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

CHAPTER 9-Hl-RES COLOR 95Thanks to the delay loop in line 1©©, you will be able to see all theaction when the program runs. The fi rst dot appears in its proper violet,but then the next dot comes on- not as the expected green, but as white,and the first dot turns white with it. The remaining dots also display white,regardless of the color set in the program. Remember, any time two consecutivedots in a row are on, they display white, so if you wish to alternatecolors across a line, you must leave one or more black dots betweeneach color and the next (see Figure 9-5).m 1 m 1 m 1 mFigure 9-5. Alternating colors.Even if you leave black dots between colored dots, you are still notcompletely safe. The next listing ostensibly plots violet in col umn ©, blue incolumn 2, violet in column 4, blue again in column 6, and so on acrossthe line.l HGR2 Cl = 2:C2 = 63 FOR X = TO 276 STEP 44 HCOLOR = Cl : HP LOT X , Y: REM VIOLET5 GOSUB l6 HCOLOR = C2 : HP LOT X + 2, Y: REM BLUE7 GOSUB l8 NEXT X9 ENDl FOR I = 1 TO 2 : NEXT I : RETURNListing 9-6.When the program runs, a violet (group 1) dot will appear in column ©,then a blue dot will appear in column 2. Since blue is in group 2, theentire byte will become group 2 and the violet dot will change to bluealso. Next, a violet dot will be plotted in column 4, turning the previoustwo dots violet, and then a blue dot will be plotted in column 6 to turneverything blue again. Column 8 is in the next byte of memory so whenthe violet dot is plotted there, the previous four dots are unaffected.Column 1 © displays blue, and then column 12 displays violet so that thethree dots in that byte are all violet. Column 14 begins the third byte, and,as it displays blue, the previous three dots remain violet. The fi nal patternis a line which has alternating violet and blue segments.This demonstrates a propensity of Hi-Res graphics called "clashing."Clashing is the inadvertent switching of a dot from one color group to theother and occu rs when you attempt to plot colors from opposite colorgroups within the same byte-the computer simply cannot and will not doit-so the attempt turns the first colors plotted into colors from the secondgroup plotted. You cannot mix green with orange or blue in the samebyte, nor can you put orange or blue in the same byte as violet. Clashing is