A colour image and the Y, Cb and Cr elements of it. Note that the Y image is essentially a greyscale copy of the main image; that the white snow is represented as a middle value in both Cr and Cb; that the brown barn is represented by weak Cb and strong Cr; that the green grass is represented by weak Cb and weak Cr; and that the blue sky is represented by strong Cb and weak Cr.

YCbCr is a family of color spaces used in video systems. Y is the luma component and Cb and Cr are the blue and red chroma components. It is often confused with the YUV color space, and typically the terms YCbCr and YUV are used interchangeably, leading to some confusion. In fact, when referring to signals in digital form, the term "YUV" probably really means "YCbCr" more often than not. YCbCr is sometimes abbreviated to YCC. When used for analogue component video, YCbCr is often called YPbPr, although the term YCbCr is commonly used for both systems. Image File history File links Size of this preview: 200 × 598 pixel Image in higher resolution (1600 × 4780 pixel, file size: 825 KB, MIME type: image/jpeg) This takes an image ( Image:Barns grand tetons. ... Image File history File links Size of this preview: 200 × 598 pixel Image in higher resolution (1600 × 4780 pixel, file size: 825 KB, MIME type: image/jpeg) This takes an image ( Image:Barns grand tetons. ... A color model is an abstract mathematical model describing the way colors can be represented as tuples of numbers, typically as three or four values or color components (e. ... Video (Latin for I see, first person singular present, indicative of videre, to see) is the technology of electronically capturing, recording, processing, storing, transmitting, and reconstructing a sequence of still images representing scenes in motion. ... As applied to analog television signals, two different words are used, luminance and luma, meaning two different things. ... Chrominance (chroma for short) comprises the two components of a television signal that encode color information. ... Example of U-V color plane, Y value = 0. ... YPbPr (also referred to as YPrPb, PrPbY, and PbPrY) is a color space used in video electronics. ...

YCbCr signals (prior to scaling and offsets to place the signals into digital form) are called YPbPr, and are created from the corresponding gamma-adjusted RGB (red, green and blue) source using two defined constants Kb and Kr as follows: YPbPr (also referred to as YPrPb, PrPbY, and PbPrY) is a color space used in video electronics. ... The RGB color model utilizes the additive model in which red, green, and blue light are combined in various ways to create other colors. ...

 YPbPr (analog version of YCbCr) from R'G'B' ==================================================== Y' = Kr * R' + (1 - Kr - Kb) * G' + Kb * B' Pb = 0.5 * (B' - Y') / (1 - Kb) Pr = 0.5 * (R' - Y') / (1 - Kr) .................................................... R', G', B' in [0; 1] Y' in [0; 1] Pb in [-0.5; 0.5] Pr in [-0.5; 0.5] 

where Kb and Kr are ordinarily derived from the definition of the corresponding RGB space. Here, the prime (') symbols mean gamma correction is being used; thus R', G' and B' and to nominally range from 0 to 1, with 0 representing the minimum intensity (e.g., for display of the color black) and 1 the maximum (e.g., for display of the color white). The resulting luma (Y) value will then have a nominal range from 0 to 1, and the chroma (Cb and Cr) values will have a nominal range from -0.5 to +0.5. The reverse conversion process can be readily derived by inverting the above equations. The RGB color model utilizes the additive model in which red, green, and blue light are combined in various ways to create other colors. ... Gamma correction is the name of an internal adjustment made in the rendering of images through photography, television, and computer imaging. ... Black cat, thought by some to cause bad luck Black is both a color and the shade of objects that do not reflect light in any part of the visible spectrum. ... White rose. ...

When representing the signals in digital form, the results are scaled and rounded, and offsets are typically added. For example, the scaling and offset applied to the Y' component per specification results in the value of 16 for black and the value of 235 for white when using an 8-bit representation. The standard has 8 bit digitized versions of Cb and Cr scaled to a different range of 16 to 240. This arbitrary "footprint" forces rescaling by the fraction 219/224 (235-16/240-16) when doing color matrixing or processing in Y Cb Cr space. This results in unnecessary quantization distortions which a choice of identical ranges would have rationalized.

