Smooth Out For The Atari
James Hague
This sophisticated graphics technique lets you smooth out of the rough lines of your graphics screens in mere seconds. For the Atari 400, 800 XL, and XE with 48K or more of RAM.
One of the strangest of the many graphics modes of the Atari eight-bit computers is GRAPHICS 9. It allows as many as 16 different shades of each color. That's twice the number of intensities available on the Atari ST. But that doesn't mean that you can get ST-quality pictures on the eight-bit machines—the resolution simply isn't high enough. While an ST's "low-resolution" screen offers 320×200 pixels, GRAPHICS 9 is limited to 80×192 pixels.
When you draw on such a low-resolution screen, you're likely to see jaggies, the stairstep effects you see on lines that are drawn at an angle (nonhorizontal or nonvertical lines). One common method used to eliminate jaggies is antiliasing, a complex smoothing method used in advanced paint programs such as Deluxe Paint II. In "Smooth Out," a related method—pixel averaging—is used to smooth the entire screen at once.
Pixel averaging smooths pictures by blurring them. As a result, the process is better suited to landscapes and portraits than to bar charts and boxes.
Typing It In
Type in Program 1 and save it to tape or disk. Do not run the program yet—it is not a complete program until you add the lines from either Program 2 or Program 3.
Program 2 is a very short demonstration of Smooth Out. Load Program 1, type in the lines from Program 2 and then save the resulting program with a unique filename. Program 3 draws an alien landscape. Follow the same procedure with Program 3 as you did with Program 2.
When you're ready to try one of the sample programs, load it and type RUN. A picture is drawn on then screen. When the picture is finished, you'll hear a tone. Press START, and Smooth Out will Smooth the screen with a machine language subroutine. Press START again to end the program.
Behind The Scene
Pixel averaging can be done in many ways. The smoothing algorithm used here adds up the values of the surrounding eight pixels and divides the total by 8 to find the new value of the pixel. This must be done for each pixel on the screen. In machine language, the task takes seconds. If i had used BASIC, it would have taken more than 30 minutes.
If you want to use this program with your own graphics screens, insert the lines to draw your GRAPHICS 9 screen between lines 100 and 5000 of Program 1. (Be sure not to remove line 100.)
For an interesting effect, try changing the GRAPHICS 9 statements in the programs to GRAPHICS 11.(See lines 100 and 5040.)
For Instructions on entering these programs, please refer to "COMPUTE!'s Guide to Typing in Programs" elsewhere in this issue.
Program 1: Smooth Out
BG 90 REM COPYRIGHT 1987 COMPUTE! PUBLICATIONS, INC. ALL RIGHTS RESERVED.
JI 92 PRINT "{CLEAR} COPYRIGHT 1987 " : PRINT "COMPUTE! PUBLICATIONS, INC." : PRINT "ALL RIGHTS RESERVED."
FE 94 FOR X = 1 TO 1500 : NEXT X
Ln 96 PRINT "{CLEAR}"
HB 100 MEMTOP = PEEK (106) : GRAPHICS 9
CJ 5000 IF PEEK (1540) <> 205 AND PEEK (1542) <>39 THEN RESTORE 5200 : FOR A = 0 TO 153 : READ B : POKE 1536 + A, B : NEXT A
AM 5010 GOSUB 5100
KP 5020 POKE 203, PEEK (88) : POKE 204, PEEK (89)
GA 5030 POKE 106, PEEK (106)-48
FD 5040 GRAPHICS 9
GC 5050 A = USR (1536)
BB 5060 GOSUB 5100
EL 5070 POKE 106, MEMTOP : GRAPHICS 0
KE 5080 END
GC 5100 FOR A = 15 TO 0 STEP –1 : SOUND 0, 100, 10, A : NEXT A
HM 5110 IF PEEK (53279)<> 6 THEN 5110
KI 5120 RETURN
OH 5200 DATA 104, 169, 0, 133, 205, 160
AC 5210 DATA 39, 177, 203, 145, 88, 136
PE 5220 DATA 16, 249, 160, 40, 177, 203
AF 5230 DATA 145, 88, 160, 79, 177, 203
AD 5240 DATA 145, 88, 169, 81, 133, 203
JC 5250 DATA 169, 0, 133, 207, 162, 7
MB 5260 DATA 165, 206, 24, 125, 146, 6
ND 5270 DATA 74, 144, 8, 168, 177, 203DI 5280 DATA 41, 15, 76, 60, 6,1 6B
KB 5290 DATA 177, 203, 74, 74, 7, 4, 74
BD 5300 DATA 24, 101, 207, 133, 207, 202
PB 5310 DATA 16, 224, 165, 206, 74, 144
KD 5320 DATA 9, 168, 165, 207, 7, 4, 74
HK 5330 DATA 74, 76, 88, 6, 168, 165
IP 5340 DATA 207, 10, 41, 240, 1, 7, 88
CO 5350 DATA 145, 88, 230, 206, 165, 206
BA 5360 DATA 201,159, 208, 186, 165, 203
DN 5370 DATA 24, 105, 40, 133, 2, 03, 165
DL 5380 DATA 204, 105, 0, 133, 2, 04, 165
JD 5390 DATA 88, 24, 105, 40, 13, 3, 88
JK 5400 DATA 165, 89, 105, 0, 13, 3, 89
EM 5410 DATA 230, 205, 165, 205, 201, 190
CE 5420 DATA 208, 136, 160, 40, 177, 203
PM 5430 DATA 145, 88, 200, 192, B0, 20B
EF 5440 DATA 247, 96, 175, 176, 177, 255
ND 5450 DATA 1, 79, 80, 81
Program 2: Smooth Out Demo
DC 110 FOR A = 1 To 25 LL 120 X = INT (RND (0) * 57)+1 DH 130 Y = INT (RND (0) * 150)+1 EG 140 COLOR INT (RND (0) * 16) DE 150 FOR B = X TO X+19 KI 160 PLOT X+10, Y :DRAWTO B, Y+39 ND 170 NEXT B :NEXT AProgram 3: Alien Landscape
CL 110 COLOR 12 :PLOT 0, 0 :DRAWTO 79, 0 :DRAWTO 79, 191 :DRAWTO 0, 191 :DRAWTO 0,0 PO 120 FOR A = 1 TO 2 DF 130 IF A = 1 THEN X=30 :Y=30 :GOTO 150 MI 140 X = 55:Y = 40 DP 150 FOR B=1 TO 78 NL 160 COLOR INT (RND (0) * 5)+2 :PLOT X, Y :DRAWTO B,135 ND 170 NEXT B :NEXT A DI 180 FOR A = 130 TO 190 :COLOR INT (RND (0) * 7) +5 :PLOT 1, A :DRAWTO 39, A—5 : DRAWTO78, A :NEXT A AF 190 FOR A = 1 TO 2 DN 200 IF A = 1 THEN X=55 :Y=80 :GOTO 220 NC 210 X = 39 : Y = 95 KL 220 FOR B = 1 TO 78 STEP 2 AI 230 COLOR INT (RND(0) * 10)+4 : PLOT X, Y : DRAWTO B,190 NB 240 NEXT B : NEXT A DH 250 FOR A = 1 TO 25 KM 260 COLOR INT (RND (0) * 10) + 5 : PLOT 1, 70 : DRAWTO A, 190 : NEXT A