edited by: John W. Moore and Ron Collins
Leonard J. Soltzbera Simmons College Boston. MA 021 15 -
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This article is intended to survey the current scene in computer graphics from the point of view of a chemistrv educatbr. ~ 2 hardware h and software for producing computer maohics are considered in the context of selected applications in the teaching of chemistry. This article will attempt to give a feeling for the scope of current applications of computer graphics in chemical education and to give sufficient information about hardware and software systems to promote communication with vendors of computer graphics equipment. A comprehensive coverage of vendors and equipment is not intended; with graphics equipment, as with computers, strong local support in a particular geographical area may favor one manufacturer over another. Particular vendors. ( l w ~ w i:lnd . p r w i ure mentioned only to put inu) perspecti5.e the reneri~ltlwrnes which this article de\,elor)s.' H'e shall rcstrict our attention to interactive devices, since output only araphics' e u u i ~ m e n tassociated with a batch-processinp computer is not usually under the direct co&rol of the chemistry instructor. Large systems ($20K and up) . graphics . are beyond the scope encompassed here. Background Joseph Weizenbaum (1)has pointed out that certain devices given us by technology-the microscope and the telescope. for examole-have led t o profound chanaes in mankind's view of himself and the unkerse. These devices have been like special windows which have enlarged man's view of the world &und him. The computer, working through special graphics inputloutputequipment, is likely to become another such window; in this case, however, the enhanced view afforded is an inward one. Through the medium of computer graphics, the computer can give pictorial form to abstract formulations of the human intellect, allowing us to look visuallv inward into our own minds. The use of graphic representation of computer output is not new. As earlv as 1951. the Whirlwind I computer a t the Massachusetts institute bf Technology made ;se of a cathoderay-tube as a graphicoutput device. Readers whose computing roots go hack to the early 1960's may recall the friendly "clack-buzz-buzz-clack" of the small CalComp drum plotters which were even then part of many computer systems; this noise, for some, was all that stood between consciousness and a sort of delirium induced by the drone of the disk drives and a multitude of cooling fans. The rapid technological evolution which has brought us time-shared, interactive computing, pocket calculators, video games, and home computers is having its effect on computer graphics. While most early devices were designed simply to Comprehensivedescriptions and specificationsof awide variety of graphics equipment are available from DATAPRO Research Corporation, 1805 Underwood Blvd., Delran, NJ 08075. The document is called "All About Graphics Display Devices" and costs 512.
- Some d t h ~ r appli~itticm.i e are bxwd im unpd~lixhed work of the aurhur and his wllra.rue, and %rudenrsnr Simmons ('ollere. Pmfesson .lnmci Plper, kllward l'renowir?, \Is. 1)omthyBell and hlr. Beth Lapin deserve special mention. 644 1 Journal of Chemical Education
plot output from a batch-processing computer system, modern devices support two-wav interaction between .graphics . . .graphic . .
the user and the c&puter. T h ~ capability s has stimulated n wide variety. of maphie applications in mans fields, includiny - . .. education. In spite of this progress, computer graphics as an instructional tool has not yet come of age. Developments in the hardware area are still coming rapidly. It appears that we are poised on the verge of anew eraof computing in which the Dowers of the comouter. . . through miniaturization and throueh graphics, will find pervasive application in education and other areas of human endeavor. Reference (2) eives a preview of the improved graphics capabilities which ma; be in store.
