Introduction A better way

After 15 years of development, much promise,but limited use, is interactive computer graphicsfinally due for a sharp growth spurt? If the moodprojected at the Fourth Annual Conference on

Computer Graphics and Interactive Techniques heldin San Jose recently is any indication, the answer

would seem to be yes. On the hardware side, costsare coming down and capability is going up,

reflecting trends that have characterized the com-

puter industry in recent years. On the softwareside, although programming for computer-aideddesign tends to be specialized and hence not broadlyavailable, many standardized display packages are

on the market.' In applications, there is activity inmore than 25 areas, ranging from stereotaxic surgery

to landfills after strip mining.The driving force behind computer graphics is

the information explosion: interactive computergraphics permits the vast quantities and complexinterrelations of information to be organized andmanipulated in a way that exploits the uniquehuman ability to work with patterns.

*Dice courtesy of Kevin Weiler and Peter Atherton, Program of Computer Graphics,Cornell University. Reprinted from Computer Graphics, Proc. 4th Annual Conf on

Computer Graphics and Interactive Techniques, (3 1977 by the Association forComputer Machinery.

Even in first-generation computers, users were

overwhelmed by stacks of printout. Combining thecomputer with a cathode-ray tube to generate lines,curves, and surfaces that could represent the datapromised a better way of transmitting informationacross that last critical inch or so inside the humanhead. Then the addition of human-operated inputdevices resulted in interactive computer graphics.23

Trends. Rapidly falling hardware costs, as exem-

plified by the microprocessor and the 16K RAM, are

affecting graphic systems, as they are other parts ofthe computer world. More "intelligence" is beingplaced at the graphic display, taking some of theprocessing load off the host computer. Anothertrend-proclaimed by some but denied by others-is toward raster-scan CRT displays. Made feasibleby lower-cost memories, the raster-scan techniqueenables graphics users to benefit from the mass-

production cost level of the television industry.Costs are even declining enough to permit thecomputer hobbyist to experiment with simpledisplays, such as interactive games and checkerboard-type art patterns.

Displays. Basically, there are four arrangementsof displays using the CRT principle: direct-viewstorage tubes,4 stroke or vector writing units, rasterscan,5 and scan converter.6 In addition, the plasmapanel, based on a different principle, has had limiteduse.7

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SPECIAL FEATUREInteract-veCompuerGraphKs:Poised for Takeoff?Ware MyersContributing Editor

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Designers would like a display capable of retainingthe image indefinitely without the need for refresh-ing and without loss of quality, while avoiding thecost of a display memory.9 They would also like theability to selectively erase and modify the image oncommand. Naturally, they want the display to beinexpensive, small, and operable from low-voltagesupplies. They would like non-flickering color thatthe user could work at comfortably for hours ata stretch.'0 To state these desires is to suggest howfar present-day technology has yet to go in achievingits promise.

Input devices. There are three common inputdevices: the keyboard, pointing device, and tablet.In addition to entering alphanumeric information,the keyboard may have keys for moving a cursor,making it comparable to a pointing device. Thepointing device serves to point to something on thedisplay as a prelude to acting on it in a way definedby a command.An intriguing version of the pointing concept is

found on the Magnavox plasma panel. Called thetouch panel, it is an array of 16 x 16 LED's, emittingbeams parallel to the flat plasma panel. A fingerinterrupting two of the beams at right angles toeach other defines one of 256 sets of x-y coordinates.The joystick, tracker ball, or rolling "mouse"

enables the user to position a screen cursor byfeedback from eye to hand, as a prelude to acommand. The tablet, available in several forms,permits the user to create on the screen the lineshe draws on the tablet. It is especially useful indigitizing existing documents.

Processing power

Processing capability may reside entirely in asingle-user computer, or it may be time-shared outof a large central processor. In the past, it has beenlargely resident in one or the other computer.However, as the hardware for distributed processingbecomes more common, part of the processing cap-ability may migrate to a unit-called the displayprocessor-associated with the display.", 2 Broadlyspeaking, there are four tasks to be accomplishedby the processing power: controlling the display,accepting input commands, managing a data base,and computing display relationships.

Display control. At the minimum, the vector andraster-scan displays have to be refreshed. Conse-quently, there must be-either in the displayprocessor or the host computer-a hardware memoryof what is currently on the screen and a means oftranslating the memory contents into signals thatcontrol the deflection of the electron beam. Forexample, in a vector-type display, a stored commandmay direct a line to be drawn from Xl, Y1 to X2, Y2.The display controller converts this digital commandinto X and Y deflection signals that turn on thebeam at Xl, Y1, move it at a computed angle toX2, Y2, and turn it off. Because of the time con-

Using an entirely different principle from that of the CRT,the plasma panel became commercially available aboutfive years ago. The flat panel consists of two sheets ofglass with a neon-based gas filling the few thousandthsof an inch between the sheets. When a voltage is appliedto the gas between two grid points, a small spot of gasglows, generating a red display. The glow is sustained bya high-frequency voltage which, by itself, will not fire aspot. In spite of its promise of low cost, the plasmapanel has not been widely commercialized so far. How-ever, over a million terminal hours per year are beinglogged on the plasma panels attached to the Universityof Illinois' PLATO system. Users are said to like theappearance of the image and the absence of eye strain.As shown above, the Magnavox Model 12,000 can beused for circuit design. The circuit and logic symbols arespecial characters stored in RAM memory. Containing aprogrammable microprocessor, the display controls512 x 512 addressable points, or 46 lines of 85 characterseach. It is also capable of vector generation.

The Tektronix 4081 interactive graphics system, a line-drawing display,combines the conventional storage-tube concept with a refresh featurethat lets the user enlarge, change, delete, rotate, or move a portion ofthe image while the rest of it holds still. When the revision is ready, thestorage screen holds it in place. Portions of the image that fall off thescreen are automatically clipped. These capabilities are achieved withtwo processors: a general-purpose processor and a special displayprocessor. This system is priced at about $27,000 (base price).

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-~~~~~~~~ _

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.~.. _~~~~~~~9..

