After quantization, it is not unusual for more than half of the DCT coefficients to equal zero. JPEG incorporates run-length coding to take advantage of this. For each non-zero DCT coefficient, JPEG records the number of zeros that preceded the number, the number of bits needed to represent the number's amplitude, and the amplitude itself. To consolidate the runs of zeros, JPEG processes DCT coefficients in the zigzag pattern shown in figure two:
The number of previous zeros and the bits needed for the current number's amplitude form a pair. Each pair has its own code word, assigned through a variable length code ( for example Huffman, Shannon-Fano or Arithmetic coding). JPEG outputs the code word of the pair, and then the codeword for the coefficient's amplitude (also from a variable length code). After each block, JPEG writes a unique end-of-block sequence to the output stream, and moves to the next block. When finished with all blocks, JPEG writes the end-of-file marker.