Section 2 - Commodore Computers

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158 COMPUTE! May. 1982. 1982, Issue 24 Extra Colors Extra Colors For Atari Through Artifacting Bill Mohn Danville, CA How can you getup to Four colors in GRAPH ICS How can you get up to four colors in GRAPHICS 8' 8? The technique is called "artifacting"; it ii makes use of an otherwise unintended characteristic of a use of an otherwise uninte nded characte ristic or a color display tube. The highest resolution mode for the Atari is GRAPHICS 8. This niode mode includes 51,200 pixels (picture elements) arranged in a rectangular area extending 320 units horizontallyy aand 160 units vertically. Unlike modes 2 through 7, GRAPHICS 8 normally all ows onl y one color at two brightnesses. 8 normally allows only one color at two brightnesses. The background hue and luminance is determined by Color Register 2. "SETCOLOR 2,hue,lum" establishes Color Register 2. If left unset, it defaults establishes Color Register 2. If left unset, it defaults to dark blue. Colorr Register I, 1. which is set by SETCOLOR SETCOI.OR 1 l,hue,lum is used onlyy to determine the luminance off graphics points. The hue off graphics points is determined by Color Register 2. A point is put on the screen by BASIC's BASIC'S PLOT or ORA DRAWTO commands. To be effective, a COLOR I 1 must preceed the first Iirst PLOT. In order to understand artif;,cting, artifacting. look closely at a color television screen. Youu will see closely spaced vertical stripes of red, , green and blue phosphor elements. If these are ill uminated equally, a phor elements. If these are illuminated equally, a white while picture is produced. Unequal illumination produces a ll of the other colors. produces all of the other colors. GRAPHICS 7 Versus 8 A pixel in GRAPHICS 8 is the size of one half off one set of three color phosphor elements. IfI I' Color Register 1I luminance is relatively high and a graphics point is plotted ploued at an odd valued horizontal coordinate, only the blue phosphor will be li ghted. coordinate, only the blue phosphor will be lighted. If I f a shape is drawn, taking care to use only odd values for X, the entire shape will be blue. On the othe r hand, ir onl y even values of X On the other hand, if only even values of X are chosen, the red and green phosphors will be selected. The proportion of these will depend on selected . The proportion of these will d e pend on the e background hue (Color Register 2) aand the resulting "even-only" figure can range from red through brown to green. Ifboth even aand odd points are plotted, the background hue will result at a luminance specified ed by Color Registerr I. 1. Of course if neither even nor odd points are plotted, ploued, the result will be background hue and luminance. An interesting side effect occurs when an "even fi gure" overlaps a n "odd fi gure" - the area of figure" overlaps an "odd figure" — the area of overlap will be clearly visible. GRAPHICS 7 has pixels four times as large as G RAPH ICS 7 has pixels four tim es as large as GRAPHICS 8. That is, they are twice as wide aand twice as high. Since artifacting requires skipping half the points in the horizontal direction, the resolution in that direction with GRAPHICS 8 approximately equals that of GRAPHICS 7. However, the vertical resolution of GRAPHICS 8 is twice that of GRAPHICS 7. The program here is a simple pie demons! monstration off these principles. It first uses GRAPHICS 8 to draw three overlappingg disks, thee upper r left using odd points only, the upper right using g even points only, and the lower using all points. Note that the lower disk does have better horizontal resolution than the otherr two. Afterr a deJa delay, the program m plots three similar ligures figures in GRAPI1ICS H ICS 7. Youu will see that these disks have horizontal resulution resolution equal to and vertical resolution worse than the previous upper disks. previous upper disks. You u may explore the variety of colors possible with artifacting by changing the two SKTCOI.OR with artifacring by cha ngin g the twO SETCOLOR stateme nts. Statements 4000 through 4 120 may be statements. Statements 4000 through ■! 