Extending the art of photocomposition. John C. Beatty of the ComputerSystems Division at Lawrence Livermore Laboratory produced hisdoctoral dissertation on a photocomposition system that he haddeveloped for the laboratory. Described by Beatty as the most sophis-ticated photocomposition system in existence-though somewhatdifficult to learn and to use-the system generates not only a variety oftype faces and mathematical symbols in color, but also enables theuser to create diagrams. Starting with a number of primitive forms-lines, circles, triangles, etc.-the user can extend them, change theirsize, join them together, and, in general, manipulate them from akeyboard to produce the diagram he desires. Once produced, inter-actively, the diagram exists as computer data and may be stored andindexed in this form, or output to an appropriate hardcopy device.

straints imposed to prevent flicker, most designershave found it cost-effective to implement thedisplay-control function in special-purpose hardware,rather than leave it in the host computer.

Input interrupts. Some of the input devices, suchas the keyboard, provide digital inputs; others,such as the mouse and the tablet, supply analogvalues that have to be converted to digital form.Both polling and interrupt procedures have beenemployed to acquire these inputs. In either case,the source and nature of the event are identified,leading to a task scheduler which invokes someprogram process in response.

Data base. In some applications, a substantialamount of information is needed in order to respondto a user query. This need forces the combinationof graphics software with data-base systems. Databases with geographical attributes are naturalcandidates for graphics queries.To test this belief, Phillips'3 has developed a

generalized high-level query language and is evalua-ting it with a data base representing 477 tractsand 25 oil leases off the coast of western Louisiana.Implemented on a subset of the Codasyl Data BaseTask Group specifications, the system can, forexample, generate a map of the tracts with the leasedtracts colored or shaded to distinguish their acreagesize.In another graphics/data-base development, Ber-

man and Stonebraker'4 have described an approachthat reduced software development costs by a factorof 30 to 40, at the expense of a performancedrop of 2 to 4. Their idea was to add a rathersmall "front end" to an existing data-base system,rather than to develop a special-purpose data-basesystem from the beginning. The front end is ageographic information retrieval and display systemthey call GEO-QUEL. Consisting of 3000 lines of sourcecode, it took 0.4 man-year to develop. The data-basesystem is INGRES, a powerful relational system of16,000 lines of source code in which 15-20 man-yearshave been invested. In general, GEO-QUEL enables auser to manipulate map-type information stored inthe INGRES data-base system.The increasing demand for map data in machine-

readable form has induced the U.S. GeologicalSurvey to embark upon the creation of a digitalcartographic data base that will eventually encom-pass the entire United States. At present, Edsonand Lee'5 are considering ways of structuring thedata categories, such as state, county, and municipalboundaries, transportation networks, and drainagechannels. They believe the data base should be setup so other organizations can add their ownspecialized information to it without having to re-digitize the existing base.

Graphic techniques are being used to generateanother kind of map at the State University ofNew York at Buffalo, where a group has combined

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machine-readable incident and arrest records keptby the police with the geographic base file of theCensus Bureau to produce crime maps relatingcrime statistics to census-tract data.'6 The systemis being implemented on a CDC 173. When thenumerous variables have been fully analyzed andvisually mapped for easy understanding, the forcesof law and order will be better able to allocatetheir resources.While the use of computer graphics in architectural

design is not new, its application from conceptualdesign through the entire life cycle of the buildingis an intriguing idea. New layouts, building modi-fications, energy saving-all can be accomplishedby calling up appropriate portions of the plans andinteractively working on them, with the ownerhimself participating in the first stages of the process.Renfrow17 reports that the data base for this systemincludes three alphanumeric elements (architecturalcriteria, cost control, and management information)and one graphics element. The graphics programresides on a dedicated PDP-15 with 32K core; theother programs are either on an IBM 370/138, oraccessed via a service bureau.

Computing display relationships

An image is represented on a two-dimensionalplanar display by elements such as dots, straightlines, and curves of various intensities, shades ofgrey, or colors' These elements are straightforwardand, at the level of the display controller, theirmanipulation is simple. At the conceptual level wherethe programmer works, it is complex for 2D repre-sentations and very complex for 3D representations.

2D processing. In manipulating a two-dimensionalimage on the two-dimensional screen, it must bepossible to translate the picture from one screenposition to another, a process called panning; toscale the picture up for more detail or down for moreof the picture, a process called zooming; to rotatethe picture through an angle; or to combine (orconcatenate) several of these processes for greatercomputing efficiency. There is also the matter ofassuring that lines that are supposed to meet do,in fact, close on each other. Moreover, in the processof translating, scaling, and rotating an image, someof the lines may fall off-screen and have to be clipped.Finally, all of these processes have to be doneinteractively.

Difficult as these processes may seem to a refugeefrom freshman geometry, they have been largelymastered. Progress consists of finding more ingen-ious algorithms that take less computer time.Earnshaw'8 has developed a method of line generationfor incremental or raster devices that compressescode for repeated straight-line patterns more than40 percent and reduces processor usage between20 and 40 percent.Perhaps future students of descriptive geometry

will be better prepared for its mind-bending manipu-

1970 CENSUS ORTR

TOTAL POPULATION

FER RCRE

0.000 10.00040.00 70.00 100.00 129

Creating crime maps through graphics. Continuouslyshaded map of Buffalo, showing population densityin census tracts, demonstrates the power of the CrimeMapping System developed by the Geographic lnfor-mation Systems Laboratory at the State University ofNew York at Buffalo. Statistical unit attributes, collect-ed from crime and census data, can be mapped onto a

spatial file containing census tracts. This mappingroutine supplies title and legend, shading symbolmechanism, class-interval algorithm, and output param-

eters.

12 CRIME CONSTRUCTS"White Collar" CrimesGamblingBribery & ExtortionSolicting & LoiteringHarassmentAssault & HomicideDriving ViolationsNarcoticsDangerous WeaponsBurglary & LarcenyArmed Robberv"Petty" Theft

CRIME IN BUFFRLO: 1971-1973OFFENSES PER 100000 POPULRTION

832.9 624.7 416.4 208.2 -0.0

The Buffalo Crime Mapping program is capable of retriev-ing a set of attributes from the statistical units andrepresenting them on an outline map. In this case, crimedensity is represented by graduated circles on censustracts. Program options include circle size and density,map lines (full or dashed), map units, titles, and legends.This varied array of automated representation techniquesis intended to make public data more usable by deci-sion-making officials. (Courtesy K. Brassel and J. Utano,SUNY Buffalo. Reprinted from Computer Graphics, Proc.4th Annual Conf. on Computer Graphics and InteractiveTechniques. © 1977 by the Association for ComputingMachinery.)

