Any Point, Any Color
The APAC System, or Any Point, Any Color was a software-driven display mode for the Atari 8-bit computer. It was originally 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.