Project

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Three-Dimensional Sketching

Master's Project
Master of Architecture Degree Program
Option III

College of Environmental Design
Department of Architecture
University of California, Berkeley

Kevin Matthews

August, 1988

Mark Smith, Assistant Professor of Architecture
Fred Stitt, Visiting Lecturer in Architecture

Three-Dimensional Sketching


Abstract .. 1

Introduction .. 2

The Problem

Design with Computers? .. 2

The Context

Getting ideas down without getting distracted .. 7

Design and Computing

An Interface Issue .. 15

Existing Two-Dimensional Interfaces .. 17

Existing Three-Dimensional Interfaces .. 22

Three-Dimensional Sketching .. 27

Putting It All Together

Looking Forward .. 40

Conclusion .. 42

References .. 44

© 1988 by Kevin Matthews. All rights reserved

Master's Project August 1988
Department of Architecture
University of California, Berkeley
Kevin Matthews

Three-Dimensional Sketching


Graphically oriented computers have become an efficient means for the production of construction documents, but despite recent large advances in computer graphics capabilities computers are still not effective tools for creative design work. This paper will examine the design process to determine what has made it resistant to computer assistance, and then discuss how to address the unique nature of architectural design work by changing the character of human-computer interaction. Observations about existing design process based on sketching with paper and pencil will show that the design utility of computer modeling systems can be advanced through appropriate innovations in interface design.

The use of sketch modeling in early design is a project distinct from the use of more elaborate computer models as visualization tools during design development. While imaging techniques have advanced, the methods for creating the fundamental three-dimensional model have lagged. The model shop metaphor suggested by previous research on architectural design interfaces is not appropriate to the crucial design phase of actually generating and capturing new ideas. In conventional practice this work is not done in a model shop, where tooling operations must be done in a premeditated sequence. Rather, early design is done freehand on paper. In general, we must look to the freehand sketching process for the best clues to a successful design interface.

The paper will examine the sketching process and delineate its fundamental characteristics. These provide the basis for a new approach to a three dimensional freehand sketching environment optimized for architectural design. The modeling space is characterized by a noun-verb syntax direct -manipulation interface, using direct mouse control of a crosshair in three dimensions and objects with click-selectable handles to achieve facile editing in three dimensions. In addition, some computer modeling tools will be suggested which would facilitate the seamless refinement of conceptual sketch models into precision models for architectural design development and design documentation.


Computers are now beneficial for the production of construction documents, but not usable for architectural design despite recent large advances in computer graphics capabilities. This paper will examine the creative design process to determine what has made it resistant to computer assistance, and then discuss how to address the unique nature of architectural design work in the character of human-computer interaction. A few key observations about existing design process will lead to the argument that a major advance in the design utility of computer modeling systems is accessible through appropriate innovations in interface design.


The Problem

Design with Computers?

Graphic computing has tremendous potential for aiding predictive visualization, which is central to the creative process of building. With the recent advent of fast raster graphics on expansive high-resolution color monitors, powered by quick processors and growing amounts of random access memory, improvements in computer imaging have spawned wide ranging new uses. These have come to public attention in computer animations on television and through the commercial success of the graphically oriented Apple Macintosh microcomputers. It is now possible, with a computer graphics system affordable to a medium sized architecture firm, to create a three-dimensional data model of a building--including landforms, vegetation, fenestration, exterior detailing, and interiors with furnishings--that can be viewed from any distance or direction, and automatically rendered in a few minutes in full shaded color with accurately

Figure 1. Color rendering from a GDS three-dimensional model.

HOK Computer Systems: demonstration video tape.computed shadow casting (figure 1). Leading firms have even used special hardware and software to produce full perspective interior and exterior "fly throughs" for video presentations.

These powerful tools for looking at buildings before they are built transcend traditional presentation techniques such as physical models and hand renderings because the computer's ability to rapidly re-image a model space from any position virtually eliminates viewing restrictions due to scale or position. Unlike most existing forms of manual presentation, a computer model may be adjusted interactively. This opens new avenues for the exploration of a building while it is still "on paper," and for as yet unexplored modes of architect-client exchange.