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lations if Hefez'9 and his co-authors at the Universityof Haifa in Israel have their way. Their TRANS pro-gram is addressed not to college freshmen but to8- to 10-year-olds. Operating on a PDP 11/40 withthe GT40 graphics system (including a light pen), itprovides drill on the translation, rotation, andreflection about an axis of lines or plane figures.3D processing. The same processes of translation,

scaling, etc., are carried out for images in threedimensions as were required of those in twodimensions. However, not only are the mathematicaloperations more voluminous, but additional processesare needed. It must be possible to construct athree-dimensional picture in perspective from two-dimensional coordinates, selecting the viewpoint,the disappearing point of the lines, and sometimesthe source of the viewing light. To make the imagerealistic, depth cues can be added; these includethe elimination of hidden lines and surfaces, theaddition of texture gradients or shading, and theuse of shadows and highlights.

Surface Representation. At the present time,interactive modification of three-dimensional surfaces-a capability of great potential value in biologicalresearch, medical diagnosis and therapy, architecture,and automobile and ship design-cannot be easilyaccomplished with existing hardware and software.Two approaches to this problem were presented byWu, Abel, and Greenberg,20 and Fuchs, Kedem,and Uselton21 in papers that were chosen by theconference program committee for publication inCACM.In the first paper, the approach was to extend

the Bezier curve method and combine it with splinetheory. In effect, sectional curves are representedby uniform B-splines and the surface interpolatedbetween sections by Cardinal splines. The result isthe ability to rapidly generate three-dimensionalsurfaces of arbitrary shapes.In the second paper Fuchs et al presented a

general solution to the problem of developing anautomatic procedure for constructing a surface,given a number of parallel plane intersections withthe surface. Their procedure provides a formula forcalculating the upper bound of the number of stepsrequired in the computation. They foresee theirtechnique being used in a number of applications:

* reconstruction of three-dimensional micro-scopic structures from two-dimensional images;

* simulation of the likely results of recon-structive surgery, starting with the pre-surgerysurface;

* calculating the volume of the organs withinthe human body for diagnostic use, basedupon computerized axial tomographic data(reconstructed cross-sectional X-rays);

* simulating low-level flying by constructingterrain surfaces from topographic maps inpilot-training simulators;

* approximating geographic surfaces fromlimited reconnaissance data;

* automatically constructing surfaces in inter-active design-for example, automobile bodies.

Hidden surfaces. Several authors at the conferencepresented new approaches to hidden-surface elimina-tion. Hamlin and Gear22 showed that two newalgorithms they had developed reduced the number,of multiplications involved in depth calculations.Weiler and Atherton23 described a hidden-surfaceremoval algorithm that recursively subdivides theimage into polygon-shaped windows until a windowis reached for which the depth order of all surfaceswithin the window is'known. The algorithm makesit possible to use several techniques to reducecomputation time, as compared to previous algor-ithms. Tanimoto24 contended that visible surfacealgorithms would be more efficient if they madeuse of information about the structure of the envir-onment in which the picture is located. (For example,the interior of the bathroom is never visible fromthe living room.) He also considered constraints onviewpoint locations and the matter of coherencebetween successive views in a sequence. His improvedmethod of surface editing is based on an applicationof graph theory.

Shadows. Of the three classifications of shadowalgorithms, Crow25 found that inclusion of shadowvolumes in the object data had more appealingcharacteristics than the other two-shadow compu-tation during scanout and the division of objectsurfaces into shadowed and unshadowed areas priorto the removal of hidden surfaces.

Light reflection. Once visibility of, a surface hasbeen established, the next stage of computation isthe determination of the intensity-or degree of lightreflection-of the surface. Blinn26 carried the tech-nique beyond the perfect-diffuser surface modelby simulating highlights based on experimentalmeasurements of how light reflects from realsurfaces, metallic and non-metallic. Many of theeffects are so subtle, he found, as to be apparentonly during movie sequences.

3D sculpting. Most of these techniques cometogether in the system for sculpting 3D data devisedby Parent.27 A PDP-11/45 with 96K 16-bit words andseveral disks was actually made to function-to adegree-as a sculptor's studio, displaying the resultthrough a Vector General display with 4096 x 4096addressable points. Keyboard inputs of coordinatesor other numbers were considered to be an unnaturalmeans of expression for an artist. Therefore, toolssuch as scaling, rotating, translating, rubber-banding,joining, or intersecting are controlled by analogdevices, such as dials, giving the sculptor a feel forwhat he is doing. In a short film, Parent showed howthese tools were used to create and color a whale foran animation sequence.

Software for interactive graphics

The user of an interactive graphics system needsa means to make his commands known to it. One

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means is the subroutine library, programmed in thelanguage of the host computer.2" At present thislanguage is oft,n Fortran. When the user wishes toaccomplish a graphics objective, he calls in thecorresponding subroutine, using the host language.The other means is the command language. Here asingle command, usually mnemonically related toits purpose, sets in motion a series of program stepsto accomplish the user's objective.At the next level up, so to speak, the programmer

of an interactive graphics system needs a procedural,or a programming, language in which to implementthe foregoing types of commands. Again, thislanguage has often been Fortran, in spite of thefact-according to Newman and Sproull-that itsperformance is "abysmal." More effective high-levelprogramming languages specifically adapted to thegraphics task are still limited mainly to universityor industry research laboratories.

