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Color appearance models


MPEG-4, MPEG-4/AVC

Digital video coding system colorimetric characteristics specified in MPEG-2 – H.262 | 13818-2 [3.15, 3.16], MPEG-4 – 14496-2 [3.17, 3.18], H.264 | MPEG-4/AVC – 14496-10 [3.19] are shown in the Tables 3.9, 3.10, 3.11, which combine according data from Tables 6-7, 6-8, 6-9 from standard 13818-2, Tables 6-8, 6-9, 6-10 from standard 14496-2 and Tables E-2, E-3, E-4 from standard 14496-10.

Primaries chromaticity coordinates and reference white for given parameter values of
color_primaries are shown it Table 3.9.

Opto-electronic conversion characteristics – transfer primaries channels characteristics for given parameter values of transfer_characteristics are shown in Table 3.10. Table specifies:

– relative luminance levels – image components;

V– relative levels of gamma-corrected signals – image components ( );

– normalized luminance signal, ;

, – color-difference signals normalized to .

Luminance signals and color-difference signals matrixes coefficients for given parameter values of transfer_characteristics are shown in Table 3.11. For maintaining form of coefficient definitions, accepted in standards, matrixes coefficients of ITU-T H.262 | ISO/IEC 13818-2 and ISO/IEC 14496-2 standards are given in 3 column; matrixes coefficients of ITU-T H.264 | ISO/IEC 14496-10 standard are given in 4 column through KR and KB, used for matrix conversion description as following:

(3.154)

(3.155)

(3.156)

with exception of cases when transfer_characteristics values are equal to 0 and 8.

Value 8 in ITU-T H.262 | ISO/IEC 13818-2, ISO/IEC 14496-2 and ITU-T H.264 | ISO/IEC 14496-10 accords to signal coding , processed by algorithms specified in these standards, where signals are in terms of .

Value 0 in ITU-T H.264 | ISO/IEC 14496-10 accords to coding space signals coding processed by algorithms specified in Annex E.2 of standard ITU-T H.264 | ISO/IEC 14496-10.


TABLE 3.9
Digital video coding system colour primaries referred to MPEG-2, MPEG-4, MPEG-4/AVC

color_primaries Systems and standards Primaries and reference white chromaticity coordinates
Forbidden (ISO/IEC 13818-2 and ISO/IEC 14486-2)  
Reserved (ISO/IEC 14486-10) For future use ITU-T/ISO/IEC
Recommendation ITU-R BT.709-5 [3.6] Recommendation INTO-R BT.1361 [3.9], conventional color gamut system and extended color gamut system IEC 61966-2-4 [3.13] SMPTE 170M [3/2] (functionally the same as the value)      
  x y
Red 0.640 0.330
Green 0.300 0.600
Blue 0.150 0.060
White D65 0.3127 0.3290
 
Unspecified Image characteristics are unknown or are determined by the application
Reserved For future use by ITU-T | ISO/IEC
ITU-R Recommendation BT.470-6 system M (historically) ITU-R Recommendation BT.1700 [3.3], 625 PAL or 625 SECAM (ISO/IEC 14486-10) NTSC 1953 Recommendation for transmission standards for color television US FCC Title 47 Code of Federal Regulations (2004) 73.682 (a) (20)      
  x y
Red 0.67 0.33
Green 0.21 0.71
Blue 0.14 0.08
White C 0.310 0.316
 
ITU-R Recommendation BT.1700 [3.3] 625 PAL or 625 SECAM ITU-R Recommendation BT.1358 [3.5] 625 ITU-R Recommendation BT.601-6 [3.4] 625 (ISO/IEC 13818-2 and 14486-2) ITU-R Recommendation BT.470-6 systems B, G (historically)      
  x y
Red 0.64 0.33
Green 0.29 0.60
Blue 0.15 0.06
White D65 0.3127 0.3290
 
                 

TABLE 3.9
(End)



