HP PCL/PJL reference (PCL 5 Printer Language) - Technical Reference Manual Part I

15-22 Raster Graphics EN
As mentioned, the offset is contained in the lower five bits of the
command byte allows for offset values from 0 to 31. Compression
mode allows offsets larger than 31 bytes as follows:
z An offset value of 0-30 indicates that the replacement bytes are
offset from the 1st byte to the 31st byte.
z A value of 31 indicates that the next byte following the command
byte is an additional offset byte which adds to the first (32) offset
value. This allows offset values larger than 31. Also, if this second
offset byte is set to 255 (all ones), additional offset bytes follow
until the required offset value is obtained. When the formatter
detects an offset byte less than 255, it is assumed to be the last
offset value and the offset bytes are then totaled (added). The
following example shows an offset larger than 31:
Figure 15-10
The total offset 414, which is the sum of the three offset values: 31 +
255 + 128.
Seed Row
The seed row is basically the current raster data row; the row
being printed. It is maintained by the printer for use by delta row
compression. The delta compression replacement bytes are applied
to the seed row to create the new row. This new data row is printed
and becomes the new seed row.
The seed row is updated by every raster graphic transfer, regardless
of the compression method. This allows delta compression method to
be mixed with other methods to achieve better compression
performance.