williams-et-al-1983-apple-ii-computer-graphics

12Byte-Move ShapesObjectivesAfter reading Chapter 12 you should be able to:• Use byte-move to perform a stationary and a vertical animation.• Draw and digitize the seven horizontal separations of a byte-movefigure.• Animate a simple byte-move figure across the screen.The standard Apple shape is a fairly usefu l construct, particularly for theBASIC programmer because it lets the user use a number of fast machinelevelroutines via the APPLESOFT shape commands (DRAW, XDRAW,SCALE, and others). Unfo rtunately, those routines are not fast enough fo rarcade type animation, so byte-move graphics are often used to gain extraspeed. To be honest, professional byte-move animation is written in machinelanguage because BASIC is too slow to gain much advantage,whereas "look-up tables" are something that a machine-level programhandles very well. These examples in BASIC serve to give you the "basic"idea behind byte-move, so if you are so inclined and endowed you canwrite these strategies into your machine-level program.It is important to realize how byte-move shapes differ from standardshapes. As you saw in Chapter 1 ([), a shape is defi ned by a series ofinstructions (vectors). For example, plot-up, skip-left, skip-left, plot-down,and so on. Each time the shape is plotted, the drawing routines followthose directions and reconstruct the shape. Since shapes can easily run toa hundred instructions or more, the vector method can lead to a timedelay as each · vector is reprocessed and reinterpreted.135