ISO/TS 18621-31:2024
(Main)Graphic technology — Image quality evaluation methods for printed matter — Part 31: Evaluation of the perceived resolution of printing systems with the Contrast–Resolution chart
Graphic technology — Image quality evaluation methods for printed matter — Part 31: Evaluation of the perceived resolution of printing systems with the Contrast–Resolution chart
This document specifies the Contrast–Resolution test chart, the requirements on the printing process needed to reproduce this test chart, the required characteristics of a high resolution scanner needed to digitize the information reproduced on printed test charts, and the requirements on the interpretation of this digitized data. It also specifies the resolution-score method for evaluating the perceptual resolution of printed material using the Contrast–Resolution test chart. The procedure specified in this document is intended for a characterization of the perceived resolution of a graphic arts production printing system using the Contrast–Resolution test chart.
Technologie graphique — Méthodes d’évaluation de la qualité d’image pour les imprimés — Partie 31: Évaluation de la résolution perçue des systèmes d’impression avec un graphique de contraste–résolution
General Information
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Standards Content (Sample)
Technical
Specification
ISO/TS 18621-31
Second edition
Graphic technology — Image quality
2024-01
evaluation methods for printed
matter —
Part 31:
Evaluation of the perceived
resolution of printing systems with
the Contrast–Resolution chart
Technologie graphique — Méthodes d’évaluation de la qualité
d’image pour les imprimés —
Partie 31: Évaluation de la résolution perçue des systèmes
d’impression avec un graphique de contraste–résolution
Reference number
© ISO 2024
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ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Requirements . 3
4.1 General .3
4.2 Apparatus requirements .3
4.2.1 Printing system requirements .3
4.2.2 Scanning system requirements.3
4.3 Procedure .3
4.3.1 Test chart.3
4.3.2 Evaluation intent .4
4.3.3 Printing and scanning .5
4.3.4 Evaluation process.6
5 Resolution-Score processing . 6
5.1 General .6
5.2 Element identification .6
5.3 Scanning signal interpretation .6
5.4 Spatial filtering .6
5.5 Normalized 2-D cross-correlation .7
5.6 Resolution-score computation.7
6 Reporting .12
Annex A (normative) Test chart and reference files — Availability . 14
Annex B (normative) Printing process and data path requirements . 17
Annex C (normative) Linearization .21
Annex D (normative) Scanner conformance requirements .23
Annex E (normative) Evaluation process conformance .29
Annex F (normative) Spatial filter — Specification .31
Bibliography .35
iii
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out through
ISO technical committees. Each member body interested in a subject for which a technical committee
has been established has the right to be represented on that committee. International organizations,
governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely
with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are described
in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types
of ISO document should be noted. This document was drafted in accordance with the editorial rules of the
ISO/IEC Directives, Part 2 (see www.iso.org/directives).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent
rights in respect thereof. As of the date of publication of this document, ISO had not received notice of (a)
patent(s) which may be required to implement this document. However, implementers are cautioned that
this may not represent the latest information, which may be obtained from the patent database available at
www.iso.org/patents. ISO shall not be held responsible for identifying any or all such patent rights.
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and expressions
related to conformity assessment, as well as information about ISO's adherence to the World Trade
Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 130, Graphic technology.
This second edition cancels and replaces the first edition (ISO/TS 18621:2020), which has been technically
revised.
The main changes are as follows:
— in subclause 5.5:
— Formula (1) has been corrected, including interchanging symbols ‘f’ and ‘g’;
— in the NOTE the ‘Sample’ and ‘Ref’ arguments are switched;
— the text for step 13 in subclause 5.6 has been concretized;
— steps 18 and 19, also in Figure 8, have been adapted with a change from ‘unacceptable’ to ‘suspect’;
— the text in Annex A, before Table A.2, has been concretized;
rd
— the text in E.2, 3 paragraph, has been concretized;
— replacement of the three PDF files in the ‘TestCharts’ folder at the ISO server for ‘Electronic Inserts’ by a
single ZIP file ‘TestCharts_2020correction.zip’.
A list of all parts in the ISO 18621 series can be found on the ISO website.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.
iv
Introduction
Perceived resolution, the capability to perceive fine detail, is a measure of full system capability and depends
upon characteristics of the printing system (substantially more than just its addressability), characteristics
of the substrate, of the viewing conditions, and of the observer. Perceived resolution depends critically upon
tonal differences between elements of an image – there is no perceived detail, hence no measure of resolution,
when there is no tonal difference in an image. The three major contributors to the perceived resolution of
a printing system are the capability of a printing system to maintain a desired spatial separation between
nearby elements printed on a substrate (the addressability of a printing system indicates what the minimum
spatial separation can be), the capability of the printing system to carry tonal differences (contrast) between
these nearby printed elements, and the capability of the human visual system to perceive the printed detail.
