To further reduce the storage requirements, the 4:2:0 sampling format is employed
by the MPEG-1 and H.261 video compression. The Cb and Cr components are sub-sampled
by a factor of two both horizontally and vertically. Therefore chrominance samples
are shared between certain adjacent lines as shown in the following illustration.
In addition to supporting 4:4:4 and Rec.601 4:2:2, the MPEG-2 video compression
standard supports the following 4:2:0 sampling pattern which is employed by the
DVD-Video standard. This pattern differs from the MPEG-1/H.261 standard only in
the position of the chroma samples. The MPEG committee changed this to make the
conversion from Rec.601 4:2:2 to MPEG-2 4:2:0 less computationally intensive
(dealing with only two Y values instead of four).