The brave new world is still far from unlimited, however. While imaging techniques have advanced, the methods for creating the fundamental three-dimensional model have lagged. For instance, rather than draw a human figure directly into a database, commeComputer Magic, video tape. rcial animators first model it physically in foam, then trace cross sections of the foam with manual digitizers. The duplication of effort involved in essentially modeling the figure twice is certainly inefficient, but it exemplifies the difficulty of making a computer model of a complex object.

In contrast to modeling a human figure for animation, design specifically calls for the modeling of as-yet-unknown objects. Design is more or less the creation and delineation of the model from scratch. Because design work starts with a mental construct rather than a physical one, the nature of the initial modeling process is far more critical to successful computer-aided design than is the quality of subsequent sophisticated imaging. The most spectacular successes of computer graphics have been of limited value for conceptual design work. Even in an advanced heavily computerized firm, design concepts will be developed from sketches on paper into simple massing models or drawings before a computer model is somewhat laboriously created for use in refininSchilling, Terrance G. and Patricia M. Schilling. Intelligent Drawings. New York: McGraw Hill, 1987, 17, 236.g a project during design development. Yet it is in the earliest stages of design, when fundamental forms are being established, that the ability to think and see in three dimensions is most important.

Williams, Greg. "The Macintosh Computer." BYTE, February 1984, 30-54.Two-dimensional drawing with computers has advanced recently, with the windows, icons, mouse and pointers interface of the Macintosh microcomputer, manifesting research done at SRI and Xerox PARC, leading the way.Editors of Time-Life Books. Input/Output. New York: Time-Life, 1986, 61-75., There is still a division in ease of use between two-dimensional applications used for general drawing tasks (i.e., MacDraw) and those used for

Figure 2. Railroad diagram of a complex GDS command.

precise drafting work ("CAD applications"). But while interfaces for two-dimensional graphics have advanced dramatically, three-dimensional computer model-making remains an exclusive bastion of complex transformations and typed numerical data entry. The syntactic complexity, alone, of most existing modeling interfaces would make them nearly useless for the most creative phases of design (figure 2). Because of the awkwardness of three-dimensional modeling, paper and pencil remain supreme for early conceptStitt, Fred A. Systems Drafting. New York: McGraw Hill, 1980, 231.ualization.

One of the ramifications of the continuing reign of paper-based sketching for design is that for any project to be developed or produced with CAD it must go through an awkward transition, in which work on paper is duplicated by some sort of computer data Schilling, Terrance G. and Patricia M. Schilling. Intelligent Drawings. New York: McGraw Hill, 1987, 186.entry process. It would be much more efficient to be able to develop a project seamlessly, without the redundant work attendant upon switching media. The time and attention saved could possibly be spent on extended problem-solving.

But if computers are not yet usable for design, why not? To answer this we need to know how architects design. It seems that the traditional sketch is highly suited to the designer's needs. An examination of what is so apt about sketching will supply us with some fundamental criteria for designing a computer-based modeler that really would work for design.

I believe it is possible to create a modeling system with an easily manipulable plastic modeling environment, on existing hardware, that can be easily integrated into the already evolving, more precise and rigid production-oriented CAD environment. This will allow new approaches to several important bottlenecks in the making of buildings. Such a system will combine the ease of use and communicative density of traditional sketching with the malleable multiple dimensions of a mathematical design space. I propose this solid sketching as an achievable computer graphics application type for architectural design.


The Context

Getting ideas down without getting distracted

Kostoff, Spiro. A History of Architecture. New York: Oxford University Press, 1985, 4-5.The schematic of a design oriented modeling system must be based first and foremost on an understanding of how people design. In general, architects design by sketching. The pencil on paper sketch provides a highly developed method of representing ambiguous ideas quickly and easily, without distracting the mind from the intuitive state required for creation and visualization (figure 3). A useful solid sketching system will intensify the fundamental intuitive relationship between the designer and his sketch pad, rather than divert the designer from his most subtle mental state.