The design of a test chart and an evaluation process for measuring a printing system’s capability to carry
fine detail must reflect these major contributors.
[1]
Fourier analysis has proven very useful in analysing the reproduction capability of image forming systems .
In this formalism, spatial separation is measured in terms of spatial frequency (e.g. cycles per millimetre)
and contrast is measured in terms of modulation (the dimensionless ratio of a change in perceived luminance
to its average luminance) at a particular spatial frequency. The ratio of the reproduced modulation to the
original (desired) modulation can be used to describe the capability of a printing system to reproduce a
sinusoidal input at a particular spatial frequency. This ratio, taken over a range of spatial frequencies is
called the modulation transfer function (MTF).
Key
X spatial fequency
1 modulation of original (constant amplitude)
2 modulation of reproduction (with limited resolution)
3 modulation transfer function (decreases due to limited resolution)
Figure 1 — Modulation transfer function of a printing system
The MTF characteristic shows the ratio of the reproduced modulation to the original (input) modulation
as a function of spatial frequency and provides a very useful description of printing system capability.
The decrease at high frequencies of the modulation transfer function shown in Figure 1 characterizes the
common degradation in printing system image detail capability at high spatial frequencies.
v
In characterizing perceived resolution, a single component of the imaging chain cannot be isolated since
we look at the results of the complete system. The printing system imaging chain starts with the process
of placing marks on a substrate. In many printing systems, the individual marks can provide only a limited
number of tonal levels and the full tonal range is provided by subsequent area modulation (screening)
of the marks. This screening process can strongly affect the image detail capability of a printing system.
The characteristics of the substrate can affect both the effectiveness of placing these marks (e.g. surface
roughness) and affect the interplay between the placed marks and the illumination required for viewing
the printed image (e.g. light scattering in the substrate). Finally, perceived resolution depends upon the
viewing conditions (illumination, viewing distance, and magnification) as well as the capability of the
human observer to perceive detail. The capability of normal human vision to perceive spatial detail can be
characterized by a modulation transfer function (see Reference [2]). This is shown in Figure 2.
Key
X spatial frequency
Y relative contrast sensitivity
a
6/6 visual limit.
b
cy/mm at 300 mm.
c
cy/mm at 400 mm.
d
cy/degree.
Figure 2 — Contrast sensitivity function of a human observer
The natural units for the perceptual contrast sensitivity function are cycles per degree, which are
independent of viewing distance. Shown as a dotted line on the right of Figure 2 is the ophthalmological
limit of visual acuity known as 6/6 vision in metric units which means a person being examined can see the
same level of detail at 6 m as a person with "normal" visual acuity would see at 6 m distance. This visual
limit corresponds to a spatial frequency of about 6 cy/mm at 300 mm viewing distance or about 4,5 cy/mm
at a viewing distance of 400 mm. At a viewing distance of 400 mm the human visual system response to
vi
spatial detail peaks at about 0,4 cy/mm (0,5 cy/mm at 300 mm), decreasing in sensitivity at both higher and
lower spatial frequencies.
Key
X spatial frequency
Y contrast
Figure 3 — Illustrative contrast sensitivity function (Reference [3])
A visual illustration of the dependence of perceptual detail reproduction capability on both spatial frequency
(horizontal axis) and contrast (vertical axis) is shown in Figure 3 (see Reference [3]). The perception of fine
detail is frequency dependent and can be perceived well at high contrast, but not as well at low contrast.
For given viewing conditions (illumination, viewing distance, magnification), measurements at extreme
spatial frequencies are irrelevant to the characterization of the perceived resolution of a printing system as
their effects cannot be seen (e.g. the far right side of Figure 2 or Figure 3).
The illustration shown in Figure 3 also illustrates the peak in visual sensitivity in the mid spatial frequency
range and is a major motivation for the test chart design utilized in this method for evaluating the perceived
resolution of a printing system. A test chart that explores modulation or contrast along one axis and spatial
frequency along an orthogonal axis covers a large fraction of the major contributors to the perceived
[4]
resolution of a printing system. Figure 4 shows the Contrast–Resolution test chart .
vii
Key
X1 contrast
X2 reference tone value = 50 %
Y resolution, line pairs per millimetre, log steps
NOTE Reproduced with permission from Sicofilm A.G.
