The final step of the
JPEG image compression process is to compress the quantized DCT values. This is done
through a three-part procedure detailed below.
1. Convert the DC coefficient to a relative value First, the DC
coefficient is changed from an absolute value to a relative value relative to the
DC coefficient in the previous 8 x 8 block, that is. Since adjacent blocks in an image
exhibit a high degree of correlation, coding the DC term of a given block as the
difference from the previous DC term typically produces a very small number that can be
stored in a fewer number of bits.
2. Reorder the DCT block in a zig-zag sequence Because so many
coefficients in the DCT image are truncated to zero values during the coefficient
quantization stage, the zeros are handled differently than non-zero coefficients. They are
coded using a Run-Length Encoding (RLE) algorithm. RLE gives a count of consecutive zero
values in the image, and the longer the runs of zeros, the greater the compression. One
way to increase the length of runs is to reorder the coefficients in the zig-zag sequence
shown in the diagram below. This way, the JPEG algorithm moves through the block selecting
the highest value elements first and eventually working its way to the lowest value
elements, thus optimizing the effect of RLE.
3. Entropy Encoding Finally, the JPEG algorithm outputs
the DCT blocks elements using an entropy encoding mechanism that combines the
principles of RLE and Huffman encoding. The output of the entropy encoder consists of a
sequence of three tokens, repeated until the block is complete. The three tokens are the
run length, the number of consecutive zeros that precede the current non-zero element in
the DCT output matrix; the bit count, the number of bits used to encode the amplitude
value that follows, as determined by the Huffman encoding scheme; and the amplitude, the
amplitude of the DCT coefficient.
The final output that results from JPEG compression can be mathematically reversed and
decompressed to produce an image that, to a computer, lacks defining data, but to a human
eye, appears identical to the original image.