Heidegger, Martin (translated by John Macquarrie and Edward Robinson). Being and Time. New York: Harper and Row, 1962, 187.This intuitive state is the subject of quite a bit of philosophical controversy. Nonetheless, it must be examined, and conclusions about it must be drawn, in order to proceed with understanding the mechanisms by which the sketch facilitates design. These are the mechanisms from which, by a sort of reverse engineering, we will derive generalized requirements for the modeling interface.

Good design is different from simple rational thought, at least as rational thought is generally envisioned. Where simple rational thinking proceeds by handling one idea at a time it a mannerly, logical way, design must handle an overwhelming and fundameRittel, Horst W. J. "Some Principles for The Design of an Educational System for Design." DMG Newsletter, Vol. 4, Number 12, December 1970.ntally inseparable welter of ideas, images, and data of non-comparable sorts. Design is concerned with what Horst Rittel has called "wicked problems." Given the multi-level ambiguity of design work itself, it is not surprising that design education has been recognized as a difficult matter, constituted of, as in

Fleig, Karl, Editor. Alvar Aalto. Zurich: Verlag, 1971, 188.Figure 3. The rough lines of this Aalto sketch convey a tremendous amount of ambiguous information.

Schon, Donald A. "The Architectural Studio as an Exemplar of Education for Reflection in Action." Journal of Architectural Education, Fall 1984.architect Joseph Esherick's paraphase of noted educator Donald Schon, "mucking about in the swamp." The swamp referred to is the "low ground" of real design problems, where our feet tend to get stuck in the rich mud of assorted decomposing but utterly vital concepts.

Yet people are particularly well equipped to deal with just the sorts of situations, complicated and without closure, which characterize the real world (as opposed for instance to the suspiciously clear and simple world of the digital computer). In fact, the real world of architectural design problems is so far beyond simple rationality that unutterability is a central characteristic of great solutions. It is the work of artists, with architects numbered among them, to succeed at handling just such intraAlexander, Christopher. The Nature of Order. Berkeley, CA: manuscript, 1986.ctable problems.

Bonta, Juan Pablo. Architecture and Its Interpretation. New York: Rizzoli, 1979.Moreover, since architectural meaning is embodied in the synthetic interweaving of functional structures with ornamentation, architectural design is concerned with an incredible array of parameters. The combination of doing a job while doing art, or sometimes even doing a job by doing art, makes architectural design problems even more complex, more simultaneously constrained and open ended, than those usually faced (for instance) either by artists or by engineers.

There is a particular kind of mental state, or a particular way of thinking about deep problems, that gets the normal verbal mind out of the way, and lets us work on issues en masse, by means of applied intuition. This kind of thinking works much faster than words can be formulated. At shallower levels it can be felt as visual thinking. At deeper levels, it can be sensed as the currents beneath the images of visual thinking. This type of thinking is related to the subconscious and unconscious, and unexamined, leads to many distortions in the works of confused designers.

Many great designers, including scientists, engineers, and artists, as well as architects, have discussed the importance of this intuitive thinking in their work process. For instance, Alvar Aalto spoke of the crucial intuitive aspect of his design process:

When I personally have some architectural problem to solve, I am constantly . . . faced with an obstacle difficult to surmount, a kind of "three in the morning feeling." The reason seems to be the complicated, heavy burden represented by the fact that architectural planning operates with innumerable elements which often conflict. Social, human, economic and technical demands combined with psychological questions affecting both the individual and the group, together with movements of human masses and individuals, and internal frictions all these form a complex tangle which cannot be unravelled in a rational or mechanical way. The immense number of different demands and component problems constitute a barrier from behind which it is difficult for the basic idea to emerge . . . I forget the entire mass of problems for a while, after the atmosphere of the job and the innumerable difficult requirements have sunk into my subconscious. Then I move on to a method of working which is very much like abstract art. I just draw by instinct, not architectural synthesis, but what are sometimes childlike compositions, and in this way, on this abstract basis, the main idea gradually takes shape, a kind of universal substance which helps me to bring innumerable contraMalcom Quantril. Alvar Aalto: A Critical Study. New York: Shocken Books, 1983, 5.dictory component problems into harmony.

