Classic Computer Magazine Archive COMPUTE! ISSUE 87 / AUGUST 1987 / PAGE 8

Atari Artifacts

When I use graphics mode 8 on an Atari 800XL, I get the color registers mixed up. Somehow, I always get red and green in the picture. How can I get rid of these colors?

Marc Canoul

Your color problem has nothing to do with color registers. The problem is that the resolution of GRAPHICS 8 is too great to display properly on a color television. This problem does not appear on monochrome or high-resolution color monitors. GRAPHICS 8 uses a blue background instead of a black background to reduce the effects of these "false" colors (known as artifact colors). Some clever software designers have taken advantage of these colors for their programs. Let's turn the screen black and take a closer look. Type in and run this program:

10 GRAPHICS 8 : POKE 710, 0
15 COLOR 1
20 FOR I = 0 TO 20 STEP 2
30 PLOT I, 0 : DRAWTO I, 20
50 FOR I = 21 TO 41 STEP 2
60 PLOT I, 0 : DRAWTO I, 20

On our 130XE, the left patch of color is red and the right patch is blue. This may be reversed in your computer, depending on when it was made. A white line divides the two patches. White occurs anytime there are two dots plotted next to each other horizontally. To avoid artifact colors, plot an extra point directly to the right (or left) of any point you plot on a GRAPHICS 8 screen.