Chroma Sub-Sampling Ratio (j:a:b)

Naturally human visual system is more capable to detect luminance than color differences. Chroma subsampling  process  perform its procedure by using this limitation of human eye . This is a process of image encoding by implementing less resolution for chroma information instead of for luma information. It is used in various video encoding schemes both digital and analog and also in JPEG encoding.

The subsampling scheme is generally expressed through three part ratio (j:a:b)


□ J means horizontal sampling reference (width of the conceptual region). Usually 4.

□ a means number of chrominance samples (Cr, Cb) in the first row of J pixels.

□ b means number of changes of chrominance samples (Cr, Cb) between a first and second row of J pixels.


This was established as a main motivation behind the early forms of digital and analog compression. Video signals would be separated into major two components. The first one is a lightness or “luma” component and second one is two color or “chroma” components. It can be compared with how the images can be separated into three red, green and blue (RGB) components. Then the luma and chroma components would be referred to as YUV (with analog) or YCbCr (with digital) as opposed to RGB.


Updated: June 22, 2019 — 6:25 PM


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  1. Hello,
    Please explain me what is Chroma.

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