YJK[1][2][3] is a proprietary color space implemented by the Yamaha V9958[4][5][6] graphic chip on MSX2+ computers.[7][8] It has the advantage of encoding images by implementing less resolution for color information than for brightness, taking advantage of the human visual systems' lower acuity for color differences.[9] This saves memory, transmission and computing power.

YJK is composed of three components: ${\displaystyle Y}$, ${\displaystyle J}$ and ${\displaystyle K}$. ${\displaystyle Y}$ is similar to luminance (but computed differently), ${\displaystyle J}$ and ${\displaystyle K}$ are the chrominance components (representing the red and green color differences). The ${\displaystyle Y}$ component is a 5-bit value (0 to 31), specified for each individual pixel.

The ${\displaystyle J}$ and ${\displaystyle K}$ components are stored together in 6 bits (-32 to 31) and shared between 4 nearby pixels (4:2:0 chroma sub-sampling).[10][11][12]

This arrangement allows for the encoding of 19,268 different colors.[10][11][12]

While conceptually similar to YUV, chroma sampling, numerical relationship between the components, and transformation to and from RGB are different in YJK.

## Formulas

The three component signals are created from an original RGB (red, green and blue) source. The weighted values of ${\displaystyle R}$, ${\displaystyle G}$ and ${\displaystyle B}$ are added together to produce a single ${\displaystyle Y}$ signal, representing the overall brightness of that pixel. The ${\displaystyle J}$ signal is then created by subtracting the ${\displaystyle Y}$ from the red signal of the original RGB, and then scaling; and ${\displaystyle K}$ by subtracting the ${\displaystyle Y}$ from the green, and then scaling by a different factor.

These formulae approximate the conversion between the RGB color space and YJK:[7]

From RGB to YJK:

${\displaystyle Y=B/2+R/4+G/8}$
${\displaystyle J=R-Y}$
${\displaystyle K=G-Y}$

From YJK to RGB:

${\displaystyle R=Y+J}$
${\displaystyle G=Y+K}$
${\displaystyle B=(5/4)Y-J/2-K/4}$

You may note that the ${\displaystyle Y}$ component of YJK is not true luminance, since the green component has less weight than the blue component.[13] Also, contrary to YUV where chrominance is based on Red-Blue differences, on YJK its calculated based on Red-Green differences.[10]

## References

1. ^ MSX Licensing Corporation (2022). "The YJK screen modes". MSX Assembly Page.
2. ^ Niemietz, Ricardo Cancho (2014). Issues on YJK colour model implemented in Yamaha V9958 VDP chip (PDF).
3. ^ "VCFe Vortrag vom 2016.04.30 - Homecomputer und Spielkonsolen - Videoarchitekturen als visuelles Medium". neil.franklin.ch. Retrieved 2022-11-13.
4. ^ IC Master. United Technical Publications. 2001.
5. ^ Martín Sesma, Sergio (2016-10-03). Arqueología informática: los ordenadores MSX en los inicios de la microinformática doméstica (Proyecto/Trabajo fin de carrera/grado thesis). Universitat Politècnica de València.
6. ^ Redazione (2008-10-20). "MSX - Vari Costruttori- 1983". CyberLudus.com (in Italian). Retrieved 2022-11-13.
7. ^ a b "V9958 MSX-VIDEO TECHNICAL DATA BOOK" (PDF). 1988.
8. ^ Alex, Wulms (1995). "Schermen op MSX - De 2+ schermen" (PDF). MSX Computer & Club Magazine (72).
9. ^ S. Winkler, C. J. van den Branden Lambrecht, and M. Kunt (2001). "Vision and Video: Models and Applications". In Christian J. van den Branden Lambrecht (ed.). Vision models and applications to image and video processing. Springer. p. 209. ISBN 978-0-7923-7422-0.`((cite book))`: CS1 maint: multiple names: authors list (link)
10. ^ a b c "The YJK screen modes". map.grauw.nl.
11. ^ a b Silveira, Marcelo (2017). MSX 2+ Colors (PDF).
12. ^ a b Nunes, Giovanni (8 June 2015). "Edição de imagens num MSX2+/MSX turbo R". retropolis.com.br.
13. ^ Chancho Niemietz, Ricardo (2014). "Issues on YJK colour model implemented in Yamaha V9958 VDP chip" (PDF).