TIFF 6.0 Specification
Final—June 3, 1992
Specifies a pair of headroom and footroom image data values (codes) for each
pixel component. The first component code within a pair is associated with
ReferenceBlack, and the second is associated with ReferenceWhite. The ordering
of pairs is the same as those for pixel components of the PhotometricInterpretation
type. ReferenceBlackWhite can be applied to images with a
PhotometricInterpretation value of RGB or YCbCr. ReferenceBlackWhite is not
used with other PhotometricInterpretation values.
Computer graphics commonly places black and white at the extremities of the
binary representation of image data; for example, black at code 0 and white at
code 255. In other disciplines, such as printing, film, and video, there are practical
reasons to provide footroom codes below ReferenceBlack and headroom codes
above ReferenceWhite.
In film applications, they correspond to the densities Dmax and Dmin. In video
applications, ReferenceBlack corresponds to 7.5 IRE and 0 IRE in systems with
and without setup respectively, and ReferenceWhite corresponds to 100 IRE
units.
Using YCbCr (See Section 21) and the CCIR Recommendation 601.1 video stan-
dard as an example, code 16 represents ReferenceBlack, and code 235 represents
ReferenceWhite for the luminance component (Y). For the chrominance compo-
nents, Cb and Cr, code 128 represents ReferenceBlack, and code 240 represents
ReferenceWhite. With Cb and Cr, the ReferenceWhite value is used to code
reference blue and reference red respectively.
The full range component value is converted from the code by:
FullRangeValue = (code - ReferenceBlack) * CodingRange
/ (ReferenceWhite - ReferenceBlack);
The code is converted from the full-range component value by:
code = (FullRangeValue * (ReferenceWhite - ReferenceBlack)
/ CodingRange) + ReferenceBlack;
For RGB images and the Y component of YCbCr images, CodingRange is de-
fined as:
CodingRange = 2 ** BitsPerSample - 1;
For the Cb and Cr components of YCbCr images, CodingRange is defined as:
CodingRange = 127;
For RGB images, in the default special case of no headroom or footroom, this
conversion can be skipped because the scaling multiplier equals 1.0 and the value
equals the code.
For YCbCr images, in the case of no headroom or footroom, the conversion for Y
can be skipped because the value equals the code. For Cb and Cr, ReferenceBlack
must still be subtracted from the code. In the general case, the scaling multiplica-
tion for the Cb and Cr component codes can be factored into the YCbCr transform
matrix.
Useful ReferenceBlackWhite values for YCbCr images are:
[0/1, 255/1,128/1, 255/1, 128/1, 255/1]
no headroom/footroom
[15/1, 235/1, 128/1, 240/1, 128/1, 240/1]
CCIR Recommendation 601.1 headroom/footroom
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