Graphic standards. At the subroutine-library endof the software range, the Graphic Standards

Vector General's 3400 series graphics display system isa high-end example of a vector-based unit. Stroke writersmove the electron beam to draw lines, curves, and char-acters of excellent quality at very high speed. The beamis controlled by analog deflection circuitry that is directed,in turn, by digital commands from a display file. Theprinciple permits almost immediate response to inter-active commands, but the analog circuitry drives up theprice, relative to recent units based on the raster-scanprinciple. The 3400 contains a microprogrammableprocessor that obtains instructions and data from thehost computer, then continues operations under micro-code control without intervention from the host until thehost makes further computations. A feature of the systemis its 3D capability. The Z-axis of the vector generatoris capable of modulating intensity and deflection signalsto simulate depth cues. Using 21 or 22-inch monitors,the 3400 can address 4096 x 4096 x 4096 locations andcreate up to 20,000 lines of 0.1 inch. The 3400 seriesat the top of the Vector General line is in the $50,000price range for a complete system; other models fall inthe $25,000 to $50,000 range. In general, the qualityand capability of vector-type displays make them acommon choice in complex interactive design.

Planning Committee of ACM/SIGGRAPH presentedthe results of its first year's work to the conferencein a morning-long report and discussion, backed upby a 136-page special issue of Computer Graphics.'The eight systems selected by the committee for

the state-of-the-art survey are described briefly in

Using graphics to simulate strength of materials. This cylindricallyshaped object represents a structure composed of different materials,represented by colors. It is not a real object. From a set of simulationequations, the effect of applied forces is computed, then output to aDicomed 48 slide maker. Thus, without building a physical model, theeffect of various phenomena can be simulated analytically.

January 1978 65

Table 1. The purpose of this survey was to collectinformation and develop criteria providing a frame-work within which the proposed graphic standard

could be formulated. This led to a draft of whatthe committee termed the "core graphics system."They intended it to be "a rich, essentially stand-alone subroutine package for most (simple) applica-tion programs on modern line-drawing plotters anddisplays." They say it should be "usable from suchhigh-level application programming languages asFortran or PL/fI." They urge that the user interface bedevice-, machine-, and operating system-independent.One of these graphic software packages29 is the

General Purpose Graphic System, a Fortran sub-routine package that is independent of input devices,output devices, and the host computer. In conse-quence, both programs and application programmersare transportable from system to system. Some ofits designers, as members of the Graphic StandardsPlanning Committee, had a significant influence onthe formulation of the proposed "core system."In a final 38-page section, the committee obliginglylisted several hundred important issues that arosein the course of preparing the draft standard. Inthe more significant instances they summarize thepros and cons. In every case the resolution of theissue, as the committee saw it, is presented.

Table 1. Typical graphic software packages.

ADAGE Supported by the IMAGE Fortran extension and GRAFX utilitysystem; fully interactive: more than 50 installations onADAGE graphics terminals: cost included in hardware.

CalComp Exhibits a large degree of host-computer and output-device independence: several thousand installations; $500to $1 500.

DISSPLA A complete set of graphing routines providing extensiveuser control for the presentation of graphical images; about100 installations: $22,000.

GCS Graphics Compatibility System includes graphs, segmentcontrol, 2D and 3D plotting: 118 installations; no charge.

GINO-F A set of routines that have evolved into host computer- anddevice-independence: widely used in Great Britain: 120installations: $15,000.

GPGS General Purpose Graphic System is a structured and well-documented package that exists in both Fortran and host-dependent versions: graphics standard in Norway: morethan 40 installations: $800.

1G Integrated Graphics system (University of Michigan)contains recent concepts: host-dependent and largelydevice-independent: uses non-ANSI Fortran to achieveconvenient user interface: eight installations: no charge.

Tektronix Terminal Control System and Advanced Graphing II havemore than 200 installations: a graphing and plottingsystem that is largely host-independent: $775 each.

Using graphics to simulate physical events that are otherwise invisibleto the naked eye. Above: a graphic simulation of the shape and structureof the magnetic elements that may enclose a nuclear fusion reaction.The shapes are computer-generated and output to a unit that producescolored slides. Through these techniques design studies can beadvanced to a sophisticated level before it is necessary to build anactual prototype. (Courtesy Lawrence Livermore Laboratory.)

Further debate on the proposal by the interactivegraphics community is expected. Meantime, thedraft is available to study groups on computergraphics recently appointed by the InternationalStandards Organization and the American NationalStandards Institute.An effective graphics standard will underpin

further growth of the simpler applications. Userscould more easily move from one system to another.

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Three-dimensional drawing is typical of capabilities ofgraphics software packages. This one was produced byDisspla, a $22,000 system of Fortran routines from Inte-grated Software System Corp., San Diego. Computer-independent, it has been interfaced to IBM, CDC, Univac,DEC, and other systems and to a number of plotters,microfilm recorders, and CRT displays.

The subroutines themselves, to the degree that theybecome device- and host-independent, will be morewidely usable. Even the applications programmershould become able to move with minimal retraining.An example of a subroutine in Fortran-actually

PFORT, a portable subset-is reported by Wright ofthe National Center for Atmospheric Research.30At the center, large computers such as the Cray-Iand CDC 7600 drive different plotters and micro-film recorders, including units from Tektronix,Calcomp, and Information International. The systemplot package outputs a device-independent codewhich must be translated into the particular codeaccepted by each device. Wright devised such atranslator, which runs on the large computersrather than the output devices. It is capable notonly of driving the simplest devices, but also ofaccepting instructions for the most sophisticatedhardware features.

Command languages. For the user who is notprepared to operate in Fortran calls, such as the 3Dsculptor mentioned earlier, a command languageprovides a more natural way of interacting withthe image. Commands may be given by means of thelight pen or other input devices; they may be selectedfrom a menu displayed at the edge of the screen;they may be entered from special-purpose functionkeys; or they may be typed in from the keyboard.According to Newman and Sproull, a commandgenerally contains a verb, defining the process; itmay specify data; and it may delimit the verb ordata. Examples of command languages includeSutherland's SKETCHPAD3' and Rand Corporation'sBIOMOD."2

