Software-driven graphics modes for the Atari 8-bit computers

This article describes software driven graphics modes for the Atari 8-bit computers; that is, pseudo-graphics modes whose capabilities are reliant on additional software rather than being supported directly in hardware.

Any Point, Any Color

The "APAC System", or "Any Point, Any Color" was a software-driven display mode capable of displaying an image using all 256 of the Atari's possible colors. By taking 80×192 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 80×96 or 80×192 pixels in 256 colors could be perceived.

APAC was created in early 1987 and later introduced in the magazine A.N.A.L.O.G. Computing, Issue #60, May 1988 in an article by Tom Tanida. The source code was written in 6502 assembly language.

APAC used a Display List Interrupt, or DLI, after each line of the screen was drawn to alternate between GTIA Graphics Mode 9 (15 hues) and 11 (15 shades of grey) of the GTIA chip. The hues and luminances would blend together on the screen (usually a television) to create the effect of a palette of 256 visible colors, with the artifact of a thinner, horizontal blank line in between each visible line.

APAC used a very basic API consisting of four functions:

* Init, used to place the computer into the APAC mode
* Exit, used to exit the APAC mode
* Plot, used to place a point of a specified color on the screen
* Draw, used to draw a line between the last plotted point to the given point

A second article for an "APAC-II" mode was hinted at in the original article. This mode would have alternated the GTIA 9 and 11 modes during a vertical blank interrupt, or VBI. The article was neither completed nor published.

APACView

In 1992, Jeff D. Potter created a GIF decoder and image viewer for the Atari called "APACView".

ColrView mode

Later, Potter 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 80×192 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 (12 bpp) at 80×192, with slight 'rolling' artifacts in solid red, green or blue fields in the image. This was called "ColrView" mode.

FlickerTerm80

In 1994, Clay Halliwell created a modem terminal program for the Atari ("FlickerTerm80") which uses 40×24 text mode, combined with two character sets with an identical 4×8 font — one with the pixels on the left half of the 8×8 grid, the other on the right. By altering where in memory ANTIC looks for graphics, and which font to display, an 80×24 character screen can be displayed. It uses less memory (about 2 KB) and can be more quickly manipulated, compared to rendering 80×24 characters using a 320×192 bitmap mode (which would require about 8 KB).

uperIRG

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 4×8 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 4×8 multi-colored text on the Atari is called IRG.)

HARD Interlacing Picture

In 1996, Atari demo coders HARD Software from Hungary created "HARD Interlacing Picture" (HIP), which can display 160×192 pixels in 30 shades of grey. It interlaces two modes — 80×192 with 16 shades of grey, 80×192 with 9 paletted colors — and utilizes a bug in the GTIA chip that causes one of the modes to be shifted ½ pixel, allowing for a perceived 160 pixels across.

Later, other demo coders created RIP graphics mode, which is similar to HIP, but can display 160×192 pixels in color.

External links

* [http://www.atariarchives.org/dere/chapt05.php De Re Atari chapter on Display List Interrupts]