Figure 4 — Elements of the Contrast–Resolution test chart
In Figure 3, contrast and spatial frequency vary continuously. In Figure 4, each circularly symmetric
element explores a particular sampled contrast and spatial frequency – the individual patches in the target.
The spatial frequency of separation of these circularly symmetric marks and spaces in each patch is varied
logarithmically along the vertical axis of the target and the contrast, or depth of modulation, is varied
logarithmically along the horizontal axis. This logarithmic spacing mimics the largely logarithmic response
characteristics of the human visual system. This representation of contrast vs. spatial frequency resembles
the Campbell and Robson illustration flipped on its side. The circularly symmetric shape, and the range
of values explored in the Contrast–Resolution test chart are well suited to the characterization of digital
printing workflows.
In a conventional printing system, there are processes at four spatial frequencies that interact with each
other to form an image on the substrate. The first frequency is the spatial frequency of detail in an imaged
scene (this is represented by the vertical axis of the Contrast–Resolution test chart). The second spatial
frequency is the sampling frequency of the pixel grid in the digital image to be reproduced. The Contrast–
Resolution test chart shown in Figure 4 is vector based, not a bitmap, therefore there are no image pixels. The
third spatial frequency is the addressability grid of the printing device. The printing system raster image
processor (RIP) maps the image pattern to the addressability grid and then decides, for each individual
addressability location, how to image that spot. For a binary printing device (e.g. offset or flexo printing),
the spot is either turned on or off. For a non-binary output device (e.g. some electrostatic or inkjet systems),
where the output spots can be imaged at more than one gray level, the RIP also determines at which gray
level the output spot needs to be imaged. These individual spots are utilized by the RIP to build the screening
pattern that carries the tone scale of the image. The spatial repetition frequency of this screen is the fourth
viii
frequency in this printing process. All of these frequencies have the potential to interfere with one another,
and hence have the potential to introduce moiré.
The Contrast–Resolution test chart was designed for visual evaluation. Evaluation starts at the top of
column A (lowest spatial frequency and highest contrast) and moves down the target towards higher spatial
frequencies – note how a moiré pattern gradually develops between the circular lines and addressability
grid of the printer. The observer is tasked to find, for each column of the target, the patch at the highest
spatial frequency at which the circular lines in the patch are still recognizably reproduced – where no lines
or spaces are missing or overlap and where the level of moiré interference does not obscure the circles. For
each column in the target, an index value that is the row number (each row is a single spatial frequency)
of the last recognizable patch is recorded. This operation maps the threshold curve along columns in the
Contrast–Resolution target where circular elements are no longer recognizable. The area enclosed by this
threshold curve can be used as a capability score for the printing process. In observation, the circular nature
of the lines in each pattern tends to average out any angular dependencies in system resolution.
Figure 5 — Enlarged portion of a Contrast–Resolution target print
Figure 5 shows an enlarged portion of a print made with a 1 200 spot per inch addressability, utilizing a 133
line per inch dot screen. The circular patterns of the 2,91 cy/mm Row in Columns A through E are clear. The
circular patterns of the 3,76 cy/mm Row in Columns A and B are clear, but are not legible in Columns C, D or E.
The circular pattern of the 4,85 cy/mm Row in Column A is present with some aliasing. The circular pattern
of the 6,25 cy/mm Row in Column A is barely legible with significant aliasing. The resolution capability of
this printer configuration degrades significantly as the contrast is lowered – none of the other patches in
Figure 5 shows a recognizably circular pattern. An illustration of an index value threshold curve (white line)
and its enclosed area (above the white line) is shown in Figure 6.
The procedure specified in this document provides an automated, objective measurement surrogate of the
detailed visual examination process previously used in the evaluation of the Contrast–Resolution test chart.
[5]
The initial form of this procedure, developed by Liensberger , provided a single valued score (L-score)
that correlated well with subjective impression, based upon the area of a threshold curve derived from
normalized cross-correlation coefficients. A refinement of this automated procedure proposed by Uno and
[6]
Sasahara and called resolution-score forms the basis for this document. An international verification test
was conducted, involving both objective measurements, using this improved procedure, and subjective
ix
evaluation of Contrast–Resolution test charts printed with a variety of printing systems. These experiments
showed very good correlation of objective measurements with subjective evaluations using the improved
resolution-score procedure.
Figure 6 — Enclosed area above an index value threshold curve
Both objective measurement and subjective evaluation of Contrast–Resolution test charts printed with
process colorants are minimally affected by the low levels of colorant mis-registration present in modern,
well maintained printing systems. The level of colorant mis-registration in printed test charts should be
verified to be low when utilizing the procedure specified in this document with process color printing.
