TIFF 6.0 Specification
Final—June 3, 1992
Section 20: RGB Image Colorimetry
Without additional information, RGB data is device-specific; that is, without an
absolute color meaning. This section describes a scheme for describing and char-
acterizing RGB image data.
Introduction
Color printers, displays, and scanners continue to improve in quality and avail-
ability while they drop in price. Now the problem is to display color images so
that they appear to be identical on different hardware.
The key to reproducing the same color on different devices is to use the CIE 1931
XYZ color-matching functions, the international standard for color comparison.
Using CIE XYZ, an image’s colorimetry information can fully describe its color
interpretation. The approach taken here is essentially calibrated RGB. It implies a
transformation from the RGB color space of the pixels to CIE 1931 XYZ.
The appearance of a color depends not only on its absolute tristimulus values, but
also on the conditions under which it is viewed, including the nature of the sur-
round and the adaptation state of the viewer. Colors having the same absolute
tristimulus values appear the same in identical viewing conditions. The more
complex issue of color appearance under different viewing conditions is ad-
dressed by [4]. The colorimetry information presented here plays an important
role in color appearance under different viewing conditions.
Assuming identical viewing conditions, an application using the tags described
below can display an image on different hardware and achieve colorimetrically
identical results. The process of using this colorimetry information for displaying
an image is straightforward on a color monitor but it is more complex for color
printers. Also, the results will be limited by the color gamut and other characteris-
tics of the display or printing device.
The following fields describe the image colorimetry information of a TIFF image:
WhitePoint
PrimaryChromaticities
TransferFunction
chromaticity of the white point of the image
chromaticities of the primaries of the image
transfer function for the pixel data
TransferRange
extends the range of the transfer function
ReferenceBlackWhite
pixel component headroom and footroom parameters
The TransferFunction, TransferRange, and ReferenceBlackWhite fields have
defaults based on industry standards. An image has a colorimetric interpretation if
and only if both the WhitePoint and PrimaryChromaticities fields are present. An
image without these colorimetry fields will be displayed in an application and
hardware dependent manner.
Note: In the following definitions, BitsPerSample is used as if it were a single
number when in fact it is an array of SamplesPerPixel numbers. The elements of
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