Editing Video Graphics Array (section)

==Color palette==
{{See also|List of monochrome and RGB color formats|List of monochrome and RGB color formats#18-bit RGB|l2=18-bit RGB|List of 16-bit computer color palettes|List of 16-bit computer hardware palettes#MCGA and VGA|l4=MCGA and VGA}}
[[file:VGA palette with black borders.svg|thumb|VGA 256 default color palette]]
[[file:VGA palette organised.svg|thumb|VGA palette organised into 4 groups]]
[[file:Torak.gif|thumb|Examples of VGA images in 640×480 with 16 colors and 320×200 with 256 colors (bottom). [[Dither]]ing is used to mask color limitations.]]
The VGA color system uses register-based palettes to map colors in various bit depths to its 18-bit output gamut. It is [[backward compatible]] with the EGA and CGA adapters, but supports extra [[Bit depth (computer graphics)|bit depth]] for the palette when in these modes.

For instance, when in EGA 16-color modes, VGA offers 16 palette registers, and in 256-color modes, it offers 256 registers.<ref>{{Cite web|title=VGA/SVGA Video Programming--Color Regsters|url=http://www.scs.stanford.edu/17wi-cs140/pintos/specs/freevga/vga/colorreg.htm|access-date=2020-08-16|website=www.scs.stanford.edu}}</ref> Each palette register contain a [[List of monochrome and RGB color formats#18-bit RGB|3×6 bit]] RGB value, selecting a color from the 18-bit gamut of the [[Digital-to-analog converter|DAC]].

These color registers are initialized to default values IBM expected to be most useful for each mode. For instance, EGA 16-color modes initialize to the default CGA 16-color palette, and the 256-color mode initializes to a palette consisting of 16 CGA colors, 16 grey shades, and then 216 colors chosen by IBM to fit expected use cases.<ref>{{Cite book|url=http://classiccomputers.info/down/IBM_PS2/documents/PS2_Hardware_Interface_Technical_Reference_May88.pdf|title=IBM PS/2 Hardware Interface Technical Reference Manual|pages=13–18}}</ref><ref>[https://retrocomputing.stackexchange.com/questions/27994/why-were-those-colors-chosen-to-be-the-default-palette-for-256-color-vga retrocomputing SE question: why-were-those-colors-chosen-to-be-the-default-palette-for-256-color-vga ?]</ref>
After initialization they can be redefined at any time without altering the contents of video RAM, permitting [[Color cycling|palette cycling]].

In the 256-color modes, the DAC is set to combine four 2-bit color values, one from each plane, into an 8-bit-value representing an index into the 256-color palette. The CPU interface combines the 4 planes in the same way, a feature called "chain-4", so that each pixel appears to the CPU as a packed 8-bit value representing the palette index.<ref>Uphoff, Matthias (1990). ''Die Programmierung der EGA/VGA Grafikkarte''; {{ISBN|3-89319-274-3}}; this whole section was learned from this book</ref>