120 may be inserted to cause all 960 possible SETCOLOR combinations to be displayed. This will step through all combinations ns as long as START is depressed. Notes On The Sample Program 1000-1200 Make an array of Y Y-values corresponding to X-values for circles. 1300-1500 Setup for the first display in GRAPHICS 8. 1600-1900 Draw upper-left figure using odd points only. 2000-2300 Draw upper-right figure using even pointsonly. only, 3000-3300 Draw lower figure using even and odd points. 3400 A delay loop to allow viewing the first display. 5000-5600 Setup and draw first figure in GRAPHICS 7. 5700-7300 Draw second and third figures. Figures. 9000-9100 Anotherr delay loop followed by a return to repeat. 4000-4120 Three nested loops sstepping through all possible hues and luminances. 4060 The ""arrow" is entered by typing ESC, CTRL, CLEAR 4090 This loops until either r START, SELECT, or OPTION is d epressed. 180 280 300 480 500 600 700 RBI RBI REM REM REM REM REM OPTION is depressed. "tr&k "it "it it it i: ■& Scm * * DEMONSTRATION OF t * * ARTIFACTING IN GRAF-HICS 8 * t * * * BY BILL MOHN * * 180 REM ***************************** 200 REM 300 REM ID1ONSTRATION ~ 4B0 REM * ~TIFICTII{; IN GRAPHICS a * 500 REM 600 REM 8Y BILL totI 700 REM * *
BgeREM* * 900 1080 1000 DIM DIM Y '1'(79) 1658 1050 ? "SHORT "SHCRT DELAY WHILE Ion-llLE CALCULATING" CAlClLATIt-G" I860 11160 ? "COORDINATES "CiXJRDlNATES FOR Fffi CIRCLE CIRClE ... . . uu 1188 1181l FOR F~ 1=1 1=1 TO TO 79 79 1260 Y(I>=SQRa6&0-C40-I)*u;"SETCOLOR 2,u;H\JE;",u;lJJ12 2/1;HUE;lV;LUH2 4870 ? "SETCOLffi "SETCOLOR 1,0, U 1,8,U;UUH1 ;llJ11 4080 4880 FOR 11.'1'= DLY=11 TO 50 58, ■• NEXT Il. DLY '1' 4090 4890 IF PEEK(53279)::7 PEEKC >=7 THEN 4890 4898 4100 4188 HEXT NEXT Lut11 LJUH1 4tHl 4110 HEXT NEXT Lut12 LUM2 4129 4128 HtXT NEXT H.JE HUE 5eOO 5008 GRAPHICS 7 5100 ? "~ICS "GRAPHICS MODE 7" 5200 5208 ca..~ COLOR 1 5300 5388 FOR FCft X=l TO 79 STEP 2 5400 5408 PlOT PLOT 48+)V2, 48+X/2,23-Y
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1\ MALA Educational Programs M\~ Fo
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May. 1982. Issue 24 COMPUTEI 101 Ma
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• the battle for the moons of Jup
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May, May. 1Q82lssue 1982, Issue 24
Page 9 and 10: i i ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~,
Page 11 and 12: • Announcing the Printing Breakth
Page 13 and 14: May. 1982. Issue 24 COMPUTEI 111 Ma
Page 15 and 16: APPLE • ALTOS • ATARI • MAXEL
Page 17 and 18: CBM Maintenance In-House DOWNEY, CA
Page 19 and 20: NEW PET/CBM SOFTWARE NEW PET/CBM SO
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Page 23 and 24: OLYMPIC SALES COMPANY 'ANY Vuthonze
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Page 27 and 28: OSI TRS-80 COLOR-80 OSI OSI TRS-80
Page 29 and 30: THEE MOSAIC 32K RAM FOR ATARI 32K R
Page 31 and 32: VOUNEED YOU NEED SpeedRead+ • Opt
Page 33 and 34: MOV, May, 1Q82, 1982, Issue 2A 24 C
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Page 73 and 74: May, 1982. 1982, Issue 24 211 COMPU
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Section 2 - Commodore Computers

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