Two views of an operating fountain in the fore-ground of a temple illustrate the animation systembeing developed by the Cornell Program of ComputerGraphics for eventual use in a production studioenvironment.36 The system is a computer version of themultiplane animation technique developed by WaltDisney in 1936 to provide depth perspective for hisfeature film Snow White, and used sparingly sincethen because of its great expense. To produce thetemple scene, a three-dimensional simulation of thefountain was worked out, complete with camera anglesand motion dynamics. Then the resulting fountain se-quence was back-projected as a continuum of two-dimensional images on the background plane of a multi-plane environment. Meanwhile, an artist had developeda two-dimensional rendering of an imaginary templeand colored it in a nighttime array of blues and deeppurples. The artist worked with computer-generatedanalogs of the traditional drawing implements, suchas pens, brushes, and washes. Next, the computercombined the two planes, preserving the interactingperspectives. Then, without the necessity of repaintingthe basic figures, a color palette was specified to thecomputer in order to depict the temple in warmmorning colors. In fact, an entire sequence of palettescould be used to show the play of light and colorin the hall as the sun rises. Finally, this sequencebecomes key frames from which an entire animatedsequence of subtle chromatic shifts can be produced.In key frame animation, the artist prepares images atintervals in a sequence, sketching them on a graphictablet. The interaction between the artist and theanimation is pictorial. Since programmed commandsare not needed, use of the system seems more naturalto non-programmers. The computer is programmedto fill in additional frames by means of linear inter-polation between successive pairs of key frames.

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Programming languages. Most graphic systemsto date have been programmed in a general-purposelanguage such as Basic or Fortran because theselanguages were already there and many graphicsubroutines were available in them. After a survey,Newman and Sproull concluded that these languagesdo not effectively meet the special needs of graphicsprograms. They decry the lack of interest in thedevelopment of languages designed to meet theseneeds, considering this lack to be a major obstacleto wider use of graphics.This need perhaps explains NSF's support of the

development of the Graphical Language for Inter-active Design (GLIDE) at Carnegie-Mellon Urniversity)._This high-level language combines both data-basefeatures and operations used in the design of physicalsystems, such as machines, buildings, or ships.

At 2.825 x 10-8 second after a simulated explosion atthe left of this picture, the material being impacted iscalculated to have deformed as shown by the displacedand twisted grid lines. This frame is one of a series thatmay be viewed as a moving picture of the explosioneffect. Note that this is not a high-speed film of an actualexplosion-it is a computer-generated simulation. Froma set of equations representing the force of an explosionand the opposing strength of a material, the grid displace-ments at a series of points in nanosecond time arecalculated and output to a Dicomed 48. This unit acceptsdata in computer format and processes it into lightdirected toward a photographic film. For colored slidesthe film is exposed three times. The slides are developedby standard photographic processing. (Courtesy LawrenceLivermore Laboratory.)

The initial application areas involve the design ofbuildings and chemical process plants. (G.IDE iSusable both as an interpreted command languagefor interactive design and as a procedural program-ming language.

Applications

The applications of interactive computer graphics- cover a variegated spectrum that is not easy to

characterize in a simple way. One dimension is thecomplexity of the equipment and systems employed,ranging from the hundreds of dollars a computerhobbyist invests to play TV games, to the million-dollar systems capable of generating animated

This series of connected points represents in three dimensions the educational movies. A second dimension is what thecrystal structure of calcite. The structure is created in space within a current installations seem to be doing, believed tolength of transparent plastic shaped into a cylinder. The integral holo- be mostly business statistics presentations andgraph is reproduced on the plastic. As the plastic rotates, the image computer-aided design. The more dramatic applica-seems to move. The color variation-from red at the top to blue at thebottom-is a distortion effect, not a property of the crystal. The integral cur y beg p Yholograph was made from a series of two-dimensional images that were one installation and a few researchers.computer-generated by Stephen R. Levine and his associates at the Table 2 represents a third dimension: a classifica-Lawrence Livermore Laboratory. tion of several scores of applications reported at this

68 COMPUTE R

Although the television monitor itself has long beencost-competitive, the decreasing prices of integratedcircuits are reducing the cost of refresh memory andlogic for this type of display. When memory was morecostly, it made economic sense to store commands inmemory and use them to activate special-purposecircuitry to generate alphanumeric characters and imageprimitives, such as a line or curve. With the falling costsof memory, it has become feasible to map the pictureelements (pixels) to be displayed on the screen intocorresponding bit positions in a local memory. The bit-states modulate the intensitv of the scanning electronbeam, transferring the image to the screen. This principlepermits the image to be panned or zoomed or to besplit up into several partial images at different degreesof blow-up. The CalComp IGT 100 interactive graphicsterminal, for example, incorporates a three-way split-screen technique that offers a graphics arei, view area,and alphanumeric message area. While a large drawingat relatively low resolution occupies most of the screen,a small section can be blown up in one of the split areasfor detailed manipulation. Under the control of a built-inmicrocomputer in stand-alone mode, the unit can gen-erate a grid, pan, zoom, and selectively erase and change,reducing host computer processing costs. The terminalis a raster-scan type, black and white, with 1024 x 680addressable points. The keyboard is built in, and an11 x 11-inch tablet is optional. Price Is approximately$14,700.

conference and elsewhere. Some of these applicationsbear further discussion.

Animation. The conference was enlivened bynumerous animated films-cartoons, TV-networkmoving logos, light-show patterns, and other imagin-ative works. Two evening sessions were devotedentirely to computer-generated films, some reminis-cent of the light show in the concluding minutesof 2001: A Space Odyssey. Recently, the millionsseeing Star Wars may not have appreciated thatthe Imperial Empire's dreaded Death Star, approach-ing from the far side of Yavin, was in reality

computer-generated by Larry Cuba and his assoc-iates. The computer-generated images in Star Warsare of relatively low resolution, at least as comparedto the millions of bits necessary to reproduce live-action photography. However, it is technicallypossible to generate more complex images. The NewYork Institute of Technology, for example, isproducing animated films of 1500 lines resolutionthat they consider "very good image quality."34Similarly, the Anima II system at Ohio StateUniversity outputs to a 4096 x 4096 Vector Generaldisplay.35

Table 2. Recent interactive computer graphicsapplications.