The power of sketching for designers lies in its ability to support thinking with "negative capability," an odd but valuable term defined by the English poet John Keats in 1817:

. . . several things dovetailed in my mind, and at once it struck me, what quality went to form a Man of Achievement especially in literature and which Shakespeare possessed so enormously--I mean negative capability, that is when man is capable of being in uncertainties, mysteries, doubts, without any irritable reaching after fact and reason.

Perkins, David (ed.). English Romantic Writers. New York: Harcourt Brace Javanovich, Inc., 1967, 1209.

If we accept that much of the meaning of art affects us subconsciously, and realize that any verbal discussion of subconscious matters will never be able to achieve a completely satisfying closure, we see that extra-verbal converse is not only real and important, but actually at the heart of sensitive design.

Awareness of subconscious resonances between diverse aspects of a problem set guides the designer toward the transcendent solution. We can't work these subconscious resonances readily with usual verbal/logical consciousness--they show up in our awareness Einstein, Albert. "Autobiographical Notes," in Schilpp, P. A. (Ed.) Albert Einstein: Philosopher Scientist. Evanston, Ill: Library of Living Philosophers, 1947, 7.of feeling, emotion, attitude, association, and intuition. A key point, however, is that these gray areas of thought are something we can definitely tune into and use--they just require a different approach than words and numbers.

In the design of a very simple project, the extra-verbal complexity of a good solution is relatively easy to handle, because the entire solution can be conceived as a simultaneous whole. This is analogous to totally experiencing a single tree. The design can be recorded all at once as a nearly complete thing, sketched with just a few evocative strokes.

In contrast, wielding the solution images of a larger, more complex project is analogous to experiencing all the trees in a forest, as individuals, small groups, regions, and as a whole. The overwhelming size of the problem requires a system for recordinAlexander, Christopher. Notes on the Synthesis of Form. Cambridge, MA: Harvard University Press, 1964.g fragmentary solutions as they develop, and then for massaging the fragments into a coherent whole.

The two-dimensional pencil and paper sketch plays a crucial role in the early stages of a typical design process. Two-dimensional sketching uses the expressiveness of pen or pencil on paper to record ideas quickly and richly. However, paper-based sketching is limited in its facility for representing three-dimensional images, and architecture is fundamentally a three dimensional art. Furthermore, the art of drawing may become an end in itself, to the detriment of the architecture being drawn.

The sketch is nonetheless well adapted to the needs of the design process, which requires the recording of ideas and images that are ambiguous or incomplete or provisional or even too complete and vivid. Getting too specific too early freezes the creative process, so needless or excessive specificity has to be avoided. A measure of ambiguity under the control of the designer allows thoughts essential plasticity. The complex graphic shorthand of the design sketch allows the designer to put down just enough.

In addition to two-dimensional sketching, some architects use a material form of three-dimensional sketching in the early stages of design, to assist real visualization (figure 4). This

Figure 4. A physical sketch model.

Figure 5. Design development study model.

sketch modeling, using clay or foam or roughly shaped cardboard, is a special type of modeling which needs to be distinguished even from study models used during "design development" (figure 5).

In clay-based three-dimensional sketching, the necessary plasticity of the mental model is retained by the analogous malleability of the material, as well as by the vagueness of simple lumpy forms. In cardboard sketch models a provisional character is allowed by a different mechanism. We can observe a roughness of forms in torn cardboard analogous to and reminiscent of the sketchy, searching, overlapping lines of a paper design drawing.

