TIFF 6.0 Specification
Final—June 3, 1992
current generation of Group 3 facsimile products and their defenses against data
corruption as the result of transmission defects.
Whichever form of encoding is preferable for a given application, there are a
number of adjustments that need to be made to account for the capture of the
CCITT binary-encoding strings as part of electronically-stored material and digi-
tal-image interchange.
PhotometricInterpretation.
An encoded CCITT string is self-photometric, de-
fined in terms of white and black runs. Yet TIFF defines a tag called
PhotometricInterpretation that also purports to define what is white and what is
black. Somewhat arbitrarily, we adopt the following convention:
The “normal” PhotometricInterpretation for bilevel CCITT compressed data is
WhiteIsZero. In this case, the CCITT “white” runs are to be interpretated as white,
and the CCITT “black” runs are to be interpreted as black. However, if the
PhotometricInterpretation is BlackIsZero, the TIFF reader must reverse the mean-
ing of white and black when displaying and printing the image.
FillOrder.
When CCITT encodings are used directly over a typical serial commu-
nication link, the order of the bits in the encoded string is the sequential order of
the string, bit-by-bit, from beginning to end. This poses the following question: In
which order should consecutive blocks of eight bits be assembled into octets
(standard data bytes) for use within a computer system? The answer differs de-
pending on whether we are concerned about preserving the serial-transmission
sequence or preserving only the format of byte-organized sequences in memory
and in stored files.
From the perspective of electronic interchange, as long as a receiver’s reassembly
of bits into bytes properly mirrors the way in which the bytes were disassembled
by the transmitter, no one cares which order is seen on the transmission link be-
cause each multiple of 8 bits is transparently transmitted.
Common practice is to record arbitrary binary strings into storage sequences such
that the first sequential bit of the string is found in the high-order bit of the first
octet of the stored byte sequence. This is the standard case specified by TIFF
FillOrder = 1, used in most bitmap interchange and the only case required in
Baseline TIFF. This is also the approach used for the octets of standard 8-bit char-
acter data, with little attention paid to the fact that the most common forms of data
communication transmit and reassemble individual 8-bit frames with the low-
order-bit first!
For bit-serial transmission to a distant unit whose approach to assembling bits into
bytes is unknown and supposed to be irrelevant, it is necessary to satisfy the ex-
pected sequencing of bits over the transmission link. This is the normal case for
communication between facsimile units and also for computers and modems
emulating standard Group 3 facsimile units. In this case, if the CCITT encoding is
captured directly off of the link via standard communication adapters, TIFF
FillOrder = 2 will usually apply to that stored data form.
Consequently, different TIFF FillOrder cases may arise when CCITT encodings
are obtained by synthesis within a computer (including Group 4 transmission,
which is treated more like computer data) instead of by capture from a Group 3
facsimile unit.
Because this is such a subtle situation, with surprisingly disruptive consequences
for FillOrder mismatches, the following practice is urged whenever CCITT bi-
level encodings are used:
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