Animation

High resolution animated films (1500 lines)34Three-dimensional color animation35Multiplane-technique color animation36Real-time playback of frames compiled at slower rates37Key-frame technique of animation38Integral holography39Pilot training simulators40

Architecture

Structural design38Design of physical systems33Facilities management'7Walking through the architectural scene in simulation41

Art

Simulating an artist's studio in a graphic system27Teaching art on a graphic system42Survey of computer art43Examples of computer art44Creation of recognizable faces by interactive techniques45

Tektronix' newest series of computer display terminals, the 4020 series,is directed at the predominantly alphanumeric user who desires growthto graphics. The 4025 is the first terminal to allow scrolling of bothalphanumerics and graphics. The 4024 (not shown) and 4025 can copyup to 53 lines of alphanumerics and graphics with a Tektronix 4631copier, according to Tektronix. The 4025 Is shown above with a 4631hardcopy unit.The base price of the 4024 with full alphanumeric capability and the

interface to the 4631 hardcopy unit is $2995. Base price of the 4025 withthe hardcopy interface and combined graphic-alphanumeric capabilityis $3595.

January 1978

Con t'd onoverleaf

69

Data charting and plotting

Data analysis and report design46Editing illustrated documents47Page layout design48

Computer-Aided Design

Energy conservation in building design38Milling ship-hull models49McDonnell Douglas FASTDRAW 11 for mechanical drawing50AAA Technology and Specialties Company's TRIFLEX for piping layout50Highway design51Example of shipbuilding application52

Cartography

Planning landfill contours after strip mining53Generating mapping grids54Distributing and representing crime data on an urban map'6Displaying ocean data, such as depth, in pseudo color55Developing the digital cartographic data base for the U.S. Geological

Survey15Representing data on maps by various shading or contouring

techniques56-57Urban and environmental systems58Land use planning59

Medical

Stereotaxic brain surgery60Planning administrative areas and emergency medical service

locations6'Modeling experimental data in the neurosciences62

Picture quality comes down basically to a trade-offwith computer time. For example, one algorithmtakes 25 minutes on a PDP-11/45 to compute a

single picture. With 1800 video frames per minute,computing time for an image of this quality wouldbe 750 hours per minute of animation. Sinceanimation need not be created in real time, qualityis limited only by cost. Or, to turn the matteraround, cost may be held down to the qualitylevel required by a particular use.

Simulation. The foregoing represents the economicfactors guiding simulator training. Because thepicture that the trainee pilot sees must be updatedin real time, the computation load of a simulatorcould be immense. To reduce this load, updatingmay be reduced to 20 frames per second from thenormal 30 frames, with some sacrifice of quality.Also, according to Evans and Sutherland, traineepilots will accept a relatively simple image as suffi-ciently lifelike. A runway, for example, may berepresented as two lines bordering white concrete,with green for the adjoining grass, brown for some

rather straight-line shaped mountains on the horizon,and blue for the sky.

Integral holography. This new medium combinesthe reality effect of holography with motion pictureanimation techniques. When the cylinder-shapedintegral hologram on transparent plastic is rotatedslowly, a light image becomes visible in thin air inthe center of the cylindrical space and moves in threedimensions. Researchers at Lawrence LivermoreLaboratory39 have converted 14 computer-generatedmovies on such subjects as chemistry, laser fusion,magnetic fusion, crystallography, biomedicine, and

Digital computer animation. Left: Rebel pilots in Star Wars prepare for mis- T.J. O'Donnell, and Tom Chromicz in Tom Defanti's GRASSsion against the Empire's Death Star, shown looming on the briefing room language, were drawn on a Vector General display andscreen. Right: simulated view of the mission, in which star pilots must skim photographed on 35mm film, which was then rear-pro-down the trench to the target. The images, programmed by Larry Cuba, jected onto a screen during the live action filming of the movie.

Table 2cont'd

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electromagnetic fields into integral holograms.Molecules can be shown moving and combiningin three dimensions to illustrate a chemical reaction.The technique is still in its infancy.

Landfill. As the energy shortage turns the UnitedStates toward coal, a group at the University ofMassachusetts53 turned to the problem of re-formingthe excavation surface after the coal is removed.It is not a matter that can be satisfactorily handledin the field with a little bulldozing, because thenew landform should be both structurally andenvironmentally sound as well as aestheticallypleasing. To meet these needs, the group developeda prototype program, ECOSITE. This program usesa structure of interpenetrating cones that-bysuitable arrangement and skilled manipulation ofbreadth, height, slope, and other features-can bemade to represent a wide variety of landforms, bothnaturalistic and formalized. Working with theseforms, the designer can create a geometricallyshaped landform sculpture on an environmentalscale, or he can blend the forms into gently rollinghills or other natural-appearing topography.

Brain surgery. Stereotaxic surgery treats pain,seizures, and brain disorders by making a thera-peutic lesion with a radio-frequency probe in aprecisely located part of the brain.60 A small sectionof the brain is mapped by up to 50 successiveelectrical stimulations to an area of about 20 mm by4 mm, noting the responses of the conscious patient.

The results are entered into the graphics systempoint by point as they are taken. From the develop-ing map, the neurosurgeon notes whether the invokedresponses match those expected from the typicalbrain. Sometimes it is necessary to calibrate thepatient's brain by shifting the map a few millimetersbefore making the therapeutic lesion. This shift isan operation easily accomplished by using thegraphics system, but difficult to perform in theoperating room by using a paper chart.

Poised for takeoff?

Interactive computer graphics is needed, all right,and it has been needed for some time. It is neededbecause burgeoning science, technology, and busin-ess generate ever more voluminous data for humanbeings to deal with. It is needed because manipulablepatterns are one of the natural ways for humansto work with this plethora of information.Much of the technology that supports interactive

computer graphics is now in place. There are avariety of input devices, four or five output tech-nologies, and various arrangements of processingpower. Advancing semiconductor technology prom-ises to make these elements smaller, faster, andcheaper.Subroutine packages to drive the hardware are

numerous, and standardization of them is getttingunder way. With processing power becoming less

Testing the Market for aComputer-Generated Color-Copier

A Xerox product-planning group created a stir ofexcitement on the exhibit floor of the Fourth AnnualConference on Computer Graphics and InteractiveTechniques by making color copies on a Xerox 6500color copier from computer-generated input data.At the show the data was on a disk-as a matterof convenience-but it was in computer formatand could just as well have been generated directlyby a computer.

Computer data for each of the three basic colorswas re-formatted by a Xerox minicomputer tocopier line and page format. Then the three sets ofdigital values were converted to analog and appliedsuccessively to the copier's laser scan input. The6500 builds up a color copy in three scans. Copiesare produced at the rate of three per minute, orapproximately 200 full-color copies per hour.

