View source for YJK

{{Short description|Color space implemented by the Yamaha V9958 graphic chip}}
[[File:Rgb vs YJK.png|300px|thumb|right|Cropped and zoomed comparison between the original true color image (left) and the YJK (right) version.]]
'''YJK<ref>{{Cite web |last=MSX Licensing Corporation |date=2022 |title=The YJK screen modes |url=http://map.grauw.nl/articles/yjk/ |website=MSX Assembly Page}}</ref><ref>{{Cite book |last=Niemietz |first=Ricardo Cancho |url=http://rs.gr8bit.ru/Documentation/Issues-on-YJK-colour-model-implemented-in-Yamaha-V9958-VDP-chip.pdf |title=Issues on YJK colour model implemented in Yamaha V9958 VDP chip |year=2014}}</ref><ref>{{Cite web |title=VCFe Vortrag vom 2016.04.30 - Homecomputer und Spielkonsolen - Videoarchitekturen als visuelles Medium |url=http://neil.franklin.ch/Articles/20160430_VCFe_Video_als_Medium.html |access-date=2022-11-13 |website=neil.franklin.ch}}</ref>''' is a proprietary [[color space]] implemented by the [[Yamaha V9958]]<ref>{{Cite book |url=https://books.google.com/books?id=8RwSAQAAMAAJ&dq=%22Yamaha+V9958%22+-wikipedia&pg=PA3984 |title=IC Master |date=2001 |publisher=United Technical Publications |language=en}}</ref><ref>{{Cite thesis |title=Arqueología informática: los ordenadores MSX en los inicios de la microinformática doméstica |url=https://riunet.upv.es/handle/10251/70909 |publisher=Universitat Politècnica de València |date=2016-10-03 |degree=Proyecto/Trabajo fin de carrera/grado |first=Sergio |last=Martín Sesma|hdl=10251/70909 |hdl-access=free }}</ref><ref>{{Cite web |last=Redazione |date=2008-10-20 |title=MSX - Vari Costruttori- 1983 |url=https://www.cyberludus.com/2008/10/msx-vari-costruttori-1983/ |access-date=2022-11-13 |website=CyberLudus.com |language=it-IT}}</ref> graphic chip on [[MSX2+]] computers.<ref name="auto">{{Cite web |date=1988 |title=V9958 MSX-VIDEO TECHNICAL DATA BOOK |url=http://map.grauw.nl/resources/video/yamaha_v9958_ocr.pdf}}</ref><ref>{{Cite journal |last=Alex |first=Wulms |date=1995 |title=Schermen op MSX - De 2+ schermen |url=http://www.msxarchive.nl/pub/msx/mirrors/hanso/hwdoityourself/msxplus.pdf |journal=MSX Computer & Club Magazine |issue=72}}</ref> 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.<ref>{{cite book |author=S. Winkler, C. J. van den Branden Lambrecht, and M. Kunt |title=Vision models and applications to image and video processing |publisher=Springer |year=2001 |isbn=978-0-7923-7422-0 |editor=Christian J. van den Branden Lambrecht |page=209 |chapter=Vision and Video: Models and Applications |chapter-url=https://books.google.com/books?id=_s2BlAeR66YC&pg=PA209}}</ref> This saves memory, transmission and computing power.[[File:YJK example image.png|300px|thumb|right|YJK converted image]]'''YJK''' is composed of three components: <math>Y</math>, <math>J</math> and <math>K</math>. <math>Y</math> is similar to [[Luminance (video)|luminance]] (but computed differently), <math>J</math> and <math>K</math> are the [[chrominance]] components (representing the red and green color differences). The <math>Y</math> component is a 5-bit value (0 to 31), specified for each individual pixel. [[File:Rgb.png|300px|thumb|right|Original full color image]]The <math>J</math> and <math>K</math> components are stored together in 6 bits (-32 to 31) and shared between 4 nearby pixels (4:2:0 [[chroma sampling|chroma sub-sampling]]).<ref name="auto1">{{Cite web|url=http://map.grauw.nl/articles/yjk/|title=The YJK screen modes|website=map.grauw.nl}}</ref><ref name=":0">{{Cite book |last=Silveira |first=Marcelo |url=http://marmsx.msxall.com/artigos/sc12en.pdf |title=MSX 2+ Colors |year=2017}}</ref><ref name=":1">{{Cite web |last=Nunes |first=Giovanni |title=Edição de imagens num MSX2+/MSX turbo R |url=https://retropolis.com.br/2015/06/08/edicao-de-imagens-num-msx2msx-turbo-r/ |website=retropolis.com.br|date=8 June 2015 }}</ref>

This arrangement allows for the encoding of 19,268 different colors.<ref name="auto1" /><ref name=":0" /><ref name=":1" />

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 <math>R</math>, <math>G</math> and <math>B</math> are added together to produce a single <math>Y</math> signal, representing the overall brightness of that pixel. The <math>J</math> signal is then created by subtracting the <math>Y</math> from the red signal of the original RGB, and then scaling; and <math>K</math> by subtracting the <math>Y</math> from the green, and then scaling by a different factor.

These formulae approximate the conversion between the [[RGB]] color space and YJK:<ref name="auto"/>

'''From RGB to YJK:'''
:<math>Y = B/2 + R/4 + G/8 </math>
:<math>J = R-Y </math>
:<math>K=G-Y </math>

'''From YJK to RGB:'''
:<math>R =  Y + J </math>
:<math>G =  Y + K </math>
:<math>B= (5/4)Y -J/2 -K/4 </math>

The <math>Y</math> component of YJK is not true luminance, since the green component has less weight than the blue component.<ref>{{Cite web |last=Chancho Niemietz |first=Ricardo |date=2014 |title=Issues on YJK colour model implemented in Yamaha V9958 VDP chip |url=http://rs.gr8bit.ru/Documentation/Issues-on-YJK-colour-model-implemented-in-Yamaha-V9958-VDP-chip.pdf}}</ref> Also, contrary to YUV where chrominance is based on Red-Blue differences, on YJK its calculated based on Red-Green differences.<ref name="auto1"/>

==References==
{{reflist}}

==See also==
*[[YUV]]
*[[MSX2+]]

{{Color space}}

[[Category:Color space]]
[[Category:MSX hardware]]
[[Category:MSX2+ microcomputer]]