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Vignettes of Graphics Applications In order to establish a context for our discussion of devices and techniaues for usine comnuter .. eranhics. let us examine . a sperrrum of current yraphics appliwtioos in chemical educntlm. Each of the idlowine items is a collaee of the u,ork of various individuals. The reierences given in this section illustrate types of instructional uses of graphics and are not intended to correspond exactly to the applications described in the text.2 Vignette 1. Using a standard printing terminal, Professor P.'s organic analysis students explore the relationship between the appearance of an nmr spectrum and the values of chemical shifts and cou~linaconstants. The students input values for the shifts and coupling constants and the c ~ r r & ~ o n dnmr i n ~line pattern is generated on the printing terminal ( 3 , 4 ) .This application uses only the simplest hardware and a BASIC program without any frills, but it gives the students a genuine interactive experience which is so important to effective learning. Vignette 2. Professor C. wants her students in general chemistry to achieve an intuitive appreciation of atomic and molecular orbitals and their rel&nship to chemical behavior. She cannot, however, find just the right illustrations to go with her lectures. Using a digital plotter, she prepares high-quality hard-copy illustrations of various orbitals, using contours ( 5 ) , boundary surfaces, and electron dot-density diagrams (6). She is able to select just the scaling and other characteristics that she needs to suit her teaching approach. Some of the illustrations will be used for overhead projection, while others will he reproduced and distributed to students. This application is non-interactive, at least for the students, hut the students benefit from the custom-tailored graphics keyed to the course development. Dr. C. uses a package of FORTRAN subroutines purchases from a commercial vendor to create the graphics. Vignette 3. Professor R. stresses to his physical chemistry class that a theoretician experiments on mathematical models in much the same way that an experimentalist deals with actual materials. To give the students hands-on experience in testing models, Dr. R. has prepared a program which allows a student
to specify a particular equation of state as a model for gas behavior. On an inexpensive storage-tube terminal, the student produces P-versus-V plots for various gases under avariety of conditions and compares how well various equations of state (van der Waals, Dieterici, etc.) model deviations from ideal eas behavior. While R.'s students e m.~ l . o va remote time-sharing computer for this work, students a t another institution do similar experiments using a self-contained microcomputer which incorporates a graphic display (7). The oroarams are written in BASIC and make use of special BASIC graphic support routines.
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Vignette 4. In advanced oreanic chemistrv. Professor M. is leadima- her class through a rigorous examination ol'the effect of suhstituents on molecular conformation. I h . M. h a made a collection 111interesting structurw from the literature and h~tqstored the atomic (:oordinates in computer files. tising a high resolution refreshed graphics terminal rapahlr of animation, the students can examine these mdecula in detail by rotating them on the screen. Interactive nmtrol of the molecular orientation is provided hy a joystick, which the students use to manipulate the molecular imaees ( 8 , .The nroerams to suonurt this aci f years by M herself and, tivity were developed over a being in many ways unique to the hardware environment, may not run anywhere else. Vignette 5. A t the University of M., there is an emphasis on instrumental methods of analysis. One especially interesting experiment involves using visible spectroscopy for multicom~ o n e nanalysis t of transition metal ions in solution. Part of ;he experimbnt is tosee how the accuracy uf the analysisdepen& on huu many data points are sampled. To facilitate t he dieitizntionof thevisit~lesoectra.~ flat-bed dizitrrl with .. dotter . graphic input is unrd. The student places a spectrum on the hrd of the dotterand transmits thecw~rdinawsot'thenelected points to icomputer using the plotter's graphic input facility. Various numbers of ooints and different spectral rezions can be examined in short-order this way. The p&rams themselves were written by students. Vignette 6. The chemistry department at C. College is fortunate to have access to the PI.A'l'O system, a comprehensive hardwarel software syaem designed specifically for graphics-oriented instructionill applirations. Several courses in the chemistry curriculum make use of the suecial rhsma- ane el PI .AT0 terminal with its touch-panel graphic input facility for interactive. .animated instruction. The applications include .pre-lab instructions, dril! in organic nomenclature and synthesis and animated examples of reaction mechanisms (9.10). Some of the programs hate been purchased and others have been mitten by department faculty members using the special TUTOR language which is part of the