Experimental Data on ISO/IEC 24790 amd 1
1 Introduction
This amendment accommodates the following two updates,­ modify the graininess and mottle measurement methods to correlate well with subjectiveimpressions,­ revise the conversion functions from scanner RGB to CIE Yd50.
Each measurement method for graininess or mottle specified in ISO/IEC 24790:2017 had shown a goodcorrelation between measured and subjective values when the standard had been developed. In eachdemonstration, the SC28/WG4 conformance test chart comprising only mid-tone patches of a variety ofgraininess or mottle levels had been evaluated.
However, one of WG4 members presented his preliminary study indicating inconsistencies between
measured and subjective values in shadow and highlight area both for graininess and mottles.
Measured values were lower than subjective in shadow and higher in highlight when CIE Y based input
images as defied in the standard. While an opposite tendency was detected in case of L* based input
images. The correlation was noticably improved by taking input images of the optimized power-of-Y of1/2.
As for conversion functions from Scanner RGB to CIE Yd50, a 4th-order even function to convert Scanner
RGB to linear sRGB, followed by a further conversion to CIE Yd50 based on the CIE matrix, gave a goodcorrelation with colourimetric measurement.
In addition, CIE Yd50 values in each patch with the above conversion were replaced by the mean CIE Yd50
value of the patch when the converted values were smaller than the mean. This reduced thediscrepancy of measured mottle values with subjective scores in a series of noise added patches.
Experimental data for this amendment are recorded in the following Clauses for the convenience ofusers of the amendment and developers in a future revision of ISO/IEC 24790.

2 Sample preparations
2.1 Graininess
The flowchart to prepare a graininess test chart to be printed both for graininess measurements andsubjective evaluation is shown in Figure 1.
Base noise images were obtained by passing a series of 2-demensional white noise images of 12 noiselevels ( = 0, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20 and 22 /255) through a spatial band pass filter of cut off
frequency from 0.2 to 4.0 cycles/mm. The graininess test chart, comprising a matrix with the 12 noiselevels and 6 gray levels (0, 32, 64, 128, 192, 224), is created by adding the base noise images to uniform
patches of the 6 gray levels.
The graininess test chart was printed by Approval XP4 (Kodak Polychrome Graphics) using a halftone
scree of 200 lpi and 45 degree.

2.2 Mottle
The flowchart to prepare a mottle test chart to be printed both for mottle measurements and subjective
evaluation is shown in Figure 1.

Base noise images were obtained by passing a series of 2-demensional white noise images of 10 noiselevels ( = 0, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5 and 5.0 /255) through a spatial band pass filter of cut off
frequency from 0 to 0.4 cycles/mm as specified in the mottle measurement method in ISO/IEC 24790.
The mottle test chart, comprising a matrix with the 10 noise levels and 6 gray levels (0, 32, 64, 128, 192,224), is created by adding the base noise images to uniform patches of the 6 gray levels.
The mottle test chart was printed by Approval XP4 (Kodak Polychrome Graphics) using a halftone screeof 200 lpi and 45 degree.
Parameter of image
Std. dev. ofnoise

Generate 1D
gaussian white noise

Convert dimension
1D to 2D
2D discreet Fourie
transform
Cut-offfrequency

Filtering
2D Invert discreet
Fourie transform

Mean Graylevel

Addition
Convert 32bit real to
8bit unsigned
Save image data

Figure 1. Flowchart to obtain graininess and mottle test chart.

3 Subjective evaluations
3.1 Graininess
The reference graininess scale was prepared by placing a series of 50% gray patches of various noiselevels in the graininess print sample on a graininess scale from 0 to 5 based on their graininess scoresmeasured in accordance with ISO/IEC 24790 as shown in Figure 2.
Ten Japanese evaluators in addition to 3 evaluators in SC 28/WG 4 experts, both comprising engineersof image quality evaluation experience, participated in the subjective graininess evaluation. Eachevaluator placed each patch in the grainiess print sample in 2.1 on the reference graininess scale basedon his subjective graininess impression by comparing with the reference patches arranged in the scale.
A score for each patch was defined as the normarized distance between 0 to 5 in the scale.
A subjective graininess score for each patch was calculated by averaging corresponding scores overevaluators.

3.2 Mottle
There were 10 evaluators comprising Japanese engineers experiencing image quality evaluation toprepare a reference mottle scale. The reference mottle scale was obtained by placing a series of 128
gray level patches of 6 noise levels in the mottle print sample in 2.2 on a mottle scale from 0 to 25, in2

which firstly the noiseless patch was placed at 0, then the patch of the highest noise level was placed at25, followed by placing the other patches between 0 to 25 based on their subjective impressions interms of mottle as shown in Figure 3.
Nine Japanese engineers of image quality evaluation experience participated in the subjective mottle
evaluation. Total of 25 patches comprising a matrix with 5 gray levels (0, 32, 64, 192 and 224) and 5
noise levels were depicted from the mottle print sample in 2.2. Each evaluator placed each patch on thereference mottle scale based on his subjective mottle impression by comparing with the reference
patches arranged in the scale. A score for each patch was defined as the normarized distance between 0to 25 in the scale.
A subjective mottle score for each patch was calculated by averaging corresponding scores overevaluators.

