原创 JPEG Chroma Subsampling

The JPEG (JFIF) compressed file format can produce significant reductions in file size through lossy compression. The techniques used to achieve these levels of JPEG compression take advantage of the limitations of the human eye. The compression algorithm saves space by discarding additional image information / detail that may not be as noticeable to the human observer.

It is widely known that we are much more sensitive to changes in luminance (brightness) than we are to chrominance (color) differences. Because of this, the JPEG format can discard a lot more color information than luminance in the compression process. To facilitate the different compression requirements of the two "channels" of image information, the JPEG file format translates 8-bit RGB data (Red, Green, Blue) into 8-bit YCbCr data (Luminance, Chroma Blue, Chroma Red). Now, with the brightness seperated into a separate data channel, it is much easier to change the compression algorithm used for one channel versus the others.

Chroma subsampling is the process whereby the color information in the image is sampled at a lower resolution than the original. The images below show various types of chroma subsampling (from 1x1 to 2x2) and the effect it has on reducing the color information. Pay special attention to the horizontal and vertical stripes.

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• 4:4:4 - The resolution of chrominance information (Cb & Cr) is preserved at the same rate as the luminance (Y) information. Also known as 1x1 (or subsampling disabled).
  
• 4:2:2 - Half of the horizontal resolution in the chrominance is dropped (Cb & Cr), while the full resolution is retained in the vertical direction, with respect to the luminance. This is also known as 2x1 chroma subsampling, and is quite common for digital cameras.
  
• 4:1:1 - Only a quarter of the chrominance information is preserved in the horizontal direction with respect to the luminance information. I don't think this format is nearly as common as the other variations.
  
• 4:2:0 - With respect to the information in the luminance channel (Y), the chrominance resolution in both the horizontal and vertical directions is cut in half. This form is also known as 2x2 chroma subsampling.