What does the 420 means in YUV420? I'm not sure if is the correct location for ask this question. If not please info me the correct StackExchange site.
As this wikipedia page show, there is one YUV format of image as YUV420 and YUV411.

I know what is YUV, but what those 420 and 411 means?
The 3-digit number describes the subsampling of the
chroma (U and V) channels.  A detailed explanation is at

http://en.wikipedia.org/wiki/Chroma_subsampling

In particular, YUV420 means that the U and V channels have half the resolution of the luma (Y) channel, in the horizontal and vertical directions. The sampling method usually present in low to medium quality JPEG images is YCrCb420 (high quality JPEGs are usually YCrCb444 (no subsampling).