YJK

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[edit]

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[edit]

See also[edit]

• YUV
• MSX2+