YCbCr

The YCbCr color model was developed for digital television according to the standard PAL, but is now also used in digital NTSC television. It is also used in the CCIR-601 standard for digital image and video recording, JPEG images, MPEG videos and thus also in DVDs, Video CDs, and most other digital video formats.

The YCbCr model divides the color information in the basic brightness, Y and two color components Cb ( Blue - Yellow Chrominance ) and Cr ( red-green chrominance ) on. With Y here the lightness axis is used from the CIE standard colorimetric system. It corresponds to the sensitivity of the eye, which is in the green spectral greatest (V- lambda curve). Chrominance or chroma short means variegation of color in general and in terms of brightness-color models.

General

Color models that do not specify a color locus as the RGB color space through three primary colors, but by the brightness and color, is called brightness-color models. The coordinates are about the basic brightness (from black to gray to white), the color with the largest share, and the saturation of the color (from " gaudy " to pale ).

As the YUV color model for analog color television, where the basic brightness corresponds to the pure black and white image, the YCbCr model is a pure brightness-color model. Comparing the two separations of the image example in YUV and YCbCr, it is evident that the brightness channel shows a completely achromatic gray scale image and that only the Chrominanzachsen U and V in the color table by cutting other shades as Cb and Cr, as the respective color chart proves it. Cb is a measure of the variation in color from gray toward blue / yellow, Cr is the corresponding measurement in the direction of red / turquoise.

Note that a color model is still no color space, because it does not determine which colors with "Red", "Green" and "Blue" are exactly meant. For a color space in addition, it still requires the cover to absolute color values. In addition, both color models from the similarly structured CIELAB color space differ in that brightness, chromaticities and color tones are not built perceptually equidistant.

Analogy to the sense of sight

The different perception of Y with respect to the Cb and Cr channels corresponds to the development of color and brightness distribution in nature. Much of the information stored in the brightness, the color varies hardly.

In the course of evolution, the human sense of sight has adapted to it. The eye can better detect small differences in brightness than small color differences, and these in turn better than small differences in saturation. So a text is written gray on black easy to read, but to red blue written very poorly with the same basic brightness.

As in any style similar to the visual perception of brightness-color system are opposite in YCbCr colors that are mutually exclusive. That is, a blue can never yellowish nuances (and vice versa) and a red never green or turquoise shades (and vice versa ) have. This structure corresponds to the processing of color information in the brain (opposite colors).

Color subsampling

The analogy to the human sense of sight is used for a great advantage of YCbCr: the color subsampling (English chroma subsampling ). In this case, the sampling rate and thus the amount of data of the chrominance channels Cr and Cb is reduced relative to the sampling rate of the luminance channel Y, without resulting in an appreciable reduction in quality. So you can save, for example, the JPEG compression 50% of the amount of data.

Conversion between RGB and YCbCr

Before the conversion, the values ​​for the RGB must with a gamma correction to the interval; are normalized [0 1]. The color triad formed is usually referred to the distinction in the literature with R'G'B '. The YCbCr values ​​are digitized to a specific resolution and if necessary, equipped with an offset to avoid negative values. Thus, before the YCbCr data can be determined are calculated from the R'G'B ' color data so-called YPbPr values. These values ​​of YPbPr are fixed to the value range [ 0, 1 ] for Y and [ -0.5; 0.5 ] for the two color components PbPr limited and subject to any limitation on the resolution, there are analog signals. Through a certain resolution, for example, 8 bits per value with a range of values ​​from {0, 1, ..., 255 }, the values ​​are calculated from the Y'CbCr Y'PbPr values ​​formed, respectively, over the range {0, 1, ..., 255 }. It can however also be Y'CbCr an image to 10 bits with a value range of {0, 1, ..., 1023 } take place. This resolution is used because of the higher color dynamics in the studio. The gamma correction of R'G'B ' affects only the luminance signal Y ' from. The notation should therefore be correctly Y'CbCr instead of YCbCr.

The following equations show the general calculation of Y'PbPr - value triples, without first taking a reference to a particular standard:

The constants occurring Kr and Kb are (formerly CCIR 601) defined in ITU- R BT 601 for television in standard definition SDTV.

Thus, they are dependent on this standard and with Kb = 0.114 and Kr = 0.299 fixed. The choice of this coefficient is determined from the display properties of the cathode ray tube. As ITU- RB.601 compliant conversion for Y'PbPr values ​​results in the following formulas.

These values ​​are digitized with a desired resolution and provided with an offset to YCbCr values ​​converted. The offset is necessary in order to keep clear of digital video data reserved values ​​such as 0 and 255 for specific control tasks (synchronization). The values ​​in the "back up" or "back down " are reserved for the transfer control of the video signals, and should not be used.

In the following overview the digitization for an 8 -bit dynamics is shown. The lower reserve is {1, 2, ..., 15 }, the top of { 236, 237, ..., 254 }, the synchronization values ​​of {0, 255 }. Y ' is therefore within the range of values ​​{ 16, 17, ..., 235 } and Cb and Cr in { 16, 17, ..., 240 }:

If the R'G'B data already digitally as R'dG'dB'd in the range {0, 1, ..., 255 } before, then the conversion to 8- bit dynamic according to the following formulas:

The small deviations of the coefficients in the matrix are formed by the factor 256/255. This type of conversion is as mentioned in the primary of the PAL in used in the digital representation of the analog NTSC color television signals in standard definition SDTV.

The HDTV standard ITU-R BT.709 other constants Kb, Kr are used, this leads to other transformation values ​​. It is Kb = 0.0722 and Kr = 0.2126 according to the display options of LCD and plasma screens. The matrices are then calculated accordingly.

For the YCbCr color conversion, there are a large number of other standards. The standard SMPTE 240M defines these constants with Kb = 0.087 and Kr = 0.212. The YCC color model is optimized for Kodak photo paper.

The YCbCr transformation for JPEG and MPEG also used this color model. As JPEG must represent no synchronization values ​​in the image data stream, the full range of values ​​can be used by 8 bits for the values ​​of YCbCr, that are { Y ', Cb', Cr '} and R, G and B in the value range { 0, 1, ..., 255 } possible.

  • CbCr surfaces with different Y value

Y = 0.5

Y = 1

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