jpg 샘플링
http://docs.gimp.org/ko/gimp-images-out.html
http://en.wikipedia.org/wiki/Chroma_subsampling
http://blog.naver.com/ketaroz/120073305265
원문 : http://software.intel.com/sites/products/documentation/hpc/ipp/ippi/ippi_ch6/ch6_image_downsampling.html
4:4:4 YUV (YCbCr) - conventional format, no downsampling, Y, U(Cb), V(Cr) components are sampled at every pixel. If each component takes 8 bits, than every pixel requires 24 bits. This format is often denoted as YUV (YCbCr) with the 4:4:4 descriptor omitted.
4:2:2 YUV (YCbCr) - uses the 2:1 horizontal downsampling. It means that the Y component is sampled at each pixel, while U(Cb) and V(Cr) components are sampled every 2 pixels in horizontal direction. If each component takes 8 bits, the pair of pixels requires 32 bits.
4:1:1 YCbCr - uses the 4:1 horizontal downsampling. It means that the Y component is sampled at each pixel, while Cb and Cr components are sampled every 4 pixels horizontally. If each component takes 8 bits, each four horizontal pixels require 48 bits.
4:2:0 YUV (YCbCr) - uses the 2:1 horizontal downsampling and the 2:1 vertical downsampling. Y is sampled at each pixel, U(Cb) and V(Cr) are sampled at every block of 2x2 pixels. If each component takes 8 bits, each four-pixel block requires 48 bits.
In JPEG compression, downsampling has specific distinctive features and is denoted in a slightly different way. In JPEG domain, sampling formats determine the structure of minimal coded units, or MCUs. Therefore, the Intel IPP functions specific for a JPEG codec, support the following sampling formats:
4:4:4 YCbCr - for every 8x8 block of Y samples, there is one 8x8 block of each Cb and Cr samples.
4:2:2 YCbCr - for every two horizontal 8x8 blocks of Y samples, there is one 8x8 block of each Cb and Cr samples.
4:1:1 YCbCr - for every four (two in horizontal and two in vertical direction) 8x8 blocks of Y samples, there is one 8x8 block of each Cb and Cr samples.
Structure of the corresponding MCU for each of these sampling formats is shown in Figure MCU Structure for Different JPEG Sampling Formats.