However, sketching of early design ideas in three dimensions is limited by the actual material problems of making even the crudest physical model. For instance, if the key early image is a balcony, the physical modeler must hold the balcony image in stasis while making a physical armature to suspend the physical model balcony against gravity. In comparison, the paper sketcher can proceed in the mind's non-linear way, drawing the balcony in space with a few quick strokes, then an important tree to the side, and perhaps then the first stroke of wall.

Christopher Alexander has for several years championed another approach to design visualization. In addition to making paper sketches and physical sketch models, he and his followers have developed a complete approach to construction in which full-size mAlexander, Christopher. The Nature of Order. Berkeley, CA: manuscript, 1986.ockups are made in location, to help the designers concentrate most directly on the actual reality of an actual situation. While this method is largely incompatible with business-as-usual in the building industry, it serves as an example of how to minimize errors of design due to faulty abstractions and misleading imagination. A full-size mockup, in place, allows the full power of applied intuition to be focused on whether the proposed solution is correct. Our modeling interface should similarly allow intuition to be applied to developing solutions.

Lam, William. Perception and Lighting as Formgivers for Architecture. New York: McGraw-Hill, 87-93.Alexander's mockups are a sort of full-sized study model. Full sized presentation models have also be used, for example in the design of the Washington D.C. subway system as documented by lighting consultant William Lam. Existing computer graphics systems also are capable of making three-dimensional study models and to an increasing extent are capable of producing very realistic presentation images based on three-dimensional computer models. It is important to be clear on the difference between all of these and the more provisional and mutable sketch models that are the object of this discussion.

There are four criteria, adequately met by the designer's traditional methods of sketching, that any working design system must meet. The system must be:

A successful "sketch" interface for computer modeling will meet these criteria.


Design and Computing

An Interface Issue

Because the character of the interaction between a designer and his design media is crucial to the design process, functional computer-based sketching is largely a matter of human/computer interface. Recently developed computers with bit-mapped displays and graphically controlled windowing user environments present new options for interfaces to three dimensional modeling and rendering. In light of the new potential of these graphic workstations we can rethink the process of entering architectural information, and develop an interface that will be easy, fast, and fluid enough to allow real design sketching in three dimensions.

The complexity of the data to be entered and the need to allow design work to proceed in an intuitively oriented mind state present conflicting requirements for design software. The prevailing expectation seems to be that these factors will be accommodatYessios, Chris. "What Has Yet to be CAD." ACADIA Workshop Proceedings, October 1986, 29.ed by building artificial intelligence into architectural graphic systems. This would help by allowing the computer to develop a significant part of the building description data set. This in turn would allow the designer to keep his thinking at a high level, on the design rather than the designing. In general this seems to be a plausible approach, though certainly not an easy one. However, the decision programming for the computer is a formidable problem. More importantly, deterministic effects on design must be avoided if the tool is to be acceptable.

I believe we need to use a different approach, developing alternatives to artificial intelligence programs that will work by facilitating the designer's work rather than by supplementing it. This can be done by improving the interface between the designer and a modeling space displayed by the computer.

The design process can break down from a failure to generate alternatives. Failure of the design process is more often due to a faulty choice among alternatives, caused by a failure to visualize them realistically (honestly, in effective detail) or caused by some other failure of evaluation. In Design for the Real World Papanek, Victor. Design for the Real World. London: Granada, 1974, 142., for example, Victor Papanek states that architects, engineers, and draftsmen fail to draw a correct perspective view of a particular object, when given front and side elevations, at a higher rate than people without a measured drawing background. Apparently this perceptual breakdown is due to habitual rather than explicit projection in the designer's mind's eye. The incorrect acceptance of an existing alternative will also short circuit the generation of new possibilities.

Choosing among alternate design paths is too important and not well enough defined to delegate to the machine, but the machine could help substantially with the visualization if it could hold images to hand, and allow the right kinds of subtle adjustments to those images without getting in the way.