The resulting 8 x 10-inch copy, made on plainpaper, was of good quality-100 lines per inchvertically and 200 dots per inch horizontally. Al-

though short of photographic quality, the copiesare adequate for many purposes and are muchcheaper than photos.

Exact cost is uncertain because the interfaceelements beyond the 6500 have not been pricedand because the number of copies that might beproduced by a user would vary. The copier itselfis about $25,000 and the additional elementsmight bring the price up to $35,000. If this costwere amortized over five years and if a user made7000 copies per year, the machine cost would beabout $1.00 per copy, plus operating costs. Colorphotos of this size cost about $9.00.

Xerox stressed that the unit shown was a "marketprobe," not an announced product. In the eventthat the company does decide to put such aproduct on the market, they will work closely withsystem houses, according to Richard E. Schneider,manager of product planning, Business and Pro-ducts Development Group at Xerox Square, Roch-ester, N.Y.

January 1978 71

expensive, there will be less necessity to optimizecomplex transformation algorithms. Similarly, withmemory becoming less expensive, higher resolutiondisplays become feasible.Some 5000 installations are numbered in Table 1,

just for the eight graphics software packagesstudied by the Graphic Standards Planning Com-mittee. The 36 applications listed in Table 2 suggestthe wide range of possibilities for future expansion.So it is tempting to conclude that the exponentialgrowth curve is about to take off. That seems tobe what the editors of Scientific American thought

This simulated molecule of transfer-RNA contains 1652 atoms of fourelements, represented by the circles of various sizes. It is one of a

series of slightly different frames which, when viewed in sequence,provide a three-dimensional moving picture effect. Determining thestructure of this organic molecule took a team at Duke Universityseven years. Then Stephen R. Levine and his associates at LawrenceLivermore Laboratory expanded a small computer program originallydeveloped by Ken Knowlton at Bell Laboratories to be capable ofhandling a structure of this magnitude. For example, this particularsequence required 41,000 vectors.

72

in 1970 when they subtitled Ivan Sutherland'sarticle2 "The art of using computers to makepictures on a screen is rapidly advancing." It seemsto have motivated Newman and Sproull in their1973 book.'° It also seems to have been in the mindsof the editors of Datamation in 1975 when theyproclaimed, "Interactive Graphics Comes Of Age."61And yet, takeoff has yet to occur.

It may be that putting together all this hardware,creating software for many uses, and persuadingpeople in different disciplines to make use of theresult-all of this may be a little more difficultthan the enthusiasts in the field hoped and dreamed.In reality, none of the display technologies is withoutdrawbacks. The general run of computers is notideally suited for graphic computations. Program-ming of parallel processors or array processors,which are better suited, is still a rare art-someclaim that programming theory in this area remainsto be worked out. Even on the familiar computers,availability of software appears to be a weak pointfor programs that do more than create a chart orplot. The spread of computer-aided design seems tobe hampered by the cost of developing softwarefor the well-nigh infinite variety of design proceduresfound in different companies, even in the sameindustry.

So, we're inclined to be a little cautious aboutthe slope of the future growth curve. It does seemthat there is a lot of work to do. f

Acknowledgments

Interviews with the following officers and membersof ACM/SIGGRAPH helped to place this field in per-spective:

S. H. Chasen, Lockheed Georgia Co.James George, Los Alamos Scientific Laboratory.Harvard Holmes, Lawrence Livermore Laboratory.Steve Levine, Lawrence Livermore Laboratory.Andries van Dam, Brown University.Robin Williams, IBM, San Jose, California.

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2. Ivan E. Sutherland, "Computer Displays," ScientificAmerican, June 1970, pp. 56-81.

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4. R. A. Frankland, ''Developments in Simplified DirectView Storage. Digest of International Symposium,Societv for Information Display, San Francisco, June1972, pp. 82-83.

5. C. Machover, '"Trends in Low-Cost Graphic DisplayTerminals, ''ibid., pp. 22-23.

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Il. Andries van Dam. George M. Stabler, and Richard J.Harrington, "Intelligent Satellites for InteractiveGraphics," Proc. IEEE. April 1974, pp. 483-492.

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13. Richard L. I'hillips, "A Query l,anguage for a NetworkData Base with Graphical Entities,'' ComputerGrapliics, Vol. 11, No. 2, Summer 1977, pp. 179-185.

14. Richard It. 13erman and Michael Stonebraker, 'GED-QUEI.: A System for the Manipulation and Displayof Geographic D)ata,'' ibid., pp. 186-191.

Io. Dean 'T'. Edson and George Y. G. Lee. "Ways ofStructuring Data within a Digital Cartographic DataBase.'' ibid.. pp. 148-157.

16. Kurt rE. Brassel. Jack ,J. Utano, and P'errv (. HansonIll. "'I'he 13uffalo Crime MNapping System: A DesignStrategy for the Display and Analysis of SpatiallyReferenced Crime Data,'" ibid., pp. 78-85.

17. Nancy 'V. lIenfrow, ''Computer Graphics for FacilitiesManagement." ibid.. pp. 42-47.

18. It. A. I.arnshaw, "Line Generation for Incrementaland Raster Devices,' ibid., pp. 199-205.

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21. 11. Fuchls, . )J. Kedem and S. P. Uselton, "OptimalSurface Reconstruction from planar Contours,'' ibid.,pp. 11-24.

22. Griffith Hlamlin .Ir. and C. WAm. Gear, ''Itaster-ScanIlidden Surface Algorithmii 'I'echniques,'' C'ompoterGraphics, Vol. I1l. No. 2, Summiier 1977, pp. 206-213.

23. Kevin Weiler and Peter Atherton, '"llidden SurfaceRemoval Using Plolygon Area Sorting," ibid., pp.2 14-222.

24. Steven L. 'I'animoto, ''A Graph-T'heoretic Real-TimeVisible Surface Fditing Technique,' ibid., pp. 223-228.

25. Franklin C. Crow, "Shadow Algorithms for ComputerGraphics." ibi(L. pp. 242-248.

Reader Service Number 4January 1978

26. James F. Blinn, "Models of Light Reflection for 48. P. B. Denes, "A Scan-Type Graphics System ForComputer Synthesized Pictures," ibid., pp. 192-198. Interactive Computing," ibid., pp. 21-24.

