The JPEG compressed file format can produce significant reductions in file size through lossy compression. The compression techniques take advantage of the limitations of the human eye by discarding additional image details that may not be as noticeable to the human observer.
Humans are much more sensitive to changes in luminance (brightness) than to chrominance (color) differences. JPEG can discard a lot more color information than luminance in the compression process. Chroma subsampling is the process whereby the color information in the image is sampled at a lower resolution than the original. JPEG translates 8-bit RGB data (Red, Green, Blue) into 8-bit YCbCr data (Luminance, Chroma Blue, Chroma Red).
The different levels of YCbCr subsampling are :
- 4:4:4 – The resolution of chrominance information is preserved at the same rate as the luminance information. (1×1, subsampling disabled)
- 4:2:2 – Half of the horizontal resolution in the chrominance is dropped, while the full resolution is retained in the vertical direction, with respect to the luminance. (2×1 chroma subsampling)
- 4:1:1 – Only a quarter of the chrominance information is preserved in the horizontal direction with respect to the luminance information
- 4:2:0 – With respect to the information in the luminance channel, the chrominance resolution in both the horizontal and vertical directions is cut in half (2×2 chroma subsampling)
JPEG chroma subsampling is not a particularly good mechanism for compressing images used in the medical field where the chrominance may be equally as important as the luminance.
Photoshop uses different chroma subsampling levels depending on the Quality settings:
- 2×2 Chroma Subsampling – Save Quality 0-6 or Save For Web Quality 0-50
- No Chroma Subsampling – Save As Quality 7-12 or Save For Web Quality 51-100
Additional informations about JPEG Chroma subsampling are available at the following links :