The complexity of editing building models as compared to text handling, numerical analysis, or even two-dimensional graphic design work remains a substantial barrier. Ability to manipulate text effectively with a computer is very recent, and it is dependent on millennia of language development for the simple abstractions used. Creative buildings remain with one foot in the realm of sculptural art, where the issues belie abstraction. A building exists as a full bodied entity out in reality. Good design solutions are even likely to be anti-abstraction, and we should reject any tool that reduces our ability to work with the fine-grain character of a project.

The best defined set of existing abstractions in architectural representation is the conventions of working drawings (figure 6). However, construction documents describe a set of rules and guidelines for the building process rather than the experience of the building and space. That full sense of experience is what needs to be held to hand by the media of design.

Figure 6. These standard symbols are part of the abstract language of working drawings.

That is exactly where existing applications fall down. The cumbersome modeling process gets in the way of the design process, and the toolkits don't readily support the sorts of model adjustments an architect is likely to want. More specifically, the standard CAD applications fall down on each of the key issues distractiveness, appropriate specificity, malleability, and subtlety. In the traditional sketch these are handled by the directness and expressiveness of pencil on paper. We must alter the methods of modeling in graphics software to achieve that level of directness and nuance. We must provide the same fundamental attributes using physically different but conceptually parallel methods.

With an appropriate interface, solid sketching could become a new, powerful kind of design tool.

Existing Two-Dimensional Interfaces

Most of the research on human/computer interaction has focused on two-dimensional modes, which represented the frontier of development in such interactions until quite recently. However, a sufficient body of work now exists to allow the generalized definition of advanced approaches to both text handling and graphic manipulation in two dimensions. Specifically, classic studies using several variations of the keystroke model for interface analysis demonstrate the effectiveness of mouse-based graphic interfFoley, James D., Victor L Wallace, and Peggy Chan. "The Human Factors of Computer Graphics Interaction Techniques." IEEE CG&A, November 1984.aces for a variety of fundamental selection and positioning tasks. For example, graphically oriented text editors successfully reduce the number of movements required to perform even simple manipulations of text, as compared to typed-command driven interfaces. For complicated graphic tasks the superiority of a graphic interface is only greater.

A good tool is sufficiently general to do a variety of work and to support creative extension, yet specific enough to facilitate the accomplishment of particular tasks. For example, the carpenter's plane is a powerful tool for smoothing wood exactly because it is constrained from making deep cuts. Innovators in the hardware side of interface research have tended to be seduced by the apparent power of very general tools. An extreme example of this, at least from a modeling interface point of view, is theFoley, James D. "Interfaces for Advanced Computing," Scientific American, October 1987, 126-135. sensor glove. These wired gloves provide the amazing facility of putting one's hand directly into the modeling space."I think we can build anything we can conceive of. So we're terribly knowledge-limited today." Jerome Weisner, in Brand, Stewart. The Media Lab. NewYork: Viking, 1987, 154. Once there, however, the hand is no more useful for drawing than fingers are for painting. Exotic input hardware may be a distraction from the fundamental conceptual problems of three-dimensional manipulation.

Within the class of graphic input devices, the conventional, economical mouse has advantages for many tasks. Since the mouse is inherently a relative positioning device, it tends to focus attention on the graphics display. The window on screen contains all the location information, as opposed to sharing it with the location of a stylus on a tablet. This works synergistically in an interface that provides continuous on-screen feedback of selection and editing status. Tablet menus, in contrast, dependent on the absolute positioning capability of a digitizing tablet, divide user attention between the screen and a tablet map. In the days of slow vector displays, when on-screen text was a liability, having a menu on a tablet was an asset, but on a windowing workstation an absolute positioning device like a graphics tablet is optimal only for the generally minor function of hand digitizing.

