The sub-sampling scheme is commonly expressed as a three part ratio J:a:b (for example, 4:2:2), that describes 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). This is usually 4.
• a: Number of chrominance samples (V, U) in the first row of J pixels.
• b: Number of (additional) chrominance samples (V, U) in the second row of J pixels.
To illustrate the most common sub-sampling schemes, This Figure introduces a graphical notation of sampling grid pixels.