PLAT0 system. The forepoing applications draw on a diverse menu of hardware and s;fti&e. Let us examine the spectrum of currently available graphics devices and types of software, keeping.in mind the demands of various instructional applications. Printing Devices It is possible to produce serviceable graphics with the most modest inputloutput equipment. A standard printing terminal, such as the Digital Equipment Corporation DECWriter or eauivalent device, can be used in such a way that the output infoimation is represented by the position rather than the identity of the printed characters. (See Vignette 1.) The cun.;triints on the use uf such equipment for graphic output are severe. Since printing terminals are designed to conform to a uniform character spacing and line spacing, there are many places on the paper which are not accessible to the printing head. Consequently, such devices cannot draw con-
tinuous curves or lines. The horizontal resolution of a standard minter is about 0:08 in. and the vertical resolution is about r~~~~~~~ 0.17 in. At this level of resolution, an attempt to depict a small circle will oroduce a "iaeeed form. and diaeonals will eenerallv look like i;regular steps. Since these terminals print one row of characters at a time, advancing the paper upward after each row, the programmer must organize the graphic output into a row-bv-row format which may be inconvenient. here are printing terminals, such as those based on the Diablo printing mechanism, which offer finer resolution (0.02 in. horizontal and 0.02 in. vertical). These terminals, which cost between $2600 and $3200, also allow bidirectional movement of both the printing head and the paper, so that the programmer is not constrained to work with a characterby-character,row-by-row organization of the data. Such terminals can produce satisfactory looking graphics and have the idvantaee that thev double as ordinarv ~ r i n t i n eterminals . ~-~ quality. They do not draw continuous with superior lines. naturallv. .. and thev are slow in vlottinr!- because of the necessity for many movements of the printing head and Daner. .. Just as a computer has no built-in direction uf its own and, without a program, flaphim devices th~.mselves thus, is are of lirtle use withuut appropriate software. "Appropriate software," in the caw of graphics, means nut only applications programs hut a h some sort of program or program segment to n~nverrurdinary nurncrical values inw a format which can he uresented manhicallv . ,bv-the -maohics . device. For examole. in using a standard printing terminal as a graphic output device for (x, y) data points, one would have to scale the data to correspond, say to seventy-two print positions along the width of the paper and sixty line positions along the paper length in order to fit the whole plot on one page. The programming techniaues for accomolishin~this task have been discussed in this2ournal (ll).; ~ r o ~ r & nmaking s use of teleprinter graphics are available fiom several sources (12,13, 14). For more specialized graphics devices, a "software package" of Dromams or routinks needed to convert numerical data into ihe&wcial format required hy the graphics device should be sought from the manufacturer. If nosuch support package is available. the woul&t)e graphics user must count un a very substantial investment n i time in prepnring the necessary software support for the graphics equipment. ~
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Screqn-Oriented Graphics Devices Discussion of computer graphics usually conjures images of devices with a screen. Screen-oriented devices can offer high resolution, high plotting speed, and considerable flexibility in applications, though all of these desiderata are not realized in every commercially available device. The disadvantage of the screen terminal is the lack of hard-copy, though hard-copy can be had by adding a relatively expensive accessory. I t sllould be noted that not all screen terminals are graphics devices. There are many such terminals in the $800-2500 price ranee which are strictlv alnhanumeric devices and others which have only limited graphics capabilities. There are. a t nresent. four ~ r i n c i ~t awle s of screen-oriented graphics terkjnals. Three 0;thesearei;ased on the cathoderay-tube (CRT) as the output medium. The fourth employs a special screen, tbe plasma-panel. Future developments in this area mav include liauid crvstal-based displays . . or screens which empciy cathodol;mine&ence. The storage-tubegraphics terminal, typified by the Tektronix 4006,4010 family, and large screen 4014 and 4016, is based on a device called a bistahle storage cathode-ray-tube. The phosphorescent screen of such a tube is part of the electronic circuitry of the terminal. When an image is written on
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3 Fourteen
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pages of notes on teleprinter graphics and on
CRT
~mnhicsfrom the 3 ~-~ d Nsfionnl ACS Comoutine . ,.Workshoo. Mont-~-. clair. NJ. are avnilat.lt from rhe author. Hcquests must 11raccompn~
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nird hy $1 A N D A SE1.V-ADDRESSKI) ESVELOPE.
