CHAPTER 3
78
Syntax
CCITT encoding is bit-oriented, not byte-oriented. Therefore, in principle,
encoded or decoded data might not end at a byte boundary. This problem is dealt
with in the following ways:
•
Unencoded data is treated as complete scan lines, with unused bits inserted at
the end of each scan line to fill out the last byte. This approach is compatible
with the PDF convention for sampled image data.
•
Encoded data is ordinarily treated as a continuous, unbroken bit stream. The
EncodedByteAlign
parameter (described in Table 3.9) can be used to cause
each encoded scan line to be filled to a byte boundary. Although this is not pre-
scribed by the CCITT standard and fax machines never do this, some software
packages find it convenient to encode data this way.
•
When a filter reaches EOD, it always skips to the next byte boundary following
the encoded data.
If the
CCITTFaxDecode
filter encounters improperly encoded source data, an
error occurs. The filter does not perform any error correction or
resynchronization, except as noted for the
DamagedRowsBeforeError
parameter
in Table 3.9.
Except where noted otherwise, all values supplied to the decoding filter by any of
these parameters must match those used when the data was encoded.
TABLE 3.9 Optional parameters for the CCITTFaxDecode filter
KEY
TYPE
VALUE
K
integer
A code identifying the encoding scheme used:
<
0 Pure two-dimensional encoding (Group 4)
0 Pure one-dimensional encoding (Group 3, 1-D)
>
0 Mixed one- and two-dimensional encoding (Group 3, 2-D),
in which a line encoded one-dimensionally can be followed
by at most
K
−
1 lines encoded two-dimensionally
The filter distinguishes among negative, zero, and positive values of
K
to determine how to interpret the encoded data; however, it does
not distinguish between different positive
K
values. Default value: 0.