One important aspect of the materiality of drawing a line on paper, and understanding how it fits in a drawing, is a sense of duration that translates to a feeling of length in the mind's eye. This implies that it is more meaningful to the designer to drag lines into a drawing, rather than just point to disembodied end points. The relative position information supplied by the mouse, without zeroing problems, is ideal for this approach to input. The sensate duration/length connection to object manipulations is equally relevant to the fundamental editing operations of resizing and repositioning, both of which can also be performed by dragging. Pointing combined with dragging works well for indicating a simple selection set.

The Apple Macintosh has made a commercial success of the graphic mode of interacting with a computer. In the Macintosh environment, normal hand and eye skills are used to simply accomplish graphic transformations that would be complex to describe in words--even complex in the special words of a tailored command language.

As indicated by personal use as well as commercial success, the mouse-based Macintosh interface is a good guide for how to handle two-dimensional graphic tasks, including manipulation of text and two-dimensional drawing. The Macintosh interface is a careful and creative implementation of many years of academic and industrial research in human factors and ergonomics, presented in an unusually consistent form on a relatively affordable platform. It does not provide a perfect solution to the problems of human-computer interaction. In particular, it has not been developed to do the key tasks of three-dimensional work. Nonetheless, the spirit of the Macintosh interface remains a guide into this uncharted territory. Both the method and the machine of the mouse can be effectively extended into three dimensions, providing an inexpensive yet appropriately flexible input and manipulation system created completely through thoughtful software design.

In the Macintosh interface in general, and in Macintosh MacDraw-type graphic applications in particular, operations are performed with noun-verb, or select-then-operate sequence. This is a natural arrangement for design work. First attention is directed at an object, and then and action is taken regarding the object. Command line interfaces tend to use a verb-noun, or command-argument syntax. The verb-noun order often puts the user in the position of remembering the object of the next action while simultaneously trying to recall the command to excute the action. This is represents a forced allocation of user attention for the convenience of the machine.

There is a difference in the fundamental program outline for these divergent interface approaches. The command-argument approach feeds input to program subroutines in a relatively simple, linear manner. In contrast, a select-then-operate application, especially if using a mouse for graphic input, is structured around a main loop waiting for input events. These programs must be able to respond to events in any sequence.

In Macintosh-style applications, graphic selections are made either by directly clicking on an object or by a selection range, defined by dragging diagonally across the desired area in the current view. Subsequent commands are applied to the current seleApple Computer, Inc. Human Interface Guidelines: The Apple Desktop Interface. Reading, MA: Addison Wesley, 1987.ction set.AutoCAD, version 2.62. Simple objects such as lines, rectangles, and circles are created with one mouse gesture, anchored at the location indicated by the mouse-down of a drag motion, at the size, direction and proportion indicated by the mouse-up concluding the gesture. A seGDS, version 4.9.lected object may be resized by dragging one of its handles, and it may be moved by dragging it other than by a handle.

Mitchell, William. "Solid Modeling and Volumetric Composition in Architecture," Design Computing, Volume 1, 1986, 123-135.In addition to this basic pattern, control options are provided through a small set of modifier keys. Mouse clicks on objects with the shift key depressed invert the selection state of the hit objects--that is, where a new click would normally drop an exiVan Norman, Mark. "A Digital Modelshop: The Role of Metaphor in a CADD User Interface," Design Computing, (1986) Vol. 1, 95-122 sting selection set and start a new set, a shift-click will add non-selected objects to the existing set and delete already selected objects from the set. The shift key also modifies resize and move operations. Unmodified

Figure 8. Sketching environment, showing three-dimensional crosshair with background grids, in perspective projection.

In some cases background grids may be objectionable, or ineffective, so consideration should be given to more technically demanding approaches to position indicating, such as depth cueing by relative intensity or indication of crosshair/object overlaps. The latter would require some degree of real-time hidden-line calculation, but with reversal of crosshair color to indicate "insideness" or "behindness," it could be very helpful. In addition, given ultra-fast intersection calculation, the point of crosshair penetration of object surfaces could be highlighted. The user could be given some control over the speed/alignment tradeoff by a variable setting for crosshair length--shorter means fewer objects to compare against, and longer means easier alignment with distant objects.