Volume 56, Number 10, October 1979 1 645
the screen by the primary cathode beam, the electrical characteristics of the written area are changed so that a low voltage flood of electrons from secondary electron guns keeps the written area glowing. Thus, the image, once written on the screen, remains there until intentionally erased. This has been an important advantage in the past (and continues to he, though to a lesser extent) because the data for the graphic image need be sent to the terminal only once. A non-storage CRT, in contrast, requires that the image repeatedly be written or "refreshed" upon the screen. A Tektronix 4006 storage-tube graphics terminal, at the low end of the price scale. costs about $3000. Its resolution is hiah. The 7.5 in. by 5.6 in. screen is divided into 1024 resoluti'n elements horizontally by 780 elemen@vertically;this correspondsto a linear resolution of about 0.007 in., permitting the rendering of very smooth curves. More expensive versions of the storage-tube terminal offer eraohic inout as well as outout. l . . s.~ e c i acharacter sets, larger screen size, and even higher resolution. The disadvantaee of a storaee-tube terminal is that one cannot selectively erase part of an image on the screen. Only the whole screen can be erased. and that Drocess takes the better Dart of one second and isaccompanied by a bright flash of light. This limitation precludes depicting moving objects on the screen, for motion on a graphic display depends on selective erasure and rapid modification of the image . on the screen. (Compare vignettes 3 and 4.) In a non-storage-tube CRTgraphics terminal, the screen itself does not store the image, so that the data representing the imaae must be stored in a memorv and the image repeatedly written on thesrreen; a refresh rdteon theonlirof [hirty rimes per srcond i i nrcrssury to avoid tlickering of the display. In the-past, the necessity i f storing the graphic inform&& and repeatedly transmitting it to the graphics terminal placed a considerable burden on the host computer. In order to remove this burden from the host, some higher cost refreshed "eraohics . svstems have included their own minicom~utersto take care of this image-handling ~ v e r h e a d The . ~ recent incor~orationof micro~rocessorsand inexpensive semicondut:tor memory inro the graphics terminal iuelf.has made this Ihml reircshina caoiltilits more available but still adds tu the cost of the ter&in& The apparent weakness of needing to refresh the image can be turned to advantage, for the computer can change the stored coordinates of the object being displayed in between refresh cycles. In this manner, the computer can move the object rapidly on the screen. In Figure 1,one sees an example ofthis effect: Real-time rotation of images of molecules is one application of this capability. (See Vignette 4.) There are two main types of refreshed cathode-ray-tube terminal. In a "directed-beain" or "uector-driven" display, the cathode beam is directed ooint to noint about the screen to display those line segments' ("vecto;~") whose coordinates are Dresent in a disolav list stored in the c o m ~ u t e or r in the terminal's own memor;. As more and more veitors are added to make a comolex disdav. the beam must be directed to more and more places on the screen during each refresh cycle; this fact mav cause the disdav to become dimmer or, in extreme cases, to flicker as moie vectors are added. A different a p ~ r o a c hto refreshed graphics is a raster-scan display. ~ e r e , t h beam e scans the whole screen in a raster pattern like that of a television during each refresh cycle. Points to be illuminated are stored in a "memory map" with, in the simplest case, one memory bit devoted to each "pixel" or picture element (point) on the screen. In this scheme, the writing beam is intensified or not a t each pixel according to ~
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Readers with dedicated laboratory computers equipped with dieital-to-analoeconverters could. eiven sufficient motivation. use
complished, see Reference (151, 646 / Journal of Chemical Education
Figure 1. A graphic image (box) displayed on a refreshed CRT, caught by the camera as the image moves diagonally across me screen. A storage CRT cannot achieve this effect.
the value of the corresponding bit in the memory map. Some terminals devote more than one bit to each pixel, so that the memory map can encode color information or a scale of gray values in a black-and-white environment. One advantage of the raster-scan approach is that it makes use of conventional television technology. Terminals using this "video" method often have output connectors for displaying the screen contents on conventional large screen TV monitors. Another advantage is that a video terminal can render whole areas lieht " or dark in addition to representing vectors. Like the directed-beam disolav, a video disdav can suooort animation. since the contents ofthe memor; map can be changed in be: tween refresh cvcles. The Hewlett-Packard 2648A and the Tektronix 4025 are recent graphics terminals which are based on video technolow. The Tektronix 4027 is a color video graphics terminal which, with three memory bits per pixel, displays any eight colors seleded from a palette of sixtv-four colors: A vid6hased 1'1.A'rO terminal is manufkcru;ed by (:untn~l Data (Iorporation. Such video terminals from maior manufacturers cost $5000-9000. The disadvantage of the standard television raster-scan format for computer graphics is that the vertical resolution of standard television video terminals is then limited by the standard TV raster oattern of 525 lines. On a 12 in. diaeonal screen, this transla& into a vertical resolution of about0.02 in., though the horizontal resolution may be on the order of 25% finer. Such resolution may be acceptable for many applications in chemical education. As can be seen in Figure 2, five hundred point resolution on a 12 in. diagonal screen is about the level a t which curved lines begin to show some distortion. If that screen is divided into one thousand resolution elements, no distortion is noticeable, while with only two hundred and fifty resolution elements, the effect becomes distracting. Thus, for graphics in which the accurate rendering of smoothcurves is n i t Erucia~and in which some small distortion of such shapes is acceptable, a video type terminal could serve.
