Colour Space Conversion (not performed in some highest quality modes)
Downsampling
- The ratio at which downsampling is ordinarily done is at either 4:4:4 (no downsampling), 4:2:2 (reduction by a factor of two in the horizontal direction) or 4:2:0 (reduction by a factor of 2 in both the horizontal and vertical directions)
Block Splitting
- Each channel is split into 8x8 blocks. Depending on Chroma Subsampling, this yields Minimum Coded Unit (MCU) blocks of size 8x8 (4:4:4), 16x8 (4:2:2) or 16x16 (4:2:0).
Discrete Fourier Transform
- Each 8x8 block of each component is converted to a frequency domain representation, using a normalized, 2D type-II Discrete Cosine Transform (DCT).
Quantization
- This is the only lossy operation (apart from chroma subsampling)
Entropy Coding
- It is a special form of lossless data compression.

It converts images from RGB to YCbCr.
The resolution of chroma is reduced by a factor of 2 or 3, by Chroma Subsampling.
The image is split into blocks of 8x8 pixels and for each block, each of the Y, Cb and Cr data undergoes the Discrete Cosine Transform. A DCT is similar to a Fourier Transform in the sence that it produces a kind of spatial frequency spectrum.
The amplitudes of the frequency components are quantized. Human vision is much more sensitive or small variations in colour or brightness over large areas than to the strength of high-frequency brightness variations. Therefore the magnitudes of the high-frequency components are stored with a lower accuracy than the low-frequency components according to the quality setting (the 0-100 setting in the Independent JPEG Group's library).
The resulting data for all 8x8 blocks is losslessly compressed with a variant of Huffman encoding.

Decoding process reverses these steps, except the quantization step, which is lossy.

Backlinks

Left-click: follow link, Right-click: select node, Scroll: zoom