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

94 APPLE II COMPUTER GRAPHICS50 HGR60 FOR Cl = 1 TO 770 FOH C2 = 1 TO 780 FOR Y = 0 TO SZ STEP 290 HCOLOR = Cl100 HPLOT 0 ,Y TO SZ,Y110 HCOLOR = C2120 HPLOT 0, Y + 1 TO SZ , Y + 1130 NEXT Y140 FOR I = 1 TO DLAY : NEXT I150 NEXT C2160 NEXT Cl170 ENDListing 9-4. Dithering.You should see a rectangle in the upper left corner of the screen whichcontinually changes color, and in the process takes on some shades otherthan the basic six. You may change the speed by altering the value ofOLAY in line 30, and the size of the rectangle is determi ned by the valueof SZ in line 40.All the program does is plot pairs of horizontal lines in two alternatingcolors (C1 and C2), then change one or both colors and do it again. As theroutine cycles, you can see the basic colors-green, blue, violet, orange,black, and white-but also some other colors such as light green and hotpink. The extra colors are generated by two colors blurring together toform a mixture, for instance white and green blurring to form light green.There are a great many variations used to produce other shades. Forexample, you might mix the colors orange and black, by alternating themin a checkerboard pattern to produce dark orange, or perhaps by drawinga pure orange line fo llowed by a line which alternates orange and black.The possibil ities are limited only by your imagination.There are a few considerations, however. For instance, if you tried toalternate green and violet dots across the screen, as in the following shortprogram, you would run into severe problems.10 HGR20 Cl = 2: C2 = 130 FOR X = 0 TO 278 STEP 240 HCOLOR = Cl : HPLOT X , Y: REM VIOLET50 GOSUB 10060 HCOLOR = C2 : HPLOT X + 1, Y: REM GREEN70 GOSUB 10080 NEXT X90 END100 FOR I 1 TO 200 : NEXT I: RETURNListing 9-5.