Composite artifact colors

Composite artifact colors is a designation commonly used to address several graphic modes of some 1970s and 1980s home computers. With some machines, when connected to an NTSC TV or monitor over composite video outputs, the video signal encoding allowed for extra colors to be displayed, by manipulating the pixel position on screen, not being limited by each machine's hardware color palette.

This mode was used mainly for games, since it limited the display's horizontal resolution more than normal. It was mostly used on the IBM PC (with CGA graphics),TRS-80 Color Computer and Apple II computers, but also possible on Atari 8-bit.

The limitations of composite video regarding horizontal resolution were also exploited on other systems. Adjacent pixel values got averaged horizontally, producing solid colors or generating transparency effects. On PAL displays this effect doesn't generate extra colors, as PAL color is limited to a mix of the original pixel values. If a higher resolution video connection is used, the graphics are displayed as dither patterns. Machines such as the ZX Spectrum or Mega Drive took advantage of this situation.

When using IBM's Color Graphics Adapter (CGA) with NTSC TV-out the separation between luminance and chrominance is imperfect, yielding cross-color artifacts, or color "smearing". This is especially a problem with 80-column text.

It is for this reason that each of the text and graphics modes described above exists twice: Once as the normal "color" version and once as a "monochrome" version. The "monochrome" version of each mode turns off the NTSC color decoding in the viewing monitor completely, resulting in a black-and-white picture, but also no color bleeding, hence, a sharper picture. On RGBI monitors, the two versions of each mode are identical, with the exception of the 320x200 graphics mode, where the "monochrome" version produces the third palette, as described above.

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