Figure 2. Three geometric figures pinned at various levels at graphic res&tian. (TOPRow) to24 point linear resolulion (Middle Row) 512 point linear resoiutian ROW) 256 point linear res(llu(ion. fgtres appear on U?lspage exactly as t e y w l d lmk on a 12 in. diaganal
anom om
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The output displays which come with personal or hobby computers are also video-based. The graphics resolution of present generation personal computers tends to be very coarse (16). Gerhold has suggested a resolution of 256 X 256 as a minimum for effective .graphics for CAI apolications (17). A .. .. self-contained micn~cornp~lt~~rtgraphics system featuring 512 X 6.40 point video zraphics has rewntlv hecomeavailable for around $6000 (18r 1tdoes appear that.,'under the influence of the home computer/video game market, capabilities of video-based displays are likely to improve. The compatibility with standard television equipment is an advantage both for the manufacturer and for the user. The use of a liquid crystal display medium (19) or a cathodoluminescent screen (20) might, within a few years, provide higher resolution memory-mapped graphics at a lower cost. The olasma- ane el terminal is a non-CRT screen-oriented graphici de!.ire. This terminal, developed in conjunction with the murh-discussed PLAT0 CAI svstem, has some remarkp k e l employs a layer able characteristics (9,JO). The of ionizable gas, such as neon, sandwiched between two alass plates. A finegrid of $12 horizontal and 512 vertical electr-des in rhrse plates allows each of the 262,124 pointsof incersertiun to be individually illuminated or extinguished. A point, once lighted, remains lighted until explicitly extinguished by the computer. Thus, the plasma panel stores an image, as does a storage CRT, but since each point on the plasma panel is individually controllable, the panel allows selective erasure and thus circumvents the limitations of the conventional storage tube. Plasma-panel terminals are currently marketed by Magnavox a t prices from $6300 up. Graphic Input Facilltles So far, our discussion has focused on graphic output. For full implementation of the power of graphic interaction, however, a two-way medium of graphic communication is required. Graphic input devices communicate with the computer graphically; that is, in terms of position rather than with typed letters or numbers. For example, graphic input ca~abilitvwould permit writhe -a nroeram . " which would display a molecule and would then request the student to point to. sav. a carhonvl group in the molecule. (See Vienette 6.) 'I'hk'light pen;shhwnin Figure 3, is a graphic input device which can be implemented with either directed-beam or video
Figure% Light pen in use with a directed-beam refreshed CRT display, Tracking cross following the pen is visible as six-pointed star.
CRT terminals. When available, it has the advantages of simplicity and low cost. The user simply points the light pen at an image on the screen. A photocell in the light pen detects the light pulse produced when the image a t that point is refreshed, and the screen cwrdinates of that image are returned to the program. Since the light pen itself is actually a sensing device, it cannot be used to point to dark areas of the screen; there must be something on the screen to point at. For drawing on the screen of a directed beam display with the light pen, the user program must generate a "trackingcmss"for the pen to point at, which complicates things for the programmer (21, 22). The tracking cross is visible as a six-pointed star in Figure 3. On a video display, light pen sensing in unwritten screen areas can be achieved by turning up the background brightness. The light pen cannot be used with a storage-tube terminal, since operation of the light pen depends on the refreshing process. The graphic input facility of storage-tube terminals like the Tektronix 4010 consists of a pair of thumbwheelonerated ~otentiometerson the kevboard. These thumh;heels move a faintly lighted cross hair around the screen. When a kevboard button is messed. the current screen coordinates of the cross hair are iransmitted to the computer from analog-to-digital converters driven by the potentiometer voltages. While less intuitive in use than a pointer like the light pen, .. this "araphic cursor" can nonetheless be an effective input med&mI Less natural is a "graphic tablet" which can he purchised as an accessory for some terminals. Here, the user draws or points with a stylus on an electrically active surface separate from the terminal. A certain amount of practice is necessary to control one's hand movement on this tablet while looking a t the screen for visual feedback. The resolutjon of the graphic cursor and graphic tablet is usually the same as the graphic output resolution of the terminal, while the light pen resolution may be coarser because of the relatively broad field of view of the pen's optics. The graphic input facility of the dasma- ane el terminal is noteworthy (9). Fbr interactive graphics applications, it represents the ultimate in convenience. The user simplv points a finger a t the screen, and sensors around the peripheGdetect Volume 56, Number lo, October 1979 1 647
