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

CHAPTER 13-ADVANCED MOVES 155Lines 120 and 130 test the value of PDL(0) and decrement or incrementthe X-coordinate as needed. Line 140 keeps everything within range, andline 150 skips the XDRAW statements if there is no movement of thesquare-this helps to avoid flicker.In line 160, if the old direction (OD) is different from the new direction(ND) then the extra XDRAW is done. (Remember that series of XDRAWstatements above?) Line 170 does the regular XDRAW and sets the oldvalues to the new values.With these changes in effect, the rectangle will respond to paddle control.When you add partial modification, pre-shifting, and pre-calculation toyour repertoire, you give the fi nal polish to your animation skil ls. That isabout all there is to know about designing and animating figures, and inthe next chapter we address a slightly diffe rent topic: collision detection.* * * One extra tid-bit for the Apple lie***One annoying source of flicker in animation stems from changing thedata being displayed while it is being scanned. The electron beam in yourmonitor or TV sweeps over the entire screen sixty times every second,much as a searchlight will sweep across the sky. The trick is to change thedisplay data when the beam is not "looking." Over one-quarter of the timein each sweep is spent getting the beam from the bottom back to the topof the screen so it can begin the next cycle. This is called Vertical Blanking.Since nothing new can be displayed during this interval (about one-200th of a second), this is the best time to switch the display data. With theApple II + there was no way to tell when the vertical blanking was occurring,but the lie has a special Vertical Blanking Location (VBL) at 49177($C019) which signals the vertical blanking interval to anyone who cares tolook. When the vertical blanking interval begins, the value in the VBLdrops to less than 128, and this value may be read using a PEEK statement.Let's suppose that the routine to animate the figure by changing thedisplayed data begins at line 1000, then line 1000 could read:l(il(il(il IF PEEK ( 49177) > 127 GOTO l(il(il(ill(ill(il Routine to POKE in new valuesLine 1000 will delay the changing of display values until the vertical blankingperiod begins, at which time it will drop through to line 1010 andbegin changing.The VBL is still too new to fairly evaluate its usefu lness, but it is gettingmixed reviews from some of the current game programmers. Although it isnice to change the display data during the vertical blanki ng interval, doing