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15-20 Raster Graphics EN
TIFF Encoding
E
C
*r1A
E
C
*b2m6W(-3)U(0)A(-1)T or
E
C
*b2m6W(-3)U(2)ATT
E
C
*rC
In the TIFF encoding example above, parenthetical expressions
are used to identify control bytes. For example, the byte (-3) is shown
to represent the control byte for a repetition (minus value) of 3. The
actual value for this position is the decimal value 253. Additional
“encoded” control bytes in this sequence include: (0) for decimal 0,
(-1) for decimal 255, and (2) for decimal 2. The raster data (pattern)
bytes are represented as by the ASCII character.
Delta Row Compression (Method 3)
Delta row compression identifies a section of bytes in a row that is
different from the preceding row, and then transmits only that data
that is different (the delta data). If a row is completely different from
its preceding row, then the entire row must be sent as the delta
(not very efficient); if only one bit is different, then only one byte is
identified and sent. To reassemble the raster data rows, the printer
takes the current row (referred to as the seed row) and makes the
changes indicated by the delta data, to create the new row. The new
row (which becomes the new seed row) is used by the next delta
compression data to create another row.
A delta compression row consists of two parts, a command byte and
the replacement bytes, as shown below:
[(Command byte)(1 to 8 Replacement bytes)]
The command byte identifies two things: 1) the number of
replacement (delta) bytes that follow; and, 2) where to position the
replacement byte string (the left offset). The replacement bytes are
some number (up to eight bytes) of consecutive bytes that are used
to create the new row from the seed row.