TIFF - Page 101

2001/10/02 

CHAPTER 2 "PCL" - 81 

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# = 5 indicates that adaptive compression is to be used.  

This compression compresses multiple rows (a block) into one transfer graphics command. 

Most other compression methods compress only one row to one transfer graphics command. 

One transfer graphic command (ESC *b##W) can contain only up to 32,767 bytes. 

As mode 5 compression compresses multiple rows, the amount of transferred data may become large. 

If compressed data exceeds this limit, reduce the number of  rows in the block. 

We can use mode 0 to mode 3 compression to compress each row. We can even compress empty (white) rows 
and duplicate rows with these compression methods. 

Command byte 

Mode 5 compressed data consists of multiple elements. Each element contains 3 header bytes at the 
beginning. 

The first byte of the header bytes is a command byte. 

First byte 

Second, third byte 

0 

Unencoded 

1 

Run-length Encoding 

Number of bytes of data 

2 

TIFF  

3 

Delta Row compression 

4 

Empty Row 

Number of empty row or repetition 

5 

Duplicate Row 

Command byte 

0 - 

This element is an uncompressed row. The next two bytes contain the byte count of the uncompressed 
data. Uncompressed data follows the header. 

1 -  

This element identifies a compressed row. Compression method  is mode 1. The next two bytes  (upper 
byte first) contain the byte count of the compressed data. Compressed data follows the header. 

2 - 

This element identifies a compressed row. Compression method is mode 2. The next two bytes  (upper 
byte first) contain the byte count of the compressed data. Compressed data follows the header 

3 -  

This element identifies a compressed row. Compression method is mode 3. The next two bytes  (upper 
byte first) contain the byte count of the compressed data. Compressed data follows the  header 

4 -  

This element identifies multiple empty rows. The next two bytes  (upper byte first) contain the empty 
row count. 

5 - 

This element identifies duplicate rows. The next two bytes  (upper byte first) contain the duplicate row 
count. 

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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)