Atari 8-bit family

Atari built a series of 8-bit home computers based on the MOS Technology 6502 CPU, starting in 1979. Over the next decade several versions of the same basic design would be released, but the models remained largely identical internally. They were, for their era, one of the most technically advanced machines on the market, but a combination of factors, largely business related, meant they did not have a major market when the Commodore 64 shipped a few years later and took over most of the market.

Graphics capabilities

Standard modes

While the ANTIC and GTIA chips allowed a variety of graphics modes to be combined, and different playfield widths to be used, the Atari's Operating System provided a basic set of graphics modes. In most cases, these were exposed to Atari BASIC via the "GRAPHICS" command, and to some other languages, via similar system calls.

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• 40x24 text modes
• 1 color of text, with each character's 8x8 pixels the same size as those in 320x192 graphics mode, with the same hue restriction. Characters with the high-bit on were represented in inverse-video.
• "Lowercase with descenders" mode, which was not available through GRAPHICS, only as part of custom display lists. In this mode characters were 10 pixels high and occupied either the upper or lower 8 pixels of that height. This was not strictly speaking a 40x24 text mode, because of the unusual height.
• Colored text, where every two bits represents a colored pixel (characters were 4x8 pixels that were the same size as those in 160x192 graphics mode). Characters with the high-bit on were displayed using a 5th color palette registered where the 4th would normally be used.
• Colored text, where every four bits represents a colored pixel (characters were 2x8 pixels that were the same size as those in 80x192 graphics mode and had the same color limitations). This mode was not directly available through GRAPHICS but required setting GTIA flags in text mode.
• 20x24 text mode
• 1 color of text, with each character's 8x8 pixels the same size as those in 160x192 graphics mode. Characters with various bits enabled or disabled (which would normally appear as 'control-characters', lower-case characters, or inverse-video) were displayed with different colored pixels.
• 20x12 text mode
• (Same as 20x24 text mode, but with larger pixels and fewer rows of text)
• 40x24 graphics mode -- 4 colors (2bpp)
• 80x48 graphics modes -- Either 2 colors (1bpp), or 4 colors (2bpp)
• 160x96 graphics modes -- Either 2 colors, or 4 colors
• 160x192 graphics modes -- Either 2 colors, or 4 colors
• 320x192 graphics mode -- 2 colors (1bpp). The pixels were a shade of the playfield color, and could not be different hue.
• 80x192 graphics modes (GTIA chip only)
• 9 colors from the color palette registers
• All 15 Atari hues, but only of one brightness (plus black)
• All 16 Atari shades, but only of one hue

Software-driven modes

In 1992, Jeff D. Potter created a GIF decoder and image viewer for the Atari called APACView. APAC, or Any Point, Any Color, was a software-driven method of display an image using all 256 of the Atari's possible colors. By taking 80x192 mode lines that displayed 16 hues, and those that displayed 16 shades, and either interlacing rows of them, quickly alternating between rows of them, or both, a screen displaying 80x96 or 80x192 pixels in 256 colors could be perceived. (This mode may have been invented by someone earlier.)

Related Topics:
1992 - Jeff D. Potter - GIF - Any Point, Any Color

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Later, Jeff created another GIF decoder, and later a JPEG decoder was created, which broke an image into the three red, green and blue channels. 16 shades of each, at 80x192 pixels, would be displayed in an interlaced and flickering fashion. The human eye's persistence of vision would allow the viewer to see 4096 colors (12bpp) at 80x192, with slight 'rolling' artifacts in solid red, green or blue fields in the image. This was called ColorView mode.

Related Topics:
GIF - JPEG - Persistence of vision - ColorView

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In 1994, Clay Halliwell created a modem terminal program for the Atari (FlickerTerm80) which uses 40x24 text mode, combined with two character sets with an identical 4x8 font -- one with the pixels on the left half of the 8x8 grid, the other on the right. By altering where in memory ANTIC looks for graphics, and which font to display, an 80x24 character screen can be displayed. It uses less memory (about 2KB) and can be more quickly manipulated, compared to rendering 80x24 characters using a 320x192 bitmap mode (which would require about 8KB).

Related Topics:
1994 - Clay Halliwell - Font

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In 1998, Bill Kendrick created a puzzle video game for the Atari (Gem Drop) which utilized a similar effect, but by using two alternating character sets (fonts) in colored text. (Each character is 4x8 pixels, each pixel being one of 4 colors.) No color palette changes occurred, and ANTIC's Display List wasn't altered -- only a vertical blank interrupt was used to change the character set. This allowed for approximately 13 colors on the screen. Solid color fields that were based on two actual colors (e.g., dark red created by flickering between red and black) had less artifacting because they could be drawn in a checkerboard fashion. This mode was called SuperIRG. (Normal 4x8 multi-colored text on the Atari is called IRG.)

Related Topics:
1998 - Font - Display List - Vertical blank interrupt - Checkerboard - SuperIRG - IRG

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In 1996, Atari demo coders HARD Software from Hungary created HARD Interlacing Picture (HIP), which can display 160x192 pixels in 30 shades of grey. It interlaces two modes -- 80x192 with 16 shades of grey, 80x192 with 9 paletted colors -- and utilizes a bug in the GTIA chip that causes one of the modes to be shifted 1/2 pixel, allowing for a perceived 160 pixels across.

Related Topics:
1996 - HARD Software - Hungary - HARD Interlacing Picture

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Later, other demo coders created RIP graphics mode, which is similar to HIP, but can display 160x192 pixels in color.

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