HP PCL 5 Reference Manual page 331

Repeating a Row
When using the delta compression method, it is possible to repeat
or copy the previous raster row using the Raster Data Transfer
command. This is accomplished by setting the Raster Data Transfer
command, value field, to zero.
Printing A Zeroed Row (Setting the Seed Row to Zero)
It is possible to print a row of all zeros using the Raster Y-Offset
command. Sending a Raster Y Offset command, with a value field
of 1, sets the seed row to zero and prints the zeroed row. Note, that
the next delta row is applied to a zeroed seed row.
Other cursor position moves set the seed row to zeros. (Remember,
non-graphic cursor moves have the same effect as an end graphics
command.)
Note If the byte count of the Transfer Raster Data command value field is
less than the number of bytes that can be replaced, the byte count
has precedence. Also, if the last byte is a control byte, it is ignored.
Therefore,
previous row to be replicated.
EN
E
*b0W
C
E
*b1Y
C
does not affect the seed row, but causes the
&esc*b1W
Set Compression Method Command 15-23