2 TIFF - Page 112

CHAPTER 2 PCL - 92 

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# = 9  Mode 9 compression 

Like mode 3 compression, this mode 9 compression data contains only the difference of the current row 
from the preceding row. 

Replacement data often consists of a repetition of a particular byte in mode 3 compression. 

Mode 9 compression is effective in such cases. 
Simple mode 9 compression data consists of a command byte and the replacement data. 
A Command byte consists of  3 parts. These are the control bit, the offset count and the  replacement count. 
The Control bit is the most significant bit (bit 7) of a control byte. 

7

6

5

4

3

2

1

0

Offset count

Replacement count-1

Control bit

0

Command byte for mode 9 compression (non repeated replacement data)

If the control bit is 0, replacement data is not repeated data. Bits 6 to 3 contain the offset count. This field 
value is from 0 to 15. If this value is 15, the next byte of the control bytes is an optional offset count. The 
value of this optional offset count byte is from 0 to 255.  If this optional offset count value is 255, then the 
next byte is another (second) optional offset count byte. Optional offset count bytes will continue until the 
value is less than 255. The total Offset value is the sum of the offset count in the control byte and all the 
optional offset bytes. 

Bits 2 to 0 of the control byte contains the replacement count - 1. The value is from 0 to 7. If this value is 7, 
optional replacement byte(s) follow the control byte or optional offset byte(s) (if any). 

If the first optional replacement count byte is 255, then a second optional count byte follows the first 
optional count byte. Optional offset count bytes will continue until the value becomes less than 255. The 
addition of the replacement count in the control byte and all optional replacement count bytes gives the total 
replacement count. 

Replacement data bytes follow the control byte and any optional offset bytes and any optional replacement 
count bytes. 

<Example> 
Seed row 

55 55 55 55 55 55 55 55 55 55 55 55 55 

Current row 

55 55 55 55 55 11 11 22 33 44 55 66 77 

<Different section  11 11 22 33 44 55 66 77> 
Offset count = 5 
Replacement count = 8 
Replacement data = 11 11 22 33 44 55 66 77 

0 0

1

0

1

1

1

1

7

6

5

4

3

2

1

0

Replacement count = 7 + 1 = 8

Offset count = 5

Optional offset count byte = 0

8 bytes of repracement data

11 11 22 33 44 55 66 77

7

6

5

4

3

2

1

0

Offset count

Replacement count

Control bit

1

Command byte for mode 9 compression (repeated replacement data)

If the control bit is 1, replacement data is repeated data. Bits 6 to 5 contain the offset count and bits 4 to 0 
contain the 'replacement count (repeat count) - 2'.