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

152 APPLE II COMPUTER GRAPHICS3 : 2 6 45 3F 3F38: 3F 3F 3F 3F 8 2D 2D 2D31 : 2D 2D 2D 18 3F 3F 3F 3F318 : 3F 3F 8 2D 2D 2D 2D 2D32 : 2D .18 3F 3F 3F 3F 3F 3F328: 8 2D 2D 2D 2D 2D 2D 1833 : 3F 3F 3F 3F 3F 3F 8 2D338: 2D 2D 2D 2D 2D 18 3F 3F34 : 3F 3F 3F 3F 24 24 24348 : 24 DF DB DB DB 6 36 3635 : 36 36 3DGBSAVE SQUARE ,A$3, L$53With the table still in memory, let's fi nd out what we actually have there.Type:POKE 232,: POKE 233,3HCOLOR = 3:ROT = :SCALE = l :HGRDRAW 1 AT 5,5The fi rst line tells APPLESOFT where the table is stored ($0300) in lobyte/hi-bytefo rm (00 and 03). The second line sets all the parameters, andthe third draws the fi rst shape, a rectangle, at 50,50 on the Hi-Res screen.The second shape in the table is the horizontal pre-shift (the what!??) ofthe original rectangle. To explain, imagine that you shift the rectangle oneplace to the right. The bulk of the figure is unchanged; there is a single lineadded to the right side, and one deleted from the left side. Pre-shifting, likepartial modification, is a way to process only that portion of the figurewhich changes while leaving the rest alone.Now type:DRAW 2 AT 51,6to draw the pre-shift below the rectangle. Notice that the pre-shift has asingle line to the right of the rectangle, and another that lines up with theleft side of the rectangle. To affect the mod ification, we will XDRAW thepre-shift on top of the rectangle.Let us digress for a moment. When used to superimpose one figure onanother, XDRAW has the effect of comparing corresponding dots of thetwo shapes and forming a resultant figure from them. The resultant dot ison if either one of the original dots were on, but not both. The chart belowsummarizes the results from the four possibilities.Dot#!#2ONONONOFFOFFONOFFOFFResultOFFONONOFF