Subjective score=1.9

Test samples

Figure 2. Subjective evaluation procedure for Graininess.

Subjective score=14.3

Test samples

Figure 3. Subjective evaluation procedure for mottle.

3

4 Dependence of subjective correlations on power-of-Yd50
As is pointed out in Clause 1, the correlations between measured values and subjective scores both forgraininess and mottle strongly depend on power-of-CIE Yd50. Changes in correlations caused only bypower-of-CIE Yd50, excluding the effect of colur conversions, are shown in this Clause.

4.1 Graininess
Figure 4 shows the relationship between subjective scores and measured graininess values based on
ISO/IEC 24790:2017. The slops were strongly depended on gray levels.
Figure 5 shows the change in the correlations of subjective scores with measured scores under three
conditions of 1/N, power-of-Yd50. The correlation in N = 2 is much better than those in N =1 and N = 3.
Figure 6 shows the correlation factors, r2 changes with N. The forcors showed a peak at around N = 2.
9Light8Graininess76

Gray level=224

5Gray level=192

4Gray level=128

3Gray level=64
Gray level=32Dark

2Gray level=0

100

123

456

Subjective score

Figure 4 Relationship between subjective and measured graininess scores with ISO/IEC 24790.
N=1

N=2

978R² = 0.5372

6R² = 0.9597

56Graininess

Graininesss

754
3432211000

1234

560

12

Subjective score

N=37R² = 0.83766Graininess

5432
100123456

Subjective score

Figure 5 Changes in correlation with power-of-Yd50 (N) in graininiess.
43Subjective score456

1R2≒0.96

0.9

R2

0.8
0.7

0.6
0.5

0.4
11.5

22.5
Parameter N

3

Figure 6 Changes in correlation factors with power-of-Yd50 (N) in graininess.

4.2 Mottle
Figure 7 shows the relationship between subjective scores and measured mottle values based onISO/IEC 24790:2017.
Figure 8 shows the change in the correlations of subjective scores with measured scores under three
conditions of 1/N, power-of-Yd50. The correlation in N = 2 is much better than those in N =1 and N = 3.
However, the factors in N = 2 is only 0.694. A further study was required.
Figure 9 shows the reason why correlation factors is insufitient in case of N = 2. It is indicated thatmottle scores measured were significanly higher than subjective scores for shadow patches.
We solved this issue by changing the conversion functions as described in Clause 1.
4.5

Light4Mottle

3.5
3Gray level=224

2.5Gray level=192

2Gray level=128

1.5Gray level=64
Gray level=32

Dark

1

Gray level=00.5
005

1015202530

Subjective score

Figure 7 Relationship between subjective and measured mottle scores with ISO/IEC 24790.

5

N=2

N=1
4.5

4.5

4

R² = 0.3973

3.5

33Mottle

Mottle4
3.5
2.5
2R² = 0.6939

2.5
2
1.5

1.5
1

1

0.5

0.5
0005

1015

202503051015202530

Subjective score

Subjective score

N=365R² = 0.3273

Mottle432
1005

1015202530

Subjective score

Figure 8 Changes in relationship with power-of-Yd50 (N) in mottle.

4.543.5

Samles (Gray level = 0 )Mottle32.521.5

Samples (Gray level >0 )10.50051015202530

Subjective score

Figure 9 Changes in slopes with gray levels at N = 2.

5 Improved correlations by colour conversion
5.1 Graininess
Figure 10 shows the relationship between subjective scores and measured graininess values based on
the modified colour conversion procedures summarized in Clase 1 for the three conditions of 1/N,
power-of-Yd50. The correlation in N = 2 is much better than those in N =1 and N = 3. The correlation in N
= 2 improved up to 0.977, which is 0.02 better than the correlation based on the conventional colour
conversion specified in ISO/IEC 24790.
Significance of new colour conversion procedures in addition to taking optimum power-of-Yd50 isproven.
6N=2N=1698

R² = 0.5176

R² = 0.9772

56

Graininess

Graininess

754
324
3
2
1

10

00

1234

560

1234

56

Subjective score

Subjective score

N=36

R² = 0.9109

Graininess5432100123456

Subjective score

Figure 10 Correlations between measured and subjective graininess under modified colour
conversion.

5.2 Mottle
Figure 11 shows the relationship between subjective scores and measured mottle values in N = 2 basedon the modified colour conversion procedures summarized in Clase 1. The correlation between
measured and subjective mottle scores is finaly improved up to r2 = 0.937 by taking both the optimum
N and modified colour conversion.
The importance of this amendment is showen.

Mottle: Amd1
3.53

R² = 0.9365Mottle

2.5
21.5
10.5
005

1015202530

Subjective score

Figure 11 Correlations between measured and subjective mottle under modified colour conversion.

76 Conclusion
In order to improve correlations between subjective and measured scores both in graininess and mottle
for highlight and shadow patches, ISO/IEC 24790: amd1 takes two measures, one is to change thepower-of-Yd50 for input image data from 1.0 to the optimum value of 1/2, and the other is to changecolour conversion procedures from Scanner RGB to CIE Yd50.
As shown in Clause 5, the coerrelation factors in terms of r2 are greatly improved from 0.537 to 0.977
for the graininess and from 0.397 to 0.937 for mottle. Significance of this amendment was confirmed.

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