The scaling that results in the use of a smaller range of digital values than what might appear to be desirable for representation of the nominal range of the input data allows for some "overshoot" and "undershoot" during processing without necessitating undesirable clipping. The term "head-room" and "toe-room" has also been proposed to be used for extension of the nominal color gamut. Look up clipping in Wiktionary, the free dictionary. ... In computer graphics, the gamut, or color gamut (pronounced ), is a certain complete subset of colors. ...

The form of YCbCr that was defined for standard-definition television use in the ITU-R BT.601 (formerly CCIR 601) standard for use with digital component video is derived from the corresponding RGB space as follows: Standard-definition television or SDTV refers to television systems that have a lower resolution than HDTV systems. ... The ITU Radiocommunication Sector (ITU-R) is a standards body subcommittee of the International Telecommunication Union (ITU) relating to radio communication. ... CCIR 601 is the old name of a standard published by the CCIR (now ITU-R) for encoding interlaced analogue video signals in digital form. ... Three cables, each with RCA plugs at both ends, are often used to carry analog component video Component video is a video signal that has been split into two or more components. ...

 Kb = 0.114 Kr = 0.299 

From the above constants and formulas, the following can be derived for ITU-R BT.601: Firstly analog YPbPr:

 YPbPr (ITU-R BT.601) ======================================================== Y' = + 0.299 * R' + 0.587 * G' + 0.114 * B' Pb = - 0.168736 * R' - 0.331264 * G' + 0.5 * B' Pr = + 0.5 * R' - 0.418688 * G' - 0.081312 * B' ........................................................ R', G', B' in [0; 1] Y' in [0; 1] Pb in [-0.5; 0.5] Pr in [-0.5; 0.5] 

Then this is digitized for YCbCr:

 YCbCr (601) from R'G'B' ======================================================== Y' = 16 + 65.481 * R' + 128.553 * G' + 24.966 * B' Cb = 128 - 37.797 * R' - 74.203 * G' + 112.0 * B' Cr = 128 + 112.0 * R' - 93.786 * G' - 18.214 * B' ........................................................ R', G', B' in [0; 1] Y' in {16, 17, ..., 235} with footroom in {1, 2, ..., 15} headroom in {236, 237, ..., 254} sync. in {0, 255} Cb, Cr in {16, 17, ..., 240} 

If R', G' and B' are given with 8 bit digital precision, then

 YCbCr (601) from "digital 8-bit R'G'B' " ======================================================================== Y' = 16 + 1/256 * ( 65.738 * R'd + 129.057 * G'd + 25.064 * B'd) Cb = 128 + 1/256 * ( - 37.945 * R'd - 74.494 * G'd + 112.439 * B'd) Cr = 128 + 1/256 * ( 112.439 * R'd - 94.154 * G'd - 18.285 * B'd) ........................................................................ R'd, G'd, B'd in {0, 1, 2, ..., 255} Y' in {16, 17, ..., 235} with footroom in {1, 2, ..., 15} headroom in {236, 237, ..., 254} sync. in {0, 255} Cb, Cr in {16, 17, ..., 240} 

This form of YCbCr is used primarily for older standard-definition television systems, as it uses an RGB model that fits the phosphor emission characteristics of older CRTs. Standard-definition television or SDTV refers to television systems that have a lower resolution than HDTV systems. ... Cathode ray tube employing electromagnetic focus and deflection Cutaway rendering of a color CRT Electron guns Electron beams Focusing coils Deflection coils Anode connection Mask for separating beams for red, green, and blue part of displayed image Phosphor layer with red, green, and blue zones Close-up of the phosphor...

