What does 0 mean in YUV420 I know whats the difference between various formats such as YUV444,YUV420,YUV422.
But what exactly these numbers mean? They are mentioned as chroma subsampling in horizontal and vertical directions. In YUV420 Cb and Cr are sampled at half the resolution of luma in both vertical and horizontal directions. So what does zero in YUV420 mean?
As explained on the Wikipedia Page on Croma Subsampling:


  The subsampling scheme is commonly expressed as a three part ratio ''J'':''a'':''b'' (e.g. 4:2:2) or four parts if alpha channel is present (e.g. 4:2:2:4), that describe the number of luminance and chrominance samples in a conceptual region that is ''J'' pixels wide, and 2 pixels high. The parts are (in their respective order):
  
  
  ''J'': horizontal sampling reference (width of the conceptual region). Usually, 4.
  ''a'': number of chrominance samples (Cr, Cb) in the first row of ''J'' pixels.
  ''b'': number of changes of chrominance samples (Cr, Cb) between first and second row of ''J'' pixels.
  
  
  …


In other words, your 0 says there's no vertical chroma subsampling at all. Instead, you just assume that columns within the four pixel wide subsampling region have identical chroma.