Exponential-Golomb coding

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An exponential-Golomb code (or just Exp-Golomb code) is a type of universal code. To encode any nonnegative integer x using the exp-Golomb code:

  1. Write down x+1 in binary
  2. Count the bits written, subtract one, and write that number of starting zero bits preceding the previous bit string.

The first few values of the code are:

 0 ⇒ 1 ⇒ 1
 1 ⇒ 10 ⇒ 010
 2 ⇒ 11 ⇒ 011
 3 ⇒ 100 ⇒ 00100
 4 ⇒ 101 ⇒ 00101
 5 ⇒ 110 ⇒ 00110
 6 ⇒ 111 ⇒ 00111
 7 ⇒ 1000 ⇒ 0001000
 8 ⇒ 1001 ⇒ 0001001
...[1]

This is identical to the Elias gamma code of x+1, allowing it to encode 0.[2]

Extension to negative numbers[edit]

Exp-Golomb coding for k = 0 is used in the H.264/MPEG-4 AVC and H.265 High Efficiency Video Coding video compression standards, in which there is also a variation for the coding of signed numbers by assigning the value 0 to the binary codeword '0' and assigning subsequent codewords to input values of increasing magnitude (and alternating sign, if the field can contain a negative number):

 0 ⇒ 0 ⇒ 1 ⇒ 1
 1 ⇒ 1 ⇒ 10 ⇒ 010
−1 ⇒ 2 ⇒ 11 ⇒ 011
 2 ⇒ 3 ⇒ 100 ⇒ 00100
−2 ⇒ 4 ⇒ 101 ⇒ 00101
 3 ⇒ 5 ⇒ 110 ⇒ 00110
−3 ⇒ 6 ⇒ 111 ⇒ 00111
 4 ⇒ 7 ⇒ 1000 ⇒ 0001000
−4 ⇒ 8 ⇒ 1001 ⇒ 0001001
...[1]

In other words, a non-positive integer x≤0 is mapped to an even integer −2x, while a positive integer x>0 is mapped to an odd integer 2x−1.

Exp-Golomb coding is also used in the Dirac video codec.[3]

Generalization to order k[edit]

To encode larger numbers in fewer bits (at the expense of using more bits to encode smaller numbers), this can be generalized using a nonnegative integer parameter  k. To encode a nonnegative integer x in an order-k exp-Golomb code:

  1. Encode ⌊x/2k⌋ using order-0 exp-Golomb code described above, then
  2. Encode x mod 2k in binary

An equivalent way of expressing this is:

  1. Encode x+2k−1 using the order-0 exp-Golomb code (i.e. encode x+2k) using the Elias gamma code), then
  2. Delete k leading zero bits from the encoding result
Exp-Golomb-k coding examples
 x  k=0 k=1 k=2 k=3  x  k=0 k=1 k=2 k=3  x  k=0 k=1 k=2 k=3
0 1 10 100 1000 10 0001011 001100 01110 010010 20 000010101 00010110 0011000 011100
1 010 11 101 1001 11 0001100 001101 01111 010011 21 000010110 00010111 0011001 011101
2 011 0100 110 1010 12 0001101 001110 0010000 010100 22 000010111 00011000 0011010 011110
3 00100 0101 111 1011 13 0001110 001111 0010001 010101 23 000011000 00011001 0011011 011111
4 00101 0110 01000 1100 14 0001111 00010000 0010010 010110 24 000011001 00011010 0011100 00100000
5 00110 0111 01001 1101 15 000010000 00010001 0010011 010111 25 000011010 00011011 0011101 00100001
6 00111 001000 01010 1110 16 000010001 00010010 0010100 011000 26 000011011 00011100 0011110 00100010
7 0001000 001001 01011 1111 17 000010010 00010011 0010101 011001 27 000011100 00011101 0011111 00100011
8 0001001 001010 01100 010000 18 000010011 00010100 0010110 011010 28 000011101 00011110 000100000 00100100
9 0001010 001011 01101 010001 19 000010100 00010101 0010111 011011 29 000011110 00011111 000100001 00100101

See also[edit]

References[edit]