TIFF 6.0 Specification
Final—June 3, 1992
2 = Unassociated alpha data
Associated alpha data is opacity information; it is fully described in Section 21.
Unassociated alpha data is transparency information that logically exists indepen-
dent of an image; it is commonly called a soft matte. Note that including both
unassociated and associated alpha is undefined because associated alpha specifies
that color components are pre-multiplied by the alpha component, while
unassociated alpha specifies the opposite.
By convention, extra components that are present must be stored as the “last com-
ponents” in each pixel. For example, if SamplesPerPixel is 4 and there is 1 extra
component, then it is located in the last component location (SamplesPerPixel-1)
in each pixel.
Components designated as “extra” are just like other components in a pixel. In
particular, the size of such components is defined by the value of the
BitsPerSample field.
With the introduction of this field, TIFF readers must not assume a particular
SamplesPerPixel value based on the value of the PhotometricInterpretation field.
For example, if the file is an RGB file, SamplesPerPixel may be greater than 3.
The default is no extra samples. This field must be present if there are extra
samples.
See also SamplesPerPixel, AssociatedAlpha.
FillOrder
The logical order of bits within a byte.
Tag
= 266 (10A.H)
Type = SHORT
N
=1
1 = pixels are arranged within a byte such that pixels with lower column values are
stored in the higher-order bits of the byte.
1-bit uncompressed data example: Pixel 0 of a row is stored in the high-order bit
of byte 0, pixel 1 is stored in the next-highest bit, ..., pixel 7 is stored in the low-
order bit of byte 0, pixel 8 is stored in the high-order bit of byte 1, and so on.
CCITT 1-bit compressed data example: The high-order bit of the first compres-
sion code is stored in the high-order bit of byte 0, the next-highest bit of the first
compression code is stored in the next-highest bit of byte 0, and so on.
2 = pixels are arranged within a byte such that pixels with lower column values are
stored in the lower-order bits of the byte.
We recommend that FillOrder=2 be used only in special-purpose applications. It
is easy and inexpensive for writers to reverse bit order by using a 256-byte lookup
table.
FillOrder = 2 should be used only when BitsPerSample = 1 and the data is
either uncompressed or compressed using CCITT 1D or 2D compression, to
avoid potentially ambigous situations.
Support for FillOrder=2 is not required in a Baseline TIFF compliant reader
Default is FillOrder = 1.
32