xvColour

from Wikipedia, the free encyclopedia
This article or the following section is not adequately provided with supporting documents ( e.g. individual evidence ). Information without sufficient evidence could be removed soon. Please help Wikipedia by researching the information and including good evidence.
No evidence of any kind
Logo XvColour

xvColour or xvYCC is an extended color space for the transmission of digital video signals , which was developed by Sony on the basis of the Kodak PhotoCD color space YCC (YC b C r ). The corresponding color standard is IEC61966-2-4.

Implementation and effects of the extended color space

YCC as the basis

The color range of the YCC color space is identical to that of the sRGB color space, as is the white point D65 and the gamma value 2.2. While sRGB colors are encoded as three gamma-corrected channels R ', G' and B ', colors in the YCC color space are encoded as three channels Y, C b and C r (hence the name). Channel Y codes the luminance (brightness), the two values ​​C b and C r code two chroma channels (chroma). C b is the difference between blue and yellow, C r represents the difference between green and red. By means of a matrix transformation , sRGB signals can easily be converted into YCC signals. With the 8-bit coding used, the luminance Y ranges from 16 to 235, the chroma values ​​each range from 16 to 240 (from theoretically possible values ​​from 0 to 255). The reason for restricting the range of values ​​in this way was the desire to avoid clipped signal peaks at the upper and lower end of the chroma and luminance channels, which can result from overshoots when transmitting analog signals . The consequence of the area reduction is that dark gray colors with luminance values ​​between 1 and 16 are displayed as black and light gray colors between 236 and 254 as white. When the 8-bit value range is fully used, only 0 is defined as black and 255 as white. With the chroma channels, the range restriction has the effect that the colors green and red or yellow and blue reach the highest saturation value at values ​​of 16 or 240. Of course, this means a strong reduction in the chroma and brightness values ​​that can theoretically be represented with 8 bits.

Innovations in the xvYCC color space

In the digital transmission of signals, which is becoming increasingly widespread in the field of television technology, it is no longer necessary to reserve an area for overshoot. The extended color space xvYCC therefore encodes luminance and chroma channels with the entire range of values ​​from 0 to 255 and includes all body colors or optimal colors , i.e. all physically realizable colors that can arise through reflection or transmission. Since the coding of xvYCC does not differ in principle from YCC, the downward compatibility of the standard is guaranteed as long as the recording standard used supports a full 8-bit value range (this is the case with MPEG-2, for example). Signals coded in xvYCC can result in negative values ​​in the sRGB color space. The result is colors that are outside the range that can be represented in RGB, i.e. colors that display devices cannot produce. Before the display can therefore be converted into colors that can be displayed with the device. The possibility of theoretically producing all representable colors with xvYCC has the consequence that 8-bit-coded signals turn out to be disadvantageous for display devices with an extended color space. A larger color space is available, but the colors can only be resolved in 255 color gradations. The effect is that no smooth color transitions are possible with large device color spaces and the image quality suffers greatly despite richer colors. The only possibility is to encode the xvYCC signals with higher bit widths. A minimum width of 10 bits up to 16 bits is recommended. But even such high bit widths can still lead to - admittedly small - perceptible color gradations, as experience with extended color spaces such as ProPhotoRGB, WideGamutRGB and others show. The use of a higher bit width naturally results in a higher transmission bandwidth required for data transmission, which can be compensated in part by more efficient compression algorithms.

requirements

The following requirements are necessary for the representation of xvYCC-coded signals:

  • A signal source suitable for xvYCC. So far, these have only been HD video camcorders and BluRay players (especially for AVCREC DVDs).
  • A digital connection between signal source and display. The only medium so far is HDMI 1.3 or higher.
  • A display device that can display xvYCC-encoded image data. Flat screens from many manufacturers now meet this requirement.

Future development

It remains to be seen whether DVDs , Blu-ray discs or satellite television will also be xvYCC-coded in the future and will also depend on the behavior of old playback devices when loaded with appropriately coded signals (downward compatibility).