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EN Raster Graphics 6-27
The seed row is updated by every raster graphic transfer, regardless
of the compression method. This allows the delta compression
method to be mixed with other methods to achieve better
compression performance.
Repeating a Row
?* b 0 W
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)
?* b 1 Y
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,
?*b1W does not affect the seed row, but causes the
previous row to be replicated.
Example: Delta Row Compression
The following example demonstrates how to compress the following
data using the delta row compression. (The bytes highlighted in bold
type indicate those bytes needing replacement – those bytes that are
different from the previous row, the seed row.)
Byte No. 0 1 2 3 4
Row 1 00000000 11111111 00000000 00000000 00000000
Row 2 00000000 11111111 11110000 00000000 00000000