Chroma subsampling is a technique used in digital image and video compression to reduce the amount of data required to represent a color image or video signal. The technique exploits the fact that the human eye is more sensitive to variations in brightness than to variations in color, by reducing the resolution of the color information.
In chroma subsampling, the color information is sampled at a lower resolution than the luminance (brightness) information. The most common chroma subsampling ratios are 4:2:2, 4:2:0, and 4:1:1. In a 4:2:2 subsampling scheme, for example, the color information is sampled at half the horizontal resolution of the luminance information, while in a 4:2:0 scheme, the color information is also sampled at half the vertical resolution.
Chroma subsampling can significantly reduce the amount of data required to represent a color image or video signal without significantly affecting the perceived quality of the image or video. However, excessive subsampling can lead to visible artifacts such as color bleeding and loss of detail in the color channels. Therefore, the choice of subsampling ratio depends on the intended use of the compressed image or video and the desired balance between image quality and compression efficiency.
The most common chroma subsampling ratios are expressed in the form of three numbers, such as 4:2:2, 4:2:0, or 4:1:1. The first number in the ratio represents the horizontal resolution of the luminance information, while the second and third numbers represent the horizontal and vertical resolutions of the chrominance (color) information, respectively.
For example, a 4:2:2 subsampling ratio means that for every four pixels of luminance information, there are two pixels of chrominance information in both the horizontal and vertical directions. This corresponds to a 2:1 reduction in the horizontal chrominance resolution compared to the luminance resolution. Similarly, a 4:2:0 subsampling ratio means that for every four pixels of luminance information, there are two pixels of chrominance information in the horizontal direction, but only one pixel of chrominance information in the vertical direction. This corresponds to a 2:1 reduction in the horizontal chrominance resolution and a 2:1 reduction in the vertical chrominance resolution compared to the luminance resolution.
The choice of subsampling ratio depends on the intended use of the compressed image or video and the desired balance between image quality and compression efficiency. Higher subsampling ratios can lead to more efficient compression, but can also result in visible artifacts such as color bleeding and loss of detail in the color channels. Lower subsampling ratios generally produce higher quality images but require more storage space or bandwidth.