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

CHAPTER 10-SHAPING UP 115printed at the bottom of the screen so you can keep up with the action.The next pass DRAWs violet on green, and around the border of the appleyou should notice white spots (mold?) caused by the violet dots of theapple plotting next to the green dots of the background. Remember, anytimeyou plot two adjacent dots, they display as white.When black2 is plotted over the green, you see the effect of clashing inall its radiant glory. Any time that a group 2 dot is plotted within a group 1byte, the entire byte becomes group 2. Hence, some of the green changesto its group 2 counterpart, orange. As the program progresses you will seeseveral more examples, both of white edges and of clashing (Oh goody!).When the program stops, change line 6(/J to read:6 SCALE = 2Run the program again and watch the fun! When it plots black1 over thegreen, it starts by only turning every other line black, and when it gets tothe lower left corner of the green field, it only plots a few dots! The reasonfor this, and later shennanigans, is that at SCALE = 2, the apple is beingplotted on every second line only. By coincidence, it happens to be plottingon only the odd columns, and green does not exist on those lines.At SCALE = 1, the shape data causes the computer to plot across onerow of the apple, and then execute a plot-down to drop to the followingrow. At SCALE = 2, the machine executes every vector twice, includingthe PD vector, so it drops two lines between plotting each row.When violet apples are plotted over the green field, the ones rotated 9(/Jdegrees (in the lower right corner of the green field) plot as white (Afarmer's nightmare!). The apple is plotting on the odd columns causing theviolet in those columns to be turned on while the field is made up of greenin the even columns. Since you then have both the odd and even columnson, the result is white! For a similar reason, plotting rotated white1 appleson a black background produces only violet apples since turning on onlythe odd columns produces violet but no green.When the program tries to DRAW rotated green apples on a group 1background, nothing is plotted, since the position of the apple causes it toattempt to plot green on odd columns, and green exists only on evencolumns. Plotting green apples on a group 2 field will cause portions ofthat field to change to the corresponding group 1 color-another instanceof clashing. The program will run through all the colors, and similarstrange combinations will occur. For fu n, you might try setting the scale tothree and watching what that does to the innocent apple.XDRA WYou saw in the last program that using DRAW to plot the shape overthe background in one color, and erasing it by using black, always left the