3 Colorimetric characteristics of tv and related systems
3.1 Introductory note
Colorimetric characteristics take a great role among video systems characteristics; they considerably influence on overall quality of transmitted and reproduced images. In this section information on colorimetric characteristics of television, multimedia and other related systems is summarized. The description of color spaces for some image compression systems is also shown.
A complete color space definition for digital video representation may include specification of the following aspects:
The chromaticity coordinates
of
the source color primaries
and coordinates
of reference white point.The opto-electronic transfer characteristics of the source components (e.g., definition of
as a function of
).Matrix coefficients for transformation of the RGB components into luma and chroma components (e.g., definition of components
and
as a function of
and 
).Definition of scaling, offsets, and quantization for digital representation.
A gamut boundary definition specifying the range of values over which effective representations of colors can be achieved.
3.2 Relationship between color coordinates in xyz color space and rgb signal space
The
correlations interrelating between
1931 color space and RGB
signal space of TV system in accordance with SMPTE RP 177 3.1
are represented
in this sub clause.
RGB
signal space coordinates are normalized in such a way, that reference
white is equi-primary signal
.
For
transformations the
matrix
of primaries chromaticity coordinates and
vector of reference white chromaticity coordinates
(3.1)
are used. vector normalization corresponds to reference white assignment with a unit luminance factor.
Signal
space in television is normalized to the unit range of
relative luminance change that corresponds to change of
primary signal levels between the values 0 and 1. It corresponds to
such
space
normalization, that
coordinate characterizing the image relative luminance values, takes
0 values on black and 1 on white.
Relationship
between
color
space and
signal space is carried out as
(3.2)
where system primaries coordinates matrix is:
(3.3)
It is calculated with use of formula:
(3.4)
The second row of normalized primary matrix represents the vector of primaries luminance factors, relative color luminance coordinates being determined as
. (3.5)
Thus
characterize primaries relative luminance. They are also named
primaries luminance factors and designated:
.
It
has been marked in 3.1
that as a result of calculations with the limited number of digits
(because of rounding) coefficients of the second row can turn out at
the calculation of
matrix, the sum of which will differ from unit. For this case it is
recommended to normalize the matrix columns so that to obtain
equality of this sum to unit.
3.3 Colorimetric characteristics of analog television systems
Colorimetric characteristics of analog television systems are presented in Table 3.1, in which it is marked:
–
relative
luminance levels of
-components;
–
gamma-corrected
-signals
relative levels (
);
–
luminance
signal;
– color-difference
signals of the PAL system;
– color-difference
signals of the SECAM system.