TIFF 6.0 Specification
Final—June 3, 1992
default for ReferenceBlackWhite is inappropriate for Class Y, it must be used
explicitly.
At first, it might seem that the information conveyed by Class Y and the RGB
Colorimetry section is redundant. However, decoding YC
b
C
r
to RGB primaries
requires the YC
b
C
r
fields, and interpretation of the resulting RGB primaries re-
quires the colorimetry and transfer function information. See the RGB Colorim-
etry section for details.
Extensions to Existing Fields
Class Y images use a distinct PhotometricInterpretation Field value:
PhotometricInterpretation
Tag
= 262 (106.H)
Type = SHORT
N
=1
This Field indicates the color space of the image. The new value is:
6 = YC
b
C
r
A value of 6 indicates that the image data is in the YC
b
C
r
color space. TIFF uses
the international standard notation YC
b
C
r
for color-difference sample coding. Y is
the luminance component. C
b
and C
r
are the two chrominance components. RGB
pixels are converted to and from YC
b
C
r
form for storage and display.
Fields Defined in Class Y
YC
b
C
r
Coefficients
Tag
= 529 (211.H)
Type = RATIONAL
N
=3
The transformation from RGB to YC
b
C
r
image data. The transformation is speci-
fied as three rational values that represent the coefficients used to compute lumi-
nance, Y.
The three rational coefficient values,
LumaRed, LumaGreen
and
LumaBlue,
are
the proportions of red, green, and blue respectively in luminance, Y.
Y, C
b
, and C
r
may be computed from RGB using the luminance coefficients
specified by this field as follows:
Y = (
LumaRed
* R +
LumaGreen
* G +
LumaBlue
* B )
C
b
= ( B - Y ) / ( 2 - 2 *
LumaBlue
)
90