A different form of YCbCr is specified in the ITU-R BT.709 standard, primarily for HDTV use. The newer form is also used in some computer-display oriented applications. In this case, the values of Kb and Kr differ, but the formulas for using them are the same. For ITU-R BT.709, the constants are: The ITU Radiocommunication Sector (ITU-R) is a standards body subcommittee of the International Telecommunication Union (ITU) relating to radio communication. ... Projection screen in a home theater, displaying a high-definition television image. ... The ITU Radiocommunication Sector (ITU-R) is a standards body subcommittee of the International Telecommunication Union (ITU) relating to radio communication. ...

 Kb = 0.0722 Kr = 0.2126 

This form of YCbCr is based on an RGB model that more closely fits the phosphor emission characteristics of newer CRTs and other modern display equipment. Cathode ray tube employing electromagnetic focus and deflection Cutaway rendering of a color CRT Electron guns Electron beams Focusing coils Deflection coils Anode connection Mask for separating beams for red, green, and blue part of displayed image Phosphor layer with red, green, and blue zones Close-up of the phosphor...

Note that the definitions of the R', G', and B' color primary signals also differ between BT.601 and BT.709. So proper conversion of YCbCr from one form to the other is not just a matter of inverting one matrix and applying the other. In fact, when YCbCr is used correctly, the values of Kb and Kr are derived from the precise specification of the RGB color primary signals, so that the luma (Y') signal corresponds as closely as possible to a gamma-adjusted measurement of luminance (typically based on the CIE 1931 measurements of the response of the human visual system to color stimuli). Gamma correction is the name of an internal adjustment made in the rendering of images through photography, television, and computer imaging. ... Luminance (also called luminosity) is a photometric measure of the density of luminous intensity in a given direction. ... The International Commission on Illumination (usually known as the CIE for its French-language name Commission Internationale de lEclairage) is the international authority on light, illumination, colour, and colour spaces. ...

Although the above two forms of YCbCr are the dominant ones, some other variants exist. For example, the SMPTE 240M standard specifies YCbCr using Kb = 0.087 and Kr = 0.212. The Society of Motion Picture and Television Engineers or SMPTE (pronounced simptee or sometimes sumptee) is a US professional association of engineers. ...

Since the equations defining YCbCr are formed in a way that rotates the entire nominal RGB color cube and scales it to fit within a (larger) YCbCr color cube, there are some points within the YCbCr color cube that cannot be represented in the corresponding RGB domain (at least not within the nominal RGB range). This causes some difficulty in determining how to correctly interpret and display some YCbCr signals. The RGB color model utilizes the additive model in which red, green, and blue light are combined in various ways to create other colors. ... The RGB color model utilizes the additive model in which red, green, and blue light are combined in various ways to create other colors. ...

JPEG allows YCbCr where Y', Cb and Cr have the full 256 values^[1]: In computing, JPEG (pronounced JAY-peg; IPA: ) is a commonly used standard method of compression for photographic images. ...

 JPEG-YCbCr (601) from "digital 8-bit R'G'B' " ======================================================================== Y' = + 0.299 * R'd + 0.587 * G'd + 0.114 * B'd Cb = 128 - 0.168736 * R'd - 0.331264 * G'd + 0.5 * B'd Cr = 128 + 0.5 * R'd - 0.418688 * G'd - 0.081312 * B'd ........................................................................ R'd, G'd, B'd in {0, 1, 2, ..., 255} Y', Cb, Cr in {0, 1, 2, ..., 255} 

YCbCr is not an absolute color space. It is a way of encoding RGB information, and the actual color displayed depends on the actual RGB colorants used to display the signal. Therefore a value expressed as YCbCr is only predictable if standard RGB colorants are used, or if an ICC profile is attached or implied which is used to translate value for the colorants in use. An absolute color space is a color space in which colors are unambiguous, where they do not depend on any external factors. ...

References

1. ^ JPEG File Interchange Format Version 1.02

External links

• Charles Poynton - Color FAQ
• Charles Poynton - Video engineering