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

CHAPTER 13-ADVANCED MOVES 15124 NEXT X25 HGR : REM SET GRAPHICS254 REM255 REM HERE GOES! !256 REM26 FOR X = 1 TO 2827 POKE A%(Q%(X) ),T%(R%(X) ):POKE A% ( Q%(X ) + 1) ,H%(R%( X) )28 NEXT X29 END294 REM295 REM DATA TABLE296 REM3 DATA 126,1,124, 3,12,7, 112,1531 DATA 96, Jl,64,63,, 127Listing 13-2.Lines 100, 160, and 200 through 270 are the only new mod ifications.The arrays Q% and R% hold the quotients and remainders fo r each ofthe 280 X-coordinates. Dividing the X value by 7 gives you the offset (0-39)used to address the correct byte ac ross the screen, and also which of theseven versions of the figure should be used (Remember?).Lines 160-180 fi ll array A% with the addresses for each of the 40 bytesacross the screen line, and lines 220-240 calculate the 280 quotients andremainders. Finally, line 250 turns on Hi-Res, and line 270 does the actualPOKEing.Sorry about the compound indexing [A%(Q%(X))], but it could not behelped. X is the coordinate number, so Q%(X) is the quotient belonging tothat coordinate, and A%(Q%(X)) is the address determi ned by that quotient.This new version of the program runs the line across the screen in sevenseconds, as opposed to nine in the earlier one. Now, seven seconds is stillpretty slow (blame BASIC), but pre-calculating did result in a significantimprovement (22 percent, since you asked.)Pre-shiing ·And now, on to perhaps the most elegant of the techniques: pre-shifting.We are going to animate using a shape table, so take a couple of minutesnow to enter it. From BASIC type:CALL -151