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There are color models which do not express a color by the additive basic colors, red, green and blue (RGB), but by other
properties, for example, the brightness-color model. Here, the criteria are the basic brightness of the colors (from black,
through gray, to white), the colors with the largest portion (red, orange, yellow, green, blue, violet, or other pure colors
that lie between them) and the saturation of the colors (‘gaudy’ to pale). This color model is based on the ability of the
eye to recognize small differences in luminosity better than small color differences, and to recognize those better than
small differences in saturation. That makes gray text written on a black background easy to read, but blue text on a red
background very hard to read, even with the same basic brightness. Such color models are called brightness-color models.
properties, for example, the brightness-color model. Here, the criteria are the basic brightness of the colors (from black,
through gray, to white), the colors with the largest portion (red, orange, yellow, green, blue, violet, or other pure colors
that lie between them) and the saturation of the colors (‘gaudy’ to pale). This color model is based on the ability of the
eye to recognize small differences in luminosity better than small color differences, and to recognize those better than
small differences in saturation. That makes gray text written on a black background easy to read, but blue text on a red
background very hard to read, even with the same basic brightness. Such color models are called brightness-color models.
The YCbCr model is a slight adaptation of such a brightness-color model. An RBG color value is divided into a basic
brightness, Y, and two components, Cb and Cr, where Cb is a measurement of the deviation from gray in the blue direction,
or if it is less that 0.5, in the direction of yellow. Cr is the corresponding measurement for the difference in the direction
of red or turquoise. This representation uses the peculiarity of the eye of being especially sensitive to green light. That is
why most of the information about the proportion of green is in the basic brightness, Y, an only the deviations for the red
and blue portions need to be represented. The Y values have twice the resolution of the other two values, Cb and Cr, in
most practical applications, such as on DVDs.
brightness, Y, and two components, Cb and Cr, where Cb is a measurement of the deviation from gray in the blue direction,
or if it is less that 0.5, in the direction of yellow. Cr is the corresponding measurement for the difference in the direction
of red or turquoise. This representation uses the peculiarity of the eye of being especially sensitive to green light. That is
why most of the information about the proportion of green is in the basic brightness, Y, an only the deviations for the red
and blue portions need to be represented. The Y values have twice the resolution of the other two values, Cb and Cr, in
most practical applications, such as on DVDs.
YUV
YUV is a
which uses two components to represent the color information, luma (the strength of the light per
area) and the chrominance, or proportion of color (chroma), where the chrominance again consists of two components. The
development of the YUV color model also goes back to the development of color television (PAL), where ways were sought
for transmitting the color information along with the black-and-white signal, in order to achieve backwards compatibility
with old black and white televisions without having to increase the available transmission bandwidth. From the YUV color
model of the analog television techiques, the YCrCb color model was developed, which is used for most kinds of digital
image and video compression. Erroneously, the YUV color model is also often spoken about in those fields, although the
YCbCr model is actually used. This often causes confusion.
development of the YUV color model also goes back to the development of color television (PAL), where ways were sought
for transmitting the color information along with the black-and-white signal, in order to achieve backwards compatibility
with old black and white televisions without having to increase the available transmission bandwidth. From the YUV color
model of the analog television techiques, the YCrCb color model was developed, which is used for most kinds of digital
image and video compression. Erroneously, the YUV color model is also often spoken about in those fields, although the
YCbCr model is actually used. This often causes confusion.
For the calculation of the luma signals, the underlying RGB data is first adjusted with the
value of the output
device, and an R’G’B’ signal is obtained. The three individual components are added together with different weights, to
form the brightness information, which also functions as the VBS signal (Video Baseband Signal, the black-and-white
signal) for the old black and white televisions.
form the brightness information, which also functions as the VBS signal (Video Baseband Signal, the black-and-white
signal) for the old black and white televisions.
Y=R+G+B
The exact calculation is more complicated, however, since some aspects of the color perception of the human eye have to
be taken into account. For example, green is perceived to be lighter than red, and this is perceived to be lighter than blue.
Furthermore, in some systems gamma correction of the basic color is first performed.
be taken into account. For example, green is perceived to be lighter than red, and this is perceived to be lighter than blue.
Furthermore, in some systems gamma correction of the basic color is first performed.
The chrominance signals, and the color difference signals also, contain the color information. They are formed by the
difference of blue minus luma or red minus luma.
difference of blue minus luma or red minus luma.
U=B-Y
V=R-Y
From the three generated components, Y, U and V, the individual color proportions of the basic color can be calculated
again later:
again later:
Y + U = Y + ( B - Y ) = Y - Y + B = B
Y + V = Y + ( R - Y ) = Y - Y + R = R
Y - B - R = ( R + G + B ) - B - R = G
Furthermore, because of the structure of the retina of the human eye, it turns out that the brightness information is perceived
at a higher resolution than the color, so that many formats based on the YUV color model compress the chrominance to
save bandwidth during transmission.
at a higher resolution than the color, so that many formats based on the YUV color model compress the chrominance to
save bandwidth during transmission.