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6-26 Raster Graphics EN
As mentioned, the offset 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:
• An offset value of 0-30 indicates that the replacement bytes
are offset from the 1st byte to the 31st byte.
• 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:
The total offset is 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.
For color raster images, the printer operates on each plane
independently, and a separate seed plane is maintained for each
graphic plane. A Y offset, however, affects all planes and seed rows
simultaneously.