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YJK

Color space implemented by the Yamaha V9958 graphic chip From Wikipedia, the free encyclopedia

YJK
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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.

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YJK converted image
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Cropped and zoomed comparison between the original true color image (left) and the YJK (right) version.

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

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Original full color image

The and 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.

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Formulas

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The three component signals are created from an original RGB (red, green and blue) source. The weighted values of , and are added together to produce a single signal, representing the overall brightness of that pixel. The signal is then created by subtracting the from the red signal of the original RGB, and then scaling; and by subtracting the 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:

From YJK to RGB:

The 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]

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