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

90 APPLE II COMPUTER GRAPHICSblue as they are in an even column (zero) and a group 2 byte. When boxD is plotted in the second column of boxes from the left, those same dotsdisplay orange since they are now in an odd column of dots (seven). Thatpattern alternates across the width of the screen.Another weakness of the program is that you are constrained to alwaysselect the apple's position in increments of a fu ll box.' That is, you may notchoose to draw the apple starting three and one-half boxes into the screen,but instead you must always use a whole number of boxes. There aretechniques which let you POKE the figure beginning with any of the 280columns and 160 rows on the screen, but they will not be pursued here, asthe results are better accomplished using the methods discussed in thechapters covering Shapes and Byte-Move Graphics.Everything You Know Is FalseIn this section we appear to contradict a portion of what we have just toldyou. We have said (repeatedly) that there are 280 positions across the Hi­Res screen, but now we attempt to convince you that there are only 40positions, and also that there are 560 positions, and sometimes 140 positions.ARRGH!! Hopefu lly, by temporari ly confusing the issue, we willclarify it. The purpose is to provide you with several clear and accu ratemodels fo r use ·with the Hi-Res screen- not physical models, but ratherconceptual ones which will help structure your thinking and simplify yourtask when designing Hi-Res output. Each model is based on a diffe rentnumber of positions across the screen, but they do use the same numberof positions down the screen: 160 rows of dots (1 92 fo r full screen).Why 4(1?The explanation of the 40 position model is easiest since you have alreadydealt with it, perhaps unknowingly. To process any dot, you must unavoidablydeal with the byte which contains that dot, and there are only 40bytes across each line of the screen. When you developed and plotted theHi-Res apple, your design was developed in one-byte increments andplotted with the consideration of having only 40 boxes or bytes across thescreen in which to draw the figure.What About 28(1?The 280-position model is still valid. Each of the 40 bytes displays sevendots, and that yields 280 dots across the screen. The 280-dot concept isusefu l when you are designing your figures, but when you begin to digitizethe data, you naturally adopt the 40-position model, perhaps without realizingit. Refer to the section where you worked with the Hi-Res apple, and