Chroma subsampling

Chroma subsampling or color subsampling is the practice of encoding images by implementing less resolution for chroma information than for luma information, taking advantage of the human visual system's lower acuity for color differences than for luminance.

It is used in many video and still image encoding schemes – both analog and digital – including in JPEG encoding.

Rationale

Digital signals are often compressed to reduce file size and save transmission time. Since the human visual system is much more sensitive to variations in brightness than color, a video system can be optimized by devoting more bandwidth to the luma component (usually denoted Y'), than to the color difference components Cb and Cr. In compressed images, for example, the 4:2:2 Y'CbCr scheme requires two-thirds the bandwidth of non-subsampled "4:4:4" R'G'B'. This reduction results in almost no visual difference as perceived by the viewer.

How subsampling works

The human vision system processes color information (hue and colorfulness) at about a third of the resolution of luminance (lightness/darkness information in an image). Therefore it is possible to sample color information at a lower resolution while maintaining good image quality.

This is achieved by encoding RGB image data into a composite black and white image, with separated color difference data (chroma). For example with

Y

′

C

b

C

r

{\displaystyle Y'C_{b}C_{r}}

, gamma encoded

R

′

G

′

B

′

{\displaystyle R'G'B'}

components are weighted and then summed together to create the luma

Y

′

{\displaystyle Y'}

component.