YJK is a proprietary color space implemented by the Yamaha V9958 graphic chip on MSX2+ computers.

YJK is composed of three components: $Y$ , $J$ and $K$ .

$Y$ is similar to luminance (but computed differently), $J$ and $K$ are the chrominance components (representing the red and green color differences). The $Y$ component is a 5-bit value (0 to 31), specified for each individual pixel. The $J$ and $K$ components are stored together in 6 bits (-32 to 31) and shared between 4 nearby pixels (chroma sub-sampling).

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

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

From RGB to YJK:

$Y=B/2+R/4+G/8$ $J=R-Y$ $K=G-Y$ From YJK to RGB:

$R=Y+J$ $G=Y+K$ $B=(5/4)Y-J/2-K/4$ You may note that the $Y$ component of YJK is not true luminance, since the green component has less weight than the blue component. Also, contrary to YUV where chrominance is based on Red-Blue differences, on YJK its calculated based on Red-Green differences.

1. ^ a b "V9958 MSX-VIDEO TECHNICAL DATA BOOK" (PDF). 1988.
2. ^ a b "The YJK screen modes". map.grauw.nl.
3. ^ Chancho Niemietz, Ricardo (2014). "Issues on YJK colour model implemented in Yamaha V9958 VDP chip" (PDF).