Some shape concepts are most naturally captured in outline, as simple planar wireframes or as sectional loops for extrusion. Therefore, it should also be easy to draw two-dimensional graphics in the model space in various orientations. To support this, the mouse table-top movement can be realigned to another plane in the model space--a temporary working "desktop" in section and elevation as well as in plan. For instance, a cross-section of a complex extrusion

Figure 9. The GDS PLANE command provides a powerful but complex means for readjusting three-dimensional axes.

perimeter can be drawn directly in the location and orientation in which it will be used, without the need to construct snap objects, and independent of the current viewpoint. With existing toolkits, drawing out of the horizontal plane requires working plane rotations which are disorienting and tedious to execute (figure 9). Due to the fundamental lack of a three-dimensional cursor, new elements can only be drawn in views orthogonal to their plane of creation, or by snapping to pre-existing objects. Existing approaches require a one-to-one relationship between the working plane and the two dimensional cursor plane. In the three-dimensional sketching environment, a single icon with multiple facets can allow a single click to orient mouse x-y to any of the three major planes for more facile drawing. A direct method for orienting the working axes non-orthogonally to the world axes would be a valuable extension to this, and three-dimensional cursor control opens many possibilities. For example, the realignment of axes could be handled by dragging the primary crosshair in the model space with a temporary secondary crosshair.

Direct x,y,z cursor control, a three-dimensional crosshair option, and iconic selection of working plane constitute the fundamental tool set. These establish an extensible paradigm for direct manipulation of solids in space. The combination of direct movement in space with clear orienters in space does the main job of liberating the architect model maker from the distraction of numerical coordinates and computer-oriented manipulations in the sketching environment. Concern is shifted from the keyboard to the modeling space and from the dimensions of forms to the forms themselves.

In addition to these fundamental tools, methods are critical, and objects need to be invented in connection with the fundamental editing method. As demonstrated for two-dimensional work by MacDraw, it should be possible to accomplish the basic editing operations without having to enter commands. For example, an object may be resized by selecting an object handle and then dragging it in any direction with the three-dimensional mouse. The select-then-operate syntax is as appropriate to three-dimensional modeling as it is to MacDraw. Object selection is indicated by

Figure 11. Resizing and reshapeing operations on rubber blocks.

profound response to the menus-vs-icons-vs-keyboard controversy. A typical way to create a cuboid would be to move the cursor to the desired starting elevation, if necessary, then drag out the diagonal of the block in plan, drag the outline upward with the up button, and release the button at the desired block height. Continuous display of the changing shape of objects during transformations is critical. For objects or groups too complex to redraw interactively,it might be adequate to continuously display the object or group coordinate box,during transformations. It is less critical whether the associated syntax follows the original MacDraw button-down/drag/button-up pattern or the click-to-start/move/click-to-finish alternative. The down-and-drag approach gives the most emphatic space-durational feedback, but the click-click syntax lets snap codes be reset in mid-transformation with maximum ease. The first favors freehand sketching, and the second favors precision model editing. The best interface option would be to allow user switching between the two methods.

Most CAD applications ignore open space (non-object) hits, but event-driven, Macintosh-style applications interpret open hits as the start of a selection-box-define procedure. Thus a blank hit undoes the current selection set. The three-dimensional cursor allows the definition of a three-dimensional (space-based) selection box, with improved selection resolution compared to two-dimensional (view-based) selection boxes.

It is not always desirable to specify every dimensional parameter graphically for every block. Tools should allow quick and independent fixing, resetting, and releasing of absolute or relative values for most creation and modification parameters. For example, a cuboid with base set at z=0, height set at relative 8'-0", width set at relative 6", and floating length, can be used like a line-type for quick drawing of ground level perimeters and partions. MacArchitrion implements a powerful but somewhat fussy approximation of this, using modal dialog boxes extensively for setting parameter groups (figure 12).

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