the position of the finger. The resolution of this scheme is necessarily low (a lfi X 16 grid), so for applications requiring finer resolution, a graphic cursor controlled from the keyboard is provided. Touch input is also available on the video version of the PLAT0 terminal. Hard-Copy Graphics Devices
Screen-oriented graphics terminals do not themselves produce a "hard-copy" which the student can carry away. There are many pedagogical applications where the lack of hard-copy is not a disadvantage, although hoth the students and instructor may require a psychological weaning from the security of hard-copy. In other situations, as when the student must make measurements on the graphic output, hard-copy is essential. Hard-coov . can he ohtained either from an add-on accessow with a screen-oriented terminal or from a stand-alone hardropy graphin device. Hard.rnpy aceewries ran he purchased for hoth stornge-tuhe and refreshed CRT trrminalq. At a price dilhout S.1:100, these devices cnn provide clear, stahle rol~irs of graphic and alphanumeric output displayed on the terminal screen. While such hard-copy units may cost as much as a graphic terminal itself, it is often possible to connect as many as four terminals to the same hard-copy unit. Special paper, amounting to perhaps ten cents per copy, can be an additional
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Thr highest quality hnrd-ropy graphics to br considered for instructimal use are produced by digital plotters, lv~~itied IIV the'l'ektronix -1li62 and thr Heu,lett-l'ackard 9872A. These plotters are true digital devices, employing stepping motors to drive the Den rather than analoe servo or slidewire mechanisms. ~ h e ' s eplotters should n o t h e confused with the socalled "time-sharing plotters," based on ordinary analog recorders, which were heing marketed a few years ago. The use of steooine motors allows digital to draw with extreme . plotters . reprdducilbility. These graphics devices employ their own microprocessors which optimize pen movements for maximum plotting speeds which could not he matched by analog plotters. The microprocessors and local memory in these plotters also permit them to produce publication quality lettering as well as graphics a t moderate cost. (See Vignette 2.) Graphic input is provided by a joystick as seen in Figure 4; with the joystick, the user moves the pen around to the desired location and, by pressing a hutton, transmits the pen position to the computer. This facility provides a convenient means for digitizing curves, such as spectra. (See Vignette 5.) Plotters of this sort cost $4000 to $5000. A digital plotter without the graphic input capability is available from Houston Instruments for about $1100; this inexpensive device does not have a built-in vector eenerator. so the seuuences of x- and v- incremental motiuns for drawing lines must be generated by ioftware.'l'he reader should be swartBthat these digital plotters must be used in conjunction with a keyboard terminal in order to oermit non-graphic communication with the host compute;. This does &tnecessarily require the purchase of
an additional terminal, since the terminal can be used for ordinary purposes when the plotter is turned off. Any ASCII type terminal (DECWriter, teletype, portables, inexpensive alphanumeric CRT's) can he used, although the use of a compatible CRT graphics terminal offers the advantage of being able to preview graphics on the screen a t relatively high drawing speed before committing a plot to paper. Electrostatic plotters such as those made by Versatec work somewhat like office copiers and are available as hard-copy graphic output devices with some small computer systems. The outout of these olotters is made uo of tinv dots or lines and appears on specially treated paper. No graphic input facilitv Hieh and mechanical sim. is oresent. . " olottine . - meed . plicity are advantages. An intriguing and inexpensive hard-copy graphics add-on for a microcomputer-based instructional system is based on a modified DECWriter-type pin matrix printer. The printing head position and each of the pins are individually controlled by the computer instead of by the usual character-oriented logic circuits; a page of graphics from a video type memory map can be printed in about 70 sec (18). It is a modern commonplace that a computer can do nothing unless it is programmed, that the hardware has no built-in direction of its own and requires software to define and direct its workincs. Indeed, as unit hardware costs fall, software is emerging as the major cost component of many computer installations. The potential user of computer maphics must . confront this reality Between the generation of a "figure" as a set of numerical (x, y) coordinates in a plane and the graphic representation of that figure on a graphic output device lies the essential step of transforming the numbers into a form which will cause the graphics hardware to produce the desired picture. This step encomoasses the universal reauirements of windowine5 and scaling'sand thespecial requirement, unique to each