Colorimetric characteristics of TV and related systems

Current television or any other related non-adaptive image systems can be presented in accordance with the scheme, depicted on the Figure 1.1, where the system is presented as a series connection of light-signal transformer (camera), electrical transmission path and signal-light transformer (reproducing device).

In the electrical path of TV-system, transmitted color is expressed with the primary signals or luminance and colour difference signals. These signals can be considered as coordinates of the three-dimensional color space of system.

Transition “light-signal” and “signal-light” in the terminal devices may be presented as the transition from (for example ) color space coordinates to the signals (for example or ) at the transmitting side and as the transition from the signals to coordinates of reproduced image color space on the receiving side.

Figure 1.2 is a block diagram of an adaptive image system, providing color reproduction, independent on used devices (regarding colorimetric transformations, realized in them) and on the shooting object observation conditions at the transmitting side and reproduced image on the receiving side, is given. The principal distinction of such a system from non-adaptive system consists in using color space in the transmitting environment independent of the colorimetric transformations in devices used and from observation conditions.

For color reproduction quality assessment in uniform color space, in which shooting object visual perception is associated with the coordinates of this colour space at the transmitting side, and reproduced image visual perception is associated with color coordinates, on the receiving side may be used. As coordinates color spaces coordinates, expressed with different degree of adequacy to the human vision characteristics, may be used. As such color spaces the CIE developed spaces CIELUV, CIELAB, CIECAM02 may be used. As a measure of color reproduction quality in such case with a certain degree of conditional character distance in the space may be used.

In multimedia applications, the principle of device independent images transmission can be realized by means of ICC profile use [1.1, 1.2], which in colorimetry literature is expanded to principle of viewing independent transmission principle.

For television applications such principles are described in [1.3‒1.10].

Shooting object

Light-signal transformer

Transmission path

Transformer signal-light

Reproduced image

Light-signal transformer   Forming of information about viewing conditions and about transformer characteristics

Shooting object

Transmission system

Signal-light transformer

Reproduced image

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.

Date: 2014-12-22; view: 1661