ANSI from Hell - NicheInterests/mistfunk GitHub Wiki

ANSI from Hell is a technique invented by PC video game developers Macrocom in the mid-'80s (presumably under a different name -- this Satanic label is courtesy of [Trixter] of [Hornet], who used the technique to strong effect in the [8088 MPH] demo) to use ANSI colour and [textmode art] to fool an early [IBM PC] with only [CGA] graphics capabilities into displaying more colourful images than it is supposed to be able to in its [high-resolution graphics] mode. The fooling is in using [textmode] graphics to pass for high resolution graphics, not an easy feat to pull off.

In a nutshell, the technique involves drawing only the top row of pixels in a line of ANSI-coloured textmode art, which allows each character cell to feature one each of sixteen foreground and background colours. These character cells are typically 9 pixels across by 14 pixels tall, but a row of "ANSI from Hell" only displays the first of the fourteen rows. The appearance of high resolution graphics is imitated by cherry-picking two-colour pixel combinations found in the top rows offered by the characters found in the CP 437 character set -- or ?? possibly remapping the characters to provide the pixel combinations needed for your wider mosaic.

VileR describes it thoroughly, in passing, at https://int10h.org/blog/2015/04/cga-in-1024-colors-new-mode-illustrated/