What is RGB Color Space? What is Chroma Subsampling? 4:4:4 vs 4:2:2 vs 4:2:0

The RGB color space is the color space used by computers, graphics cards and monitors or LCDs.  It consists of three components, red, green and blue, the so-called base (or Primary) colors.

4:4:4 is another name for RGB color space and it’s a digital image or video in which all color components have the same sampling rate, thus not using chroma subsampling.  This scheme is sometimes used in high-end film scanners and cinematic post production. 

Chroma subsampling is a type of compression that reduces the color information in a signal in favor of luminance data. This reduces bandwidth without significantly affecting picture quality.

A video signal is split into two different aspects: luminance information and color information. Luminance, or luma for short, defines most of the picture since contrast is what forms the shapes that you see on the screen. For example, a black and white image will not look less detailed than a color picture. Color information, chrominance or simply chroma is important as well but has less visual impact. What chroma subsampling does is reduce the amount of color information in the signal to allow more luminance data instead. This allows you to maintain picture clarity while effectively reducing the file size up to 50%.  In the YUV format, luma is only 1/3rd of the signal, so reducing the amount of chroma data helps a lot. Because of bandwidth limitations from internet speeds and HDMI, this makes for much more efficient use of current systems.

4:4:4 vs 4:2:2 vs 4:2:0

The first number (in this case 4), refers to the size of the sample. The two following numbers both refer to chroma. They are both relative to the first number and define the horizontal and vertical sampling respectively.

A signal with chroma 4:4:4 has no compression (so it is not subsampled) and transports both luminance and color data entirely.  In a four by two array of pixels, 4:2:2 has half the chroma of 4:4:4, and 4:2:0 has a quarter of the color information available. The 4:2:2 signal will have half the sampling rate horizontally, but will maintain full sampling vertically. 4:2:0, on the other hand, will only sample colors out of half the pixels on the first row and ignores the second row of the sample completely.

Color subsampling is a method of compression that greatly reduces file size and bandwidth requirements with practically no quality loss. Unless you are going to use your TV as a primary PC monitor where lots of text is going to be read, there shouldn't be a need to worry about it.  In simple, the higher the number, the better it is.   You’ll see it most visible impact in the following devices.


*source www.rtings.com
updated 2018/04/02