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

CHAPTER 14-COLLISION COU RSE 16 1always miss the spider so you do not have to check either coordinateagain, just let the missile make its way to the top of the screen. This sort ofspecialized adjustment can often streamline your animation.The box method used in the example is fairly simple, but it is a littlesloppy since it declares a collision based on some imaginary squ are insteadof actual contact with the target. Further, we only checked for the centerof the missile, and the center may actually miss the spider when one of theedges hits it. Unfair! If you have ever played a game which has sloppycollision detection (and there are a bunch of them) you know how frustratingthat can be.Checking across the full width of the missile is pretty easy, so we leavethat to you. The problem of detecting an actual contact is more interesting.To accomplish that, you will have to take a PEEK at Hi-Res memory to seeif the missile is about to move onto a dot which is already turned on. (Inthis example, the only dots turned on are in the target.) Dealing directlywith memory from BASIC is always complex, and it gets worse because weneed to look at ind ividual bits. Type the following changes to the existingprogram.DEL 4, 75 GOSUB 1 : REM CALC Y ADDR .515 Q% = M I 7:R% = M-7 * Q% :R% R% + 1536 V% = PEEK (Y%(Y) + Q%)538 FOR I = 7 TO R% STEP -154 P% = 2 A I542 IF V% > P% THEN V% = V%-P%544 NEXT I546 IF V% > 2 A I THEN c = 11 REM11 REM CALCULATE Y12 REM COORDINATES13 REM14 DIM Y% (192)15 FOR I = TO 184 STEP 8: READ SA%16 FOR J = TO 7:Y%(I + J) = SA% +J * 12417 NEXT J,I18 DATA 8192,832, 8448,8576,874,8832, 896,98819 DATA 8232,836, 8488, 8616 ,8744,8872, 9,912811 DATA 8272,84,8528,8656,8784,8912, 94,9168111 RETURNLines 5 and 1000 to 1100 should be fam iliar from previous chapters; they