TIFF 6.0 Specification
Final—June 3, 1992
TransferRange
Tag
= 342 (156.H)
Type = SHORT
N
=6
Expands the range of the TransferFunction. The first value within a pair is associ-
ated with TransferBlack and the second is associated with TransferWhite. The
ordering of pairs is the same as for pixel components of the
PhotometricInterpretation type. By default, theTransferFunction is defined over a
range from a minimum intensity, 0 or nominal black, to a maximum intensity,(1
<< BitsPerSample) - 1 or nominal white. Kodak PhotoYCC uses an extended
range TransferFunction in order to describe highlights, saturated colors and
shadow detail beyond this range. The TransferRange expands the
TransferFunction to support these values. It is defined only for RGB and YCbCr
PhotometricInterpretations.
After ReferenceBlackWhite and/or YCbCr decoding has taken place, an RGB
value can be represented as a real number. It is then rounded to create an index
into the TransferFunctiontable. In the absence of a TransferRange tag, or if the
tag has the default values, the rounded value is an index and the normalized inten-
sity value is:
index = (int) (value + (value < 0.0? -0.5 : 0.5));
intensity = TF[index] / 65535;
If the TransferRange tag is present and has non-default values, it provides an
offset to be used with the rounded index. It also describes a scaling. The normal-
ized intensity value is:
index = (int) (value + (value < 0.0? -0.5 : 0.5));
intensity = (TF[index + TransferRange[Black]] -
TF[TransferRange[Black]])
/ (TF[TransferRange[White]] - TF[TransferRange[Black]]);
An application can write a TransferFunction with a non-defaultTransferRange as
follows:
black_offset = scale_factor * Transfer(-TransferRange[Black]ar /
(TransferRange[White] - TransferRange[Black]));
for (i = 0; i < (1 << BitsPerSample); i++)
TF[i] = floor(0.5 - black_offset + scale_factor
* Transfer((i - TransferRange[Black])
/ (TransferRange[White] - TransferRange[Black])));
The TIFF writer chooses scale_factor such that the TransferFunction fits into a
16-bit unsigned short, and chooses the TransferRange so that the most important
part of the TransferFunction fits into the table.
Default is [0, NV, 0, NV, 0, NV] where NV = (1 <<BitsPerSample) - 1.
ReferenceBlackWhite
Tag
=532 (214.H)
Type = RATIONAL
N
=6
86