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

160 APPLE II COMPUTER GRAPHICSHPLOT 137,38 TO 153 ,38, HPLOT TO 153 , 51HPLOT TO 137, 51HPLOT TO 137 ,38As we move the missile up the screen we will check to see if it movesinside that box, and if so- BOOM! To get that to happen, add these linesto your program:4(i' REM5(i' REM SET BOUNDARIES6(i' REM7(i' YMAX = 51:MINX = 137:MAXX 15351(i' PRINT CHR$ (7) : REM BELL52(i' C = ii': REM COLLISION FLAG53ii' FOR Y = 149 TO ii' STEP -154(i' IF Y < YMAX THEN IF M > MINXAND M < MAXX THEN C = 1: REM COLLISION ! !55ii' IF C THEN PRINT CHR$ (7) ; CHR$ (7) ;CHR$ (7) : XDRAW 2 AT M,Y + 1: GOTO 62ii'561i' XDRAW 2 AT M, Y + 1: XDRAW 2 AT M , Y571i' NEXT Y58ii' REM ERASE MISSILE @) TOP591i' y = y + 16(i'fi' XDRAW 2 AT M ,Y61fi' REM DRAW NEXT MISSILE62fi' XDRAW 2 AT M, 15ii'63fi' RETURNListing 14-2.Line 7(/J sets the boundaries for the square. The upper boundary is ignoredin this example because the missiles always come from below.Line 54(/J is the line that actually detects any collision. It first checks theY-coordinate at the tip of the missile to see if it is high enough to possiblystrike the box. If so, then the X position is checked to see if it is in thecorrect horizontal range. If the answer to all of these is "yes," then wehave a coll ision and the flag (C) is set to 1 for later reference.Line 55(/J is where you jump to get your nifty explosion routine, but sincethis is only an example, all we do is set off the bells and whistles. We thenerase the missile from its last position and jump to where a new missile idrawn at the bottom- big deal!In this case, since the target is stationary, it would be advantageous tocheck the X-coordinate before the Y. If the X value is within the range ofthe target, there is bound to be a collision. In that case you need to checkonly the Y-coordinate as the missile rises in order to tell you when themissile strikes the target. If the X is not within range, then the missile will