TIFF 6.0 Specification
Final—June 3, 1992
Bytes 8-11
The Value Offset, the file offset (in bytes) of the Value for the field.
The Value is expected to begin on a word boundary; the correspond-
ing Value Offset will thus be an even number. This file offset may
point anywhere in the file, even after the image data.
IFD Terminology
A
TIFF field
is a logical entity consisting of TIFF tag and its value. This logical
concept is implemented as an
IFD Entry,
plus the actual value if it doesn’t fit into
the value/offset part, the last 4 bytes of the IFD Entry. The terms
TIFF field
and
IFD entry
are interchangeable in most contexts.
Sort Order
The entries in an IFD must be sorted in ascending order by Tag. Note that this is
not the order in which the fields are described in this document. The Values to
which directory entries point need not be in any particular order in the file.
Value/Offset
To save time and space the Value Offset contains the Value instead of pointing to
the Value if and only if the Value fits into 4 bytes. If the Value is shorter than 4
bytes, it is left-justified within the 4-byte Value Offset, i.e., stored in the lower-
numbered bytes. Whether the Value fits within 4 bytes is determined by the Type
and Count of the field.
Count
Count—called
Length
in previous versions of the specification—is the number of
values. Note that Count is not the total number of bytes. For example, a single 16-
bit word (SHORT) has a Count of 1; not 2.
Types
The field types and their sizes are:
1 = BYTE
2 = ASCII
3 = SHORT
4 = LONG
5 = RATIONAL
8-bit unsigned integer.
8-bit byte that contains a 7-bit ASCII code; the last byte
must be NUL (binary zero).
16-bit (2-byte) unsigned integer.
32-bit (4-byte) unsigned integer.
Two LONGs: the first represents the numerator of a
fraction; the second, the denominator.
The value of the Count part of an ASCII field entry includes the NUL. If padding
is necessary, the Count does not include the pad byte. Note that there is no initial
“count byte” as in Pascal-style strings.
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