Chroma Subsampling can be found in video, digital images and in color television. It affects the visual information portrayed in them.
Chroma Subsampling is the act of encoding image information in a way that reduces the amount of data in the respective image or video file. This helps saving hardware space as well as bandwidth when uploading, streaming or broadcasting the file. The video or image file, however, needs to contain both chroma (color) and luma (brightness) information independent from one another. The reason for this is that the Chroma Subsampling, as the name suggests, only targets the color and thus the chroma data in the file.
The rate of the subsampling is calculated considering luma and chroma information. The loss of color information, however, does not visibly reduce the image quality for the human eye, since they are perceiving color in a less sensitive way than a brightness contrast.
The Chroma Subsampling is typically displayed as YCbCr followed by a sequence of numbers, such as 4:2:0 or 4:4:4. The first 4 represents the luma value that is not affected by Chroma Subsampling. The following two numbers can be 4 (no change in color) or lower (a color-reduction took place).