1. FAX and TIFF Modified Huffman coding is used in fax machines.
TIFF also uses this technique.
It combines the variable length codes of Huffman coding with the coding of repetitive data in run length encoding.
4. 1

2. FAXes Faxes have only white bits and black bits.
Each line is coded as a series of alternating runs of white and black bits.
Runs of 63 or less are coded with a terminating code.
Runs of 64 or greater require that a makeup code prefix the terminating code.
The makeup codes are used to describe runs in multiples of 64 from 64 to 2560.
4. 2

3. Terminating codewords
Run Length
White bits
Black bits 32 1
000111
010 33 2
0111 11 34 3
1000 10 35 4
1011
011 36 5
1100
0011 37 6
1110
0010 38 7
1111
00011 39 8
10011
000101 40 9
10100
000100 41
00111 42
01000 43 12
001000 44 13
000011 45 14
110100 46 15
110101 47 16
101010 48 17
101011 49 18 50 19 51 20 52 21 53 22 54 23 55 24 56 25 57 26 58 27 59 28 60 29 61 30 62 31
4. 3

4. Makeup Codewords 64
11011
128
10010
192
010111
256
320
384
448
512
576
640
704
768
832
896
960
1024
1088
1152
1216
1280
1344
1408
1472
1536
1600
1664
011000
1728
1792
1856
1920
1984
2048
2112
2170
2240
2304
2368
2432
2496
2560
EOL
4. 4

5. White and Black pixels
Studies have shown that most facsimiles are 85 percent white.
Faxes assume that any line begins with a run of white bits.
If it does not, a run of white bits of 0 length must begin the encoded line.
4. 5

6. Error Recovering
Each scan line ends with a special EOL (end of line) character consisting of eleven zeros and a 1 ( ).
There is no code that has more than seven leading zeroes.
There is no code that has more than three ending zeros.
4. 6

7. Error Recovering (Cont.)
If the decoder sees eleven zeros, it will recognize the end of line and continue scanning for a 1.
Upon receiving the 1, it will then start a new line.
If a bit in a scanned line gets corrupted, the most that will be lost is the rest of the line.
If the EOL code gets corrupted, the most that will get lost is the next line.
4. 7

8. Example
0 white black white black white black white EOL
4. 8