一、色度采样
Color images require three channels of information. In computer graphics, a pixel’s color is typically defined by R, G, and B values. In traditional digital video, a pixel is represented by Y’, CB, and CR values, where Y’ is the “luma” or grayscale value and CB and CR contain the “chroma” or color-difference information. Because the eye is less sensitive to fine chroma detail, it is possible to average together and encode fewer CB and CR samples with little visible quality loss for casual viewing. This technique, known as chroma subsampling, has been used widely to reduce the data rate of video signals. However, excessive chroma subsampling can degrade quality for compositing, color correction, and other image-processing operations.
彩色图像需要3个通道的信息，在计算机图像中，一个像素由RGB三个值定义。在传统的数码视频中，一个像素由Y、Cb、Cr三个值定义，其中Y代表亮度或者灰阶的值，Cb、Cr包含色度或者色差信息。
因为人眼对色度信息没有那么敏感，因此可以对Cb、Cr信号进行平均和编码，也就是色度的下采样，这被广泛的应用于降低视频的码率。
但是，过度的色度下采样会降低色彩准确度和其他图片处理的质量。
4:4:4 is the highest-quality format for preserving chroma detail.
In 4:4:4 image sources, there is no subsampling, or averaging, of chroma information. There are three unique samples, either Y’, CB, and CR or R, G, and B, for every pixel location. Apple ProRes 4444 XQ and Apple ProRes 4444 fully support 4:4:4 image sources, from either RGB or Y’CBCR color spaces. The fourth “4” means that
Apple ProRes 4444 XQ and Apple ProRes 4444 can also carry a unique alpha-channel sample for every pixel location. Apple ProRes 4444 XQ and Apple ProRes 4444 are intended to support 4:4:4:4 RGB+Alpha sources exported from computer graphics applications such as
Motion, as well as 4:4:4 video sources from high-end devices such as dual-link HDCAM-SR.
444采样能够最好的保留色度信息的细节。在444的图片中，没有下采样、平均处理。
ProRes4444中的第4个4，指的是透明度通道。
4:2:2 is considered a high-quality, professional video format in which the chroma values of Y’CBCR images are averaged together such
that there is one CB and one CR sample, or one “CB/CR chroma pair,” for each Y’ (luma) sample. This minimal chroma subsampling has traditionally been considered adequate for high-quality compositing and color correction, although better results can be achieved
using 4:4:4 sources. 4:2:2 sources are generated by many popular higher-end video camcorder formats, including DVCPRO HD, AVC-Intra/100, and XDCAM HD422/50. All Apple ProRes 422 family members fully support the chroma resolution inherent in 4:2:2 video formats.
422被认为是高质量的、专业的视频格式，在这种格式里，每个亮度Y信号都平均有1个Cb信号和Cr信号。
4:2:0 and 4:1:1 have the least chroma resolution of the formats mentioned here, with just one CB/CR chroma pair for every four
luma samples. These formats are used in a variety of consumer and professional video camcorders. Depending on the quality of a camera’s imaging system, 4:2:0 and 4:1:1 formats can provide excellent viewing quality. However, in compositing workflows it can be difficult to avoid visible artifacts around the edges of a composited element. HD 4:2:0 formats include HDV, XDCAM HD, and AVC-Intra/50. 4:1:1 is used in DV. All Apple ProRes 422 formats can support 4:2:0 or 4:1:1 sources if the chroma is upsampled to 4:2:2 prior to encoding.
420和411的色度分辨率最小。平均每4个亮度Y信号中，才有1对Cb和Cr信号。