Clause 4 specifies the requirements of the workflow settings needed to effectively print the Contrast–
Resolution test chart, the setup requirements of the printer utilized to reproduce these test charts, the
requirements of the scanner characteristics needed to effectively digitize the information reproduced on the
printed test charts, and the requirements of the scanner data processing path needed to properly represent
this information for automated evaluation.
Clause 5 specifies the resolution-score measurement procedure.
Clause 6 specifies the reporting of results obtained with the process specified in Clause 5.
x
Technical Specification ISO/TS 18621-31:2024(en)
Graphic technology — Image quality evaluation methods for
printed matter —
Part 31:
Evaluation of the perceived resolution of printing systems
with the Contrast–Resolution chart
1 Scope
This document specifies the Contrast–Resolution test chart, the requirements on the printing process
needed to reproduce this test chart, the required characteristics of a high resolution scanner needed to
digitize the information reproduced on printed test charts, and the requirements on the interpretation of
this digitized data. It also specifies the resolution-score method for evaluating the perceptual resolution of
printed material using the Contrast–Resolution test chart.
The procedure specified in this document is intended for a characterization of the perceived resolution of a
graphic arts production printing system using the Contrast–Resolution test chart.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes
requirements of this document. For dated references, only the edition cited applies. For undated references,
the latest edition of the referenced document (including any amendments) applies.
ISO 5 (all parts), Photography and graphic technology — Density measurements
ISO 13655, Graphic technology — Spectral measurement and colorimetric computation for graphic arts images
ISO 14524, Photography — Electronic still-picture cameras — Methods for measuring opto-electronic conversion
functions (OECFs)
ISO 16067-1, Photography — Spatial resolution measurements of electronic scanners for photographic images
— Part 1: Scanners for reflective media
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
aliasing
output image artifacts that occur in a sampled imaging system for input images having significant energy at
frequencies higher than one half the spatial sampling frequency of the system
3.2
luminance factor
CIE Y
Y
dimensionless ratio of the luminance of the surface element, in the given direction, to that of the perfect
reflecting diffuser identically illuminated and viewed
Note 1 to entry: It is defined by the CIE and denoted as CIE Y.
Note 2 to entry: The luminance factor of the perfect reflecting diffuser identically illuminated is 100.
3.3
CIE L*
L*
metric lightness
function of luminance factor, defined by the CIE which approximates the human visual system response to
achromatic stimuli
1/3
Note 1 to entry: For luminance factors greater than 0,008 856, L* = 116(Y/Y ) – 16. For luminance factors less than
n
or equal to 0,008 856, L* =903,3 (Y/Y ).
n
Note 2 to entry: Y is the luminance factor of a white achromatic reference, typically the perfect reflecting diffuser.
n
3.4
ICC profile
International Color Consortium's file format used to store transforms from one colour encoding to another,
e.g. from device colour coordinates to profile connection space, as part of a colour management system
[7]
Note 1 to entry: The colour management system is standardized as ISO 15076-1 .
3.5
modulation
difference between the minimum and maximum signal levels divided by the sum of these levels
3.6
modulation transfer function
MTF
ratio, as a function of spatial frequency, of the measured modulation response in a print produced by a
printing system, to the stimulus modulation presented to that printing system
3.7
opto-electronic conversion function
OECF
relationship between the input levels and the corresponding digital output levels for an opto-electronic
digital image capture system
3.8
perceived resolution
subjective impression of the capability of an imaging system to depict fine detail
3.9
reflectance factor
dimensionless ratio of the radiant or luminous flux reflected in the directions delimited by the given cone
to that reflected in the same directions by a perfect reflecting diffuser identically irradiated or illuminated
3.10
resolution
measure of the ability of a digital imaging system to depict fine detail
3.11
scanner dynamic range
difference of the maximum density where the incremental gain is higher than 0,5 and the minimum density
that appears unclipped
Note 1 to entry: The dynamic range is determined according to ISO 21550.
4 Requirements
4.1 General
The method specified in this document provides an evaluation of the fine detail carrying capability of a
printing system comprising both workflow and printing that correlates with perceived resolution. Sample
1)
prints of the Contrast–Resolution test chart, ISO_ConRes20 , produced by a printing system are scanned by
a high resolution scanner. Evaluation of the fine detail carrying capability of the printing system is done by
comparing these high resolution scans of the Contrast–Resolution test chart prints with a defined reference.
Effective practice of this method places requirements on both the printing system and the scanning system.
This method can be applied to most printing technologies and substrates.
4.2 Apparatus requirements
4