Encyclopedia of Computer Science and Technology

computer graphics 105By the late 1970s, the microcomputers from Apple, RadioShack, Commodore, and others either included CRT monitorsor had adapters that allowed them to be hooked upto regular television sets. These machines generally camewith a version of the BASIC language that included commandsfor plotting lines and points and filling enclosedfigures with color. While crude by modern standards, thesegraphics capabilities meant that spreadsheet programscould provide charts while games and simulations couldshow moving, interacting objects. Desktop computers thatshowed pictures on television-like screens seemed less forbiddingthan giant machines spitting out reams of printedpaper (see graphics card).Research at the Xerox PARC laboratory in the 1970sdemonstrated the advantages of a graphical user interfacebased on visual objects, including menus, windows, dialogboxes, and icons (see user interface). The Apple Macintosh,introduced in 1984, was the first commercially viablecomputer in which everything displayed on the screen(including text) consisted of bitmapped graphics. Microsoft’ssimilar Windows operating environment becamedominant on IBM architecture PCs during the 1990s.Today Apple, Microsoft, and UNIX-based operating systemsinclude extensive graphics functions. Game and multimediadevelopers can call upon such facilities as AppleQuickDraw and Microsoft DirectX to create high resolution,realistic graphics (see also game console).Basic Graphics PrinciplesThe most basic capabilities needed for computer graphics arethe ability to control the display of pixels (picture elements)on the screen and a way to specify the location of the spotsto be displayed. A CRT screen is essentially a grid of pixelsthat correspond to phosphors (or groups of colored phosphors)that can be lit up by the electron beam(s). The firstIBM PCs, for example, often displayed graphics on a 320(horizontal) by 200 (vertical) grid, with 4 available colors.A memory buffer is set up whose bytes correspond to thevideo display. (A simple monochrome display needs onlyone bit per pixel, but color displays must use additionalspace to store the color for each pixel.) A screen image isset up by writing the data bytes to the buffer, which thenis sent to the video system. The video system uses the datato control the display device so the corresponding pixelsare shown (in the case of a CRT, this means lighting up the“on” pixels with the electron gun[s]).In most cases screen locations are defined in coordinateswhere point 0,0 is the upper left corner of the screen.The coordinates of the lower right corner depend on thescreen resolution, At 320 by 200, the lower right cornerwould be 319,199.For example, many versions of BASIC use statementssuch as the following:PSET 50,50 ’ draws a dot at X=50, Y=50LINE (100,50)-(150,100), B ’ draw squarewith UL’ corner at 100,50 and LR’ corner at 150,100Some example figures plotted by BASIC graphics statements usingscreen coordinates.CIRCLE (100,150), 50, 4 ’ draw a circle ofradius 50’ with center at 200,200 and’ color 4 (red)Languages such as C, C++ and Java don’t have built-ingraphics commands, but functions can be provided in programlibraries (see library, program). They would be usedmuch like the BASIC commands given above.More commonly, however, programmers use languageindependentgraphics platforms (see api). With Windows,this usually means DirectX, which includes Direct2D for3D graphics, as well as a variety of multimedia librariesfor sound, user interfacing, and networking. A competitorthat is particularly popular in the Mac and UNIX/Linuxworlds is OpenGL (Open Graphics Library). Both DirectXand OpenGL run on a wide variety of supported hardware.Graphics Models and EnginesModern applications (such as drawing programs and games)go well beyond simple two-dimensional objects. Indeed,multimedia developers typically use graphics enginesdesigned to work with C++ or Java. A graphics engineprovides a way to define and model 2D and 3D polygons.(Curves can be constructed by specifying “control points”for bicubic curves.)Complex objects can be built up by specifying hierarchies(for example, a human figure might consist of a head,neck, upper torso, arms, hands, lower torso, legs, feet, and