27. Richard E. Parent, "A System for Sculpting 3-DData," ibid., pp. 138-147.

28. A. P. Lucido, "Software Systems for ComputerGraphics," Computer, August 1976, pp. 23-32.

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30. Thomas Wright, "Machine-Independent MetacodeTranslation," ibid., pp. 128-130.

31. Ivan E. Sutherland, "SKETCHPAD: A Man-MachineGraphical Communication System," AFIPS ConfProc., Vol. 22, 1963, SJCC, p. 329 (MIT LincolnLab, Cambridge, MA, TR 296, May 1965).

32. G. F. Groner, R. L. Clark, R. A. Berman, and E. C.Deland, "BIOMOD: An Interactive Graphics Systemfor Modeling," AFIPS Conf. Proc., Vol. 39, 1971FJCC, pp. 369-378.

33. Charles Eastman and Max Henrion, "GLIDE: ALanguage For Design Information Systems," Com-puter Graphics Vol. 11, No. 2, Summer 1977, pp.24-33.

34. Edwin E. Catmull, "High Quality Computer Anima-tion," ibid., p. 37.

35. Ronald J. Hackathorn, "Anima II: A 3-D ColorAnimation System," ibid., pp. 54-64.

36. Marc Levoy, "A Color Animation System Based OnThe Multiplane Technique," ibid., pp. 65-7 1.

37. M. J. Potel, "Real-Time Playback In AnimationSystems," ibid., pp. 72-77.

38. Donald P. Greenberg, "An Interdisciplinary LaboratoryFor Graphics Research and Applications," ibid.,pp. 90-97.

39. D. L. Vickers, G. S. Smith, S. R. Levine, and L. G.Cross, "Moving, Computer-Generated Images ViaIntegral Holography," ibid., p. 120.

40. John E. Warnock, "Dynamic Modeling," ibid., p. 176.

41. Donald P. Greenberg, "Computer Graphics in Archi-tecture, " Scientific American, June 1974, pp. 98-106.

42. Grace C. Hertlein, "Computer Art For ComputerPeople-A Syllabus," Computer Graphics, Vol. 11,No. 2, Summer 1977, pp. 249-254.

43. Charles Csuri, "Computer Graphics and Art," Proc.IEEE, April 1974, pp. 503-515.

44. R. D. Resch, "Portfolio of Shaded Computer Images,"ibid., pp. 496-502.

45. Michael L. Rhodes, "Text Input, Graphics, and DataStructures," Proc. Conf: Computer Graphics, PatternRecognition, and Data Structure, May 1975, pp. 31-37.

46. Wm. H. Benson and Bernard Ketous, "InteractiveAnalysis and Display of Tabular Data," ComputerGraphics, Vol. 11, No. 2, Summer 1977, pp. 48-53.

47. B. A. Laws, "A Gray-Scale Graphic Processor UsingRun-Length Encoding," Proc. Conf Computer Graph-ics, Pattern Recognition, and Data Structure, May1975, pp. 7-10.

49. Steven G. Satterfield, Francisco Rodriguez andDavid F. Rogers, "A Simple Approach to ComputerAided Milling with Interactive Graphics," ComputerGraphics, Vol. 11, No. 2, Summer 1977, pp. 107-111.

50. W. Barkley Fritz and Charles R. Lansberry, "ShipModeling With Interactive Graphics," Datamation,Dec. 1975, pp. 54-58.

51. T. J. Moffett, "Building Highway Systems WithComputer Graphic Simulations," Proc. IEEE, April1974, pp. 429-436.

52. Chuan Lee, "A Graphics Control Algorithm forComputer-Aided Design," Proc. Conf ComputerGraphics, Pattern Recognition, and Data Structure,May 1975, pp. 147-152.

53. Robert Mallary and Michael Ferraro, "ECOSITE:An Application of Computer-Aided Design to theComposition of Landforms For Reclamation," Com-puter Graphics, Vol. 11, No. 2, Summer 1977, pp.1-7.

54. Fred T. Tracy, "Graphical Pre- and Post-Processorfor 2-Dimensional Finite Element Method Programs,"ibid., pp. 8-12.

55. Lawrence E. McCleary, "Techniques For The Displayof Ocean Data on a Raster-Driven Color CRT,"ibid., pp. 98-101.

56. Geoffrey Dutton, "An Extensible Approach to Imageryof Gridded Data," ibid., pp. 159-169.

57. C. M. Gold, T. D. Charters, and J. Ramsden,"Automated Contour Mapping Using TriangularElement Data Structures and an Interpolant OverEach Irregular Triangular Domain," ibid., pp. 170-175.

58. R. L. Phillips, "Computer Graphics in Urban andEnvironmental Systems," Proc. IEEE, April 1974,pp. 437-452.

59. Andrew B. Whinston and Jon Furdek, "ComputerGraphics in Land Use Planning: A PreliminaryPlanning System," Proc. Conf Computer Graphics,Pattern Recognition, and Data Structure, May 1975,pp. 244-247.

60. P. A. Hawrylyshyn, R. R. Tasker, and L. W. Organ,"CASS: Computer-Assisted Stereotaxic Surgery,"Computer Graphics Vol. 11, No. 2, Summer 1977,pp. 13-17.

61. Christopher Willard and Jay R. Hamann, "InteractiveGraphics: An Aid To Health Planning and DecisionMaking," ibid., pp. 38-41.

62. R. J. Sclabassi, R. Buchness, and T. Estrin, "Inter-active Graphics in the Analysis of Neuronal SpikeTrain Data," Proc. Conf. Computer Graphics, PatternRecognition, and Data Structure, May 1975, pp. 47-49.

63. Eric Teicholz, "Interactive Graphics Comes Of Age,"Datamation, Dec. 1975, pp. 50-53.

64. Udo W. Pooch, "Computer Graphics, InteractiveTechniques, and Image Processing 1970-1975: ABibliography," Computer, August 1976, pp. 46-64.

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