YJK is a proprietary

color space A color space is a specific organization of colors. In combination with color profiling supported by various physical devices, it supports reproducible representations of colorwhether such representation entails an analog or a digital represent ...

implemented by the

Yamaha V9958 The Yamaha V9958 is a Video Display Processor used in the MSX2+ and MSX turbo R series of home computers, as the successor to the Yamaha V9938 used in the MSX2. The main new features are three graphical YJK modes with up to 19268 colors and hori ...

graphic chip on

MSX2+ MSX is a standardized home computer architecture, announced by Microsoft and ASCII Corporation on June 16, 1983. It was initially conceived by Microsoft as a product for the Eastern sector, and jointly marketed by Kazuhiko Nishi, then vice-p ...

computers. 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. This saves memory, transmission and computing power. YJK is composed of three components:

Y

J

and

K

Y

is similar to

luminance Luminance is a photometric measure of the luminous intensity per unit area of light travelling in a given direction. It describes the amount of light that passes through, is emitted from, or is reflected from a particular area, and falls withi ...

(but computed differently),

J

and

K

are the

chrominance Chrominance (''chroma'' or ''C'' for short) is the signal used in video systems to convey the color information of the picture (see YUV color model), separately from the accompanying luma signal (or Y' for short). Chrominance is usually represente ...

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 (4:2:0 chroma sub-sampling). While conceptually similar to

YUV YUV is a color model typically used as part of a color image pipeline. It encodes a color image or video taking human perception into account, allowing reduced bandwidth for chrominance components, compared to a "direct" RGB-representation. His ...

, chroma sampling, numerical relationship between the components, and transformation to and from

RGB The RGB color model is an additive color model in which the red, green and blue primary colors of light are added together in various ways to reproduce a broad array of colors. The name of the model comes from the initials of the three additiv ...

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

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.

Formulas

References

See also