4:2:0

4:2:0 is an expression of chroma subsampling, a practice of encoding images by compressing their color information in order to reduce the overall storage size and bitrate of video files.

This reduced chroma subsampling takes two chroma samples from the top 'a' row of pixels and none from the bottom 'b' row. Instead, the bottom row shares choma information from the top row sampling. This reduces the overall chroma information to approximately 25 percent of the uncompressed chroma.

The configuration of 4:2:0 subsampling is common in HDV and AVCHD (file-based formats for the digital recording and playback of high-definition video), DVD-video, Blu-ray, and in many encoded video formats used by DSLR cameras.