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

138 APPLE II COMPUTER GRAPHICScaused by the need to initialize a number of variables and arrays beforebeginning the animation. In Listing 12-1, line 4(/) sets the seven Y coordinatesused in the figure, and line 16(/) assigns the corresponding memoryaddress to each of those coordinates. Lines 8(/) through 11(/) initialize anarray that contains seven data values for each of the four versions of thepropeller.The actual animating is done in lines 21(/) through 25(/) which POKE thevalues for each frame into Hi-Res memory. The variables and arrays usedin the POKEs tend to obscure some of the mechanics of the program, bu'tthey also enhance the execution, since it is much faster for the machine tolook up the value of Y%(4) stored in memory than to generate the correspondingvalue of 12288.In general it is faster to look something up in a table than to generate iteach time it is needed...Line 22(/) POKEs the first and seventh bytes of the figure [V%(1, 1) andV%(1,7)], 23(/) the second and sixth, and 24(/) the third, fifth, and fourthbytes. POKEing the values in that weird order improves the resulting effect,but you might try changing the order to POKE them sequentially just to seewhat happens.Another reason fo r the variables is to make it easy to mod ify this programso that the figure will plot at different Y coordinates on the screen.The array Y% will contain the starting addresses for each of the 192 screenlines, and Yl % through Y7% will be the coordinates for the seven linesused in the shape.The propeller is an example of stationary animation. That is, animationthat takes place within a fixed region, and the different frames of thatregion are alternated to simulate movement. There are many applicationsfor stationary animation. The APPLEVISION program on your DOS 3.3master is an excellent example, and so is a Hi-Res scoreboard for a game.(The ten diffe rent digits compose ten possible frames for display.)Drop the PropMost shapes used in a game are required to move about the screen. Wewill now alter our program to allow for this. With Listing 12-1 still inmemory, type in the fo llowing changes.16 GOSUB 1 : REM CALC ADDRESSES245 Y1% Y1% + 1: Y2% Y2% + 1:Y3% Y3% + 1: Y4% Y4% + 1:Y5% Y5% + 1: Y6% Y6% + 1:Y7% Y7% + 1246 IF Y7% > 192 THEN END1 REM