TIFF 6.0 Specification
Final—June 3, 1992
2 = ImageLength (height) of this chroma image is half the ImageLength of the associ-
ated luma image.
4 = ImageLength (height) of this chroma image is one-quarter the ImageLength of the
associated luma image.
Both C
b
and C
r
have the same subsampling ratio. Also,
YC
b
C
r
SubsampleVert
shall
always be less than or equal to
YC
b
C
r
SubsampleHoriz.
ImageWidth and ImageLength are constrained to be integer multiples of
YC
b
C
r
SubsampleHoriz
and
YC
b
C
r
SubsampleVert
respectively. TileWidth and
TileLength have the same constraints. RowsPerStrip must be an integer multiple
of
YC
b
C
r
SubsampleVert.
The default values of this field are [ 2, 2 ].
YC
b
C
r
Positioning
Tag
= 531 (213.H)
Type = SHORT
N
=1
Specifies the positioning of subsampled chrominance components relative to
luminance samples.
Specification of the spatial positioning of pixel samples relative to the other
samples is necessary for proper image post processing and accurate image presen-
tation. In Class Y files, the position of the subsampled chrominance components
are defined with respect to the luminance component. Because components must
be sampled orthogonally (along rows and columns), the spatial position of the
samples in a given subsampled component may be determined by specifying the
horizontal and vertical offsets of the first sample (i.e. the sample in the upper-left
corner) with respect to the luminance component. The horizontal and vertical
offsets of the first chrominance sample are denoted Xoffset[0,0] and Yoffset[0,0]
respectively. Xoffset[0,0] and Yoffset[0,0] are defined in terms of the number of
samples in the luminance component.
The values for this field are defined as follows:
Tag value YC bCr Positioning
1 centered
2 cosited
X and Y offsets of first chrominance sample
Xoffset[0,0] =
ChromaSubsampleHoriz
/ 2 - 0.5
Yoffset[0,0] =
ChromaSubsampleVert
/ 2 - 0.5
Xoffset[0,0] = 0
Yoffset[0,0] = 0
Field value 1 (centered) must be specified for compatibility with industry stan-
dards such as PostScript Level 2 and QuickTime. Field value 2 (cosited) must be
specified for compatibility with most digital video standards, such as CCIR Rec-
ommendation 601-1.
As an example, for
ChromaSubsampleHoriz
= 4 and
ChromaSubsampleVert
= 2,
the centers of the samples are positioned as illustrated below:
92