TIFF 6.0 Specification
Final—June 3, 1992
entirety. The main complication for readers is in properly restoring T.6-encoding
after the uncompressed sequence is laid down in the current row.
Readers that have no provision for uncompressed mode will generally reject any
case in which the flag is set. Readers that are able to process uncompressed-mode
content within T.6-encoding strings can safely ignore this flag and simply process
any uncompressed-mode occurences correctly.
Writers that are unable to guarantee the absence of uncompressed-mode material
in any of the T.6-encoded segments must set the flag. The flag should be cleared
(or defaulted) only when absence of uncompressed-mode material is assured.
Writers that are able to inhibit the generation of uncompressed-mode extensions
are encouraged to do so in order to maximize the acceptability of their T.6-encod-
ing strings in interchange situations.
Because uncompressed-mode is not commonly used, the following description is
best taken as suggestive of the general machinery. Interpolation of fine details can
easily vary between implementations.
Uncompressed mode is signalled by the occurence of the 10-bit extension code
string
0000001111.B
outside of any run-length make-up code or extension. Original unencoded image
information follows. In this unencoded information, a 0-bit evidently signifies a
white pixel, a 1-bit signifies a black pixel, and the TIFF PhotometricInterpretation
will influence how these bits are mapped into any final uncompressed bitmap for
use. The only modification made to the unencoded information is insertion of a 1-
bit after every block of five consecutive 0-bits from the original image informa-
tion. This is a transparency device that allows longer sequencences of 0-bits to be
reserved for control conditions, especially ending the uncompressed-mode se-
quence. When it is time to return to compressed mode, the 8-bit exit sequence
0000001t.B
is appended to the material. The 0-bits of the exit sequence are not considered in
applying the 1-bit insertion rule; up to four information 0-bits can legally precede
the exit sequence. The trailing bit, ‘t,’ specifies the color (via 0 or 1) that is under-
stood in the next run of compressed-mode encoding. This lets a color other than
white be assumed for the 0-width pixel on the left of the edge between the last
uncompressed pixel and the resumed 2-dimensional scan.
Writers should confine uncompressed-mode sequences to the interiors of indi-
vidual rows, never attempting to “wrap” from one row to the next. Readers must
operate properly when the only encoding for a single row consists of an
uncompressed-mode escape, a complete row of (proper 1-inserted) uncompressed
information, and the extension exit. Technically, the exit pixel, ‘t,’ should prob-
ably then be the opposite color of the last true pixel of the row, but readers should
be generous in this case.
In handling these complex encodings, the encounter of material from a defective
source or a corrupted file is particularly unsettling and mysterious. Robust readers
will do well to defend against falling off the end of the world; e.g., unexpected
EOFB sequences should be handled, and attempted access to data bytes that are
not within the bounds of the present segment (or the TIFF file itself) should be
avoided.
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