YCbCr
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 analog component video,
YCbCr is often called YPbPr, although the term YCbCr is commonly used for
both systems.
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 (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.
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.
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:
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.
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:
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.
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).
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.
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.
JPEG allows YCbCr where Y', Cb and Cr have the full 256 values:
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.
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "YCbCr".
