TIFF 6.0 Specification
Final—June 3, 1992
Section 21: YC
b
C
r
Images
Introduction
Digitizers of video sources that create RGB data are becoming more capable and
less expensive. The RGB color space is adequate for this purpose. However, for
both digital video and image compression applications a color difference color
space is needed. The television industry depends on YC
b
C
r
for digital video. For
image compression, subsampling the chrominance components allows for greater
compression. TIFF YC
b
C
r
(which we shall call
Class Y)
supports these images and
applications.
Class Y is based on CCIR Recommendation 601-1, “Encoding Parameters of
Digital Television for Studios.” Class Y also has parameters that allow the de-
scription of related standards such as CCIR Recommendation 709 and technologi-
cal variations such as component-sample positioning.
YC
b
C
r
is a distinct PhotometricInterpretation type. RGB pixels are converted to
and from YC
b
C
r
for storage and display.
Class Y defines the following fields:
YC
b
C
r
Coefficients
YC
b
C
r
SubSampling
YC
b
C
r
Positioning
transformation from RGB to YC
b
C
r
subsampling of the chrominance components
positioning of chrominance component samples relative
to the luminance samples
In addition, ReferenceBlackWhite, which specifies coding range expansion, is
required by Class Y. See Section 20.
Class Y YC
b
C
r
images have three components: Y, the luminance component, and
C
b
and C
r
, two chrominance components. Class Y uses the international standard
notation YC
b
C
r
for color-difference component coding. This is often incorrectly
called YUV, which properly applies only to composite coding.
The transformations between YC
b
C
r
and RGB are linear transformations of
uninterpreted RGB sample data, typically gamma-corrected values. The
YC
b
C
r
Coefficients field describes the parameters of this transformation.
Another feature of Class Y comes from subsampling the chrominance compo-
nents. A Class Y image can be compressed by reducing the spatial resolution of
chrominance components. This takes advantage of the relative insensitivity of the
human visual system to chrominance detail. The YC
b
C
r
SubSampling field de-
scribes the degree of subsampling which has taken place.
When a Class Y image is subsampled, each C
b
and C
r
sample is associated with a
group of luminance samples. The YC
b
C
r
Positioning field describes the position of
the chrominance component samples relative to the group of luminance samples:
centered or cosited.
Class Y requires use of the ReferenceBlackWhite field. This field expands the
coding range by describing the reference black and white values for the different
components that allow headroom and footroom for digital video images. Since the
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