ITU-R Recommendation BT.1700 [3.3] NTSC ITU-R Recommendation BT.1358 [3.5] 525 (ISO/IEC 13818-2, 14486-2, 14486-10) or 625 (only ISO/IEC 14486-2) SMPTE 170M [3.2] (functionally the same as the value 7) ITU-R Recommendation BT.601-6 [3.4] 525 (ISO/IEC 13818-2)      
  x y  
Red 0.630 0.340
Green 0.310 0.595
Blue 0.155 0.070
White D65 0.3127 0.3290
   
SMPTE 240M [3.20] (functionally the same as the value 6)      
  x y  
Red 0.630 0.340
Green 0.310 0.595
Blue 0.155 0.070
White D65 0.3127 0.3290
   
Reserved (ISO/IEC 13818-2)   For future use by ITU-T | ISO/IEC  
ISO/IEC 14486-2 ISO/IEC 14486-10 Generic film (color filters using standard illuminant C) (ISO/IEC 14486-2, 14486-10)      
  x y  
Red 0.681 0.319 (Wratten 25)
Green 0.243 0.692 (Wratten 58)
Blue 0.145 0.049 (Wratten 47)
White C 0.310 0.316  
   
9-255 Reserved   For future use by ISO/IEC  
               

 


TABLE 3.10
Digital video coding system transfer signal source characteristics referred to MPEG-2, MPEG-4, MPEG-4/AVC

transfer_characteristic Systems and standards Transfer characteristic  
Forbidden (ISO/IEC 13818-2 and ISO/IEC 14486-2)    
Reserved (ISO/IEC 14486-10) For future use ITU-T/ISO/IEC  
Recommendation ITU-R BT.709-5 [3.6] Recommendation ITU-R BT.1361 [3.9] conventional color gamut system (functionally the same as the value 6)  
Unspecified Image characteristics are unknown or are determined by the application  
Reserved For future use by ITU-T/ISO/IEC  
ITU-R Recommendation BT.470-6 system M (historically) Recommendation BT.1700 [3.3] 625 PAL or 625 SECAM (ISO/IEC 14486-2) US NTSC 1953 Recommendation for transmission standards for color television US FCC Title 47 Code of Federal Regulations (2004) 73.682 (a) (20) Assumed displayed gamma 2.8 (this value conflicts with ITU-R Recommendation BT.1700 (2007 revision) and accordingly to this Recommendation has to be changed to 2.2)  
ITU-R Recommendation BT.1700 [3.3] 625 PAL or 625 SECAM (ISO/IEC 13818-2, 14496-10) ITU-R Recommendation BT.470-6 systems B, G (historically) Assumed displayed gamma 2.8 (this value conflicts with ITU-R Recommendation BT.1700 (2007 revision) and accordingly to this Recommendation has to be changed to 2.2)
         

 


TABLE 3.10
(End)

ITU-R Recommendation BT.1700 [3.3] NTSC ITU-R Recommendation BT.1358 [3.6] 525 or 625 SMPTE 170M [3.2] (functionally the same as the value 1) ITU-R Recommendation BT.601-6 [3.4] 625 (ISO/IEC 13818-2 and 14486-2) See Eq. 3.141  
SMPTE 240M (1987) [3.21] Opto-electronic conversion:  
Linear transfer characteristic    
(ISO/IEC 13818-2, ISO/IEC 14486-2)  
(ISO/IEC 14486-10)  
Logarithm transfer characteristic (100:1 range)  
Logarithm transfer characteristic (316.22777:1 range ) (in ISO/IEC 14486-2: )  
IEC 61966-2-4 [3.13]  
Recommendation ITU-R BT.1361 [39], extended color gamut system (3.164)
9-255   Reserved for future use ITU-T/ISO/IEC