graphics device. of codine the coordinates into a form which will cause the graphics hardware to make points or lines in the right places. For example, one common coding scheme involves using sets of alphanumeric characters to encode graphic positions. Thus, the coordinate position (130, 107) might he encoded as the characters $B and #k. Thus, the computer would have to transmit the characters $B# k to the graphics device to plot a point a t the location. It is usually software which performs the translation. Thus, the would-be purchaser of graphics equipment must give careful attention to the question of where the software will come from. This question really involves three levels of software. The most basic level of graphics software consists of small routines. often (thoueh not necessarilv) . .. written in machine language for specific computers, which service the fundamental needs of the graphics device; for example, the generation of the graphics codes mentioned above and other special codes to nerform functions like erasine the screen. Unless the user is piepared to sit down with thchardware manuals for the graphics,equipmentand to invest a considerable amount of time just to get the system running, this software should he ourchased from the eraphics eauioment vendor or from the - . ;.ornputtr manufacturer. ith he; w&, this software has to he tailored hoth to the maohin device and to the cornouter heing. .. used. The second level of software is a package of routines which allows the user to perform necessary graphic functions like scaling and windowing and, perhaps, other operations like rotation, without having to do the detailed programming for those tasks. This software is typically a collection of routines
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"Windowine" is determinine what soatial region of the disolav Figure 4. Interactive digital ploner. CIaphic inpvi to me host computer is provided by the joystick at the lower right, giving bidirectional graphic communication. 648 / Journal of ChemicalEducation
window.
written in a high level language like BASIC, FORTRAN, or APL. A statement like CALL SCALE(O,10,-5,5) might then he used in an application program to scale a plot so that the plotting area would represent x-values from 0 to 10 and yvalues from -5 to 5. Graphics support packages providing these capabilities are often available from graphic equipment manufacturers, especially in FORTRAN. Graphics support in other languages is more difficult to come hy, and the prospective graphics user should be certain of the level of software support &ail:ihle from the equipment vendor." In some tc,rminali, such as the Hrwlett-Packanl2648A. the ernnhics ( v d i w task may he handled hy the trrminnl hardware itself, making it possible to plot a graph simply by sending a set of numerical (x, y) data pairs to the terminal. Also, higher level functions like zooming (expanding or contracting the image) may be done by the hardware. The third level of graphics software is the application program itself, the program which'actually accomplishes the task for which the graphics equipment is purchased. Because of the variety of graphics equipment and computers, graphics applications programscan be very difficult to transport from one system to another. Some systems, notably PLATO, already have a library of graphics applications in chemical education, hut the more usual circumstance is that the educator will have to write hisher own applications programs. This task is not prohibitively difficult for an individual who ,is a competent (not a novice but not necessarily a whizbang) programmer. The program development task may he facilitated by a language like TUTOR, which is designed to take optimum advantage of graphics equipment, or hindered by the exigencies of, say, compiler FORTRAN, which requires explicitly linking a package of graphics support subroutines to the application program and necessitates repeated recompiling and relinking as the application program is developed and debugged. Luehrmann has recently pointed out that the present generation of mainstream computer languages is woefully lacking in graphics orientation; however, he adds that we live and work in t h e present and that much can he accomplished with existing capabilities (26). At Simmons College, for example, about ten members out of a science faculty of thirty as well as several upper-class students have been actively engaged in writing graphics applications using BASIC as the primary language and an inexpensive graphics support package purchased from the terminal vendor. Future developments in the area of graphics software should have as their goals easier development of graphics applications programs and better transportability of graphics programs. Both will be some time in coming. Integrated Systems The marketplace incentives of the homehohhy computer induitry mqy t)ecome a major driving force fur t'uturr developments in graphics tor ed~~ration. Many of these small systems now kature a computer, graphics displav, and high level language ill1 t r r ~ p h i r s< , x t m s r o n s (built-in graphics statemmtsl in one, integrated, guaranteed-t
