RGB, YUV, and YCbCr are all human-defined color models or color spaces (sometimes called color systems or color spaces). Its purpose is to describe color in a generally acceptable way under certain standards. Essentially, a color model is an elaboration of a coordinate system and subspaces.
1. RGB
RGB (red, green, and blue) is a space defined by colors recognized by the human eye and can represent most colors. However, RGB color space is generally not used in scientific research, because its details are difficult to digitally adjust. It expresses hue, brightness, and saturation together, and it is difficult to separate them. It is the most versatile hardware-oriented color model. Since the RGB mode requires three independent image signals to be transmitted at the same time, the bandwidth occupation is high. RGB mode is usually used for the most primitive video data and images.
2. YUV and YCbCr
The YUV model is derived from the RGB model. In YUV space, each color has a luminance signal Y, and two chrominance signals U and V. The luminance signal is a sense of intensity, which is separate from the chrominance signal and can be changed without affecting the color. YUV uses RGB information, but it produces a black and white image from a full-color image, and then extracts the three main colors into two additional signals to describe the color. Combining these three signals back produces a full-color image.
The YCbCr model is derived from the YUV model. YCbCr is a part of the ITU-R BT.601 recommendation during the development of the World Digital Organization video standard, and is actually a scaled and offset replica of YUV. Among them, Y has the same meaning as Y in YUV, which refers to the luminance component, Cb refers to the blue chrominance component, and Cr refers to the red chrominance component, but the representation methods are different. In the YUV family, YCbCr is the most widely used member in computer systems, and its application fields are very wide. The YUV we use in encoding standards such as h.264 and mpeg is actually YcbCr.
The main sampling formats of YCbCr are YCbCr 4:4:4, YCbCr 4:2:2, YCbCr 4:1:1 and YCbCr 4:2:0. Among them, YCbCr 4:1:1 is more commonly used, and its meaning is: each point saves an 8-bit brightness value (that is, the Y value), and every 2 x 2 points saves a Cb and Cr value, the image does not feel in the naked eye. would make a big difference. Therefore, the original RGB (R, G, B are all 8bit unsigned) model, each point needs 8x3=24 bits, but now only needs 8+(8/4)+(8/4)=12bits, on average each Points account for 12bits. This compresses the image data in half.
The boundaries between the two concepts of YUV and YCbCr are blurred and partially overlapping, because YUV is used to encode analog color information in TV systems, while YCbCr is suitable for video and still pictures represented by MPEG and JPEG. Digital encoding of color information in scenarios such as compression and transmission.
3. Color value range
One is Full Range [0,255], which is the number of series divided between white and black of RGB color, which maximizes the display of color information.
One is Video Range(Limited Range) [16, 235], the principle is:
Originally a color section of 0-255, through extrusion, the black 0 is squeezed inward by 16 cells, and the white 255 is squeezed inward by 20 cells. The resulting color section is shortened by 36 cells, and the shortened part is gray. (middle color) part. It is then displayed with a color segment length of 16-235 stretched to 0-255. At this time, because the color section is stretched, there will be faults in different positions, that is, color loss. Specifically, grays that are close to black will be darker. Light grays that are close to white become whiter, i.e. loss of detail. Because the soft intermediate color is squeezed and lost most of it, although the color looks more vivid (colors close to solid colors will become close to solid colors such as red, green and blue), but details such as shadows and highlights are lost, and black is more likely to appear. A phenomenon of one piece and one piece of white.
Fourth, the color conversion standard
1982年CCIR(International Radio Consultative Committee国际无线电咨询委员会)制定了彩色视频数字化标准，称为CCIR 601标准，现改为ITU-R BT.601标准，简称BT.601。该标准规定了彩色视频转换成数字图像时使用的采样频率，RGB和YCbCr两个彩色空间之间的转换关系等，是标准数字电视（SDTV）的国际标准。
1990年发布了高清数字电视（HDTV）的国际标准ITU-R BT.709，简称BT.709.
2012年发布了超高清数字电视（Ultra HDTV）的国际标准：ITU-R BT.2020，简称BT.2020。
小结
1、颜色标准的选择会影响主观画质
2、不同平台对颜色标准的支持不一样
3、BT.601标准兼容性更好，但是BT.709及后续标准更先进