Table 3.11
Matrix coefficients defined for video coding system referred to MPEG-2, MPEG-4, MPEG-4/AVC

matrix_coefficients Systems and standards Matrix
ISO/IEC 13818-2 ISO/IEC 14486-2 ISO/IEC 14486-10
  Forbidden RGB
Recommendation ITU-R BT.709-5 [3.6] Recommendation ITU-R BT.1361 [3.9], conventional color gamut system and extended color gamut system IEC 61966-2-4 xvYCC709 [3.13] SMPTE RP 177 Annex B [3.1]
  Unspecified. Image characteristics are unknown or determined by the application
  Reserved. For future use ITU-T/ISO/IEC
US NTSC 1953 Recommendation for transmission standards for color television US FCC Title 47 Code of Federal Regulations (2004) 73.682 (a) (20)
ITU-R Recommendation BT.1700 [3.3] (2007 revision) 625 PAL and 625 SECAM ITU-R Recommendation BT.1358 [3.5] 625 IEC 61966-2-4 xvYCC709 [3.13] ITU-R Recommendation BT.470-6 systems B, G (historical) (Functionally the same as the value 6) ITU-R Recommendation BT.601-6 [3.4] 625 (Functionally the same as the value 6) (ISO/IEC 13818-2, ISO/IEC 14486-2)

 


TABLE 3.11
(Continued)

ITU-R Recommendation BT.1700 [3.3] (2007 revision) NTSC ITU-R Recommendation BT.1358 [3.5] 525 SMPTE 170M [3.2] IEC 61966-2-4 xvYCC601 [3.13] (Functionally the same as the value 5) ITU-R Recommendation BT.470-6 systems B, G (historical) (Functionally the same as the value 6) ITU-R Recommendation BT.601-6 [3.4] 525 (Functionally the same as the value 5) (ISO/IEC 13818-2, 14496-2)
SMPTE 240M (1987) [3.21]
  YCgCo
9-255   Reserved. For future use ITU-T/ISO/IEC

TABLE 3.11
Continued)

is analog with the value 0 associated with nominal black and the value 1 associated with nominal white; and are analog with the value 0 associated with both nominal black and nominal white; When transfer_characteristics is not equal to 11 or 12, has values between 0 and 1; When transfer_characteristics is not equal to 11 or 12, and have values between -0.5 and 0.5; When transfer_characteristics is not equal to 11 (IEC 61966-2-4) or 12 (ITU-R Recommendation BT.1361 extended color gamut system), are analog with a larger range not specified in this International Standard; are related to by the following formulae:
If video_range is equal to 0:  
If video_range is equal to 1: for n bit video.
– If matrix_coefficients is equal to 8 , the following applies:
If video_range is equal to 0:

TABLE 3.11
(End)

Note 1 – For purposes of the nomenclature used in Table, and d in the above equations may be referred to as and , respectively. The inverse conversion for the above three equations should be computed as: Note 2 – The decoding process given in the Recommendation | International standard limits output sample values for Y, Cr and Cb to the range [0:255]. Thus, sample values :outside the range implied by the above equations may occasionally occur at the output of the decoding process/ In particular the sample values 0 and 255 may occur In the case that sequence_display_extension() is not present in the bitstream or color_description is zero the matrix coefficients are assumed to be implicitly defined by the application. Note 3 – In applications which may have signals with more than one set of color primaries, transfer characteristics, and/or matrix coefficients, it is recommended to transmit a sequence display extension with color_description set to one, and to specify the appropriate values for the colorimetry parameters. If video_range is equal to 0: for n bit video. are related to R, G and B by the following formulae: Note – For purposes of the nomenclature used in Table, and in the above equations may be referred to as and , respectively. The inverse conversion for the above three equations should be computed as: In the case that video_signal_type() is not present in the bitstream or color_description is zero the matrix coefficients are assumed to be those corresponding to matrix_coefficients having the value 1. In the case that video_signal_type() is not present in the bitstream, video_range is assumed to have the value 0 (a range of Y from 16 to 235 for 8-bit video).

 

Color appearance models

4.1 General requirements for color appearance models

One of the crucial question for imaging systems developers is the ensuring high level of quality of image capture and reproduction, and, particularly, high level of colorimetric quality. The traditional way of assessing the reproduction quality of imaging systems is subjective assessment. Though being the most exact, it requires significant costs, time resources and computational burden. It cannot be used for real time quality assessment. There are many objective image quality assessment methods, but most of them don’t take into account colorimetric component of overall image quality.

As it was previously stated, the perception of colors plays a major part in overall image quality perception. R.W.G. Hunt in [4.1] has formulated six approaches to color reproduction (see section 9). Two of them seem to be suitable for implementation in TV systems:

- Equivalent color reproduction. Approach, by which the goal is achieving equality of chromaticities and absolute and relative luminances of colors of original scene and reproduced image being viewed under different conditions.

- Preferable color reproduction. The purpose of such approach no achievement of strict equality of color perception of display and standard images, but reproduction of colors in such a way that the colors of the estimated image were more pleasant for an observer, than colors of original scene.

It should be noted that though the reproduction of memory colors has substantial influence on judgments about reproduced image; however it cannot be used as independent criterion.

Color spaces are used for mathematical representation of colors independently on spectral power distribution of the optical radiation. The CIE 1931 color space was standardized by Comission Internationale de l’Eclairage for color patches of 2 degree angular subtense and CIE 1964 for patches of 10 degree angular subtense. These color spaces are used for color representation only, they do not allow account of the state of observer’s adaptation to viewing conditions. To allow accounting of viewing conditions (that is necessary for color transforms and colorimetric distortion correction) various color appearance model have been developed.

According to the definition of CIE Technical Committee 1-34 [4.2], color appearance model is the any model predicting at least the relative color appearance characteristics, such as lightness, chroma and hue. To make accurate prediction of these parameters the model should account for chromatic adaptation. It is desirable for a model to predict also brightness and colorfulness perception accounting for color vision phenomena that make a great contribution into color perception.

Requirements for color appearance models are defined by applications in which they are used. Requirements, formulated by TC 1-34, are following [4.2]:

1 The model should be as comprehensive as possible so that it can be used in a variety of applications; but at this stage only static states of adaptation should be included because of the great complexity of dynamic effects.

2 The model should cover a wide range of stimulus intensities from very dark object colors to very bright self–luminous color. This means that the dynamic responds function must have a maximum and can’t be simple logarithmic or power function.

3 The model should cover a wide range of adapting intensities, from very low scotopic levels such as occur in starlight to very high photopic levels, such as occur in sunlight. This means that rod vision should be included in the model; but because many applications will be such that rod vision is negligible, the model should be useable in a mode that does not include rod vision.

4 The model should cover a wide range of viewing conditions including backgrounds of different luminance factors, and dark, dim and average surrounds. It is necessary to cover the different surrounds because of their widespread use in projected and self–luminous displays.

5 For easer use the spectral sensitivities of the cones should be a linear transformation of the CIE or functions; and functions should be used for the spectral sensitivity of the rods. Because scotopic photometric data is often unknown, methods of providing approximate scotopic values should be provided.

6 The model should be able to provide for any degree of adaptation between complete and none for cognitive factors and for the Helson-Judd effect, as options.

7 The model should give predictions of hue (both as hue – angle, and as hue – quadrature), brightness, lightness, saturation, chroma and colorfulness.

8 The model should be capable of being operated in the reverse mode.

9 The model should be no more complicated than is necessary to meet the above requirements.

10 Any simplified version of the model intended for particular applications should give the same predictions as the complete model for some specified set of conditions.

11 The model should give predictions of color appearance that are not appreciably worse than those given by the model that is the best in each application.

12 A version of a model should be available for application to unrelated colors (those seen in dark surrounds in isolation from other colors).

Description of adopted CIE models [4.1, 4.3-4.5] CIELUV, CIELAB, and CIECAM02
[4.2, 4.4] is shown below with examples of color appearance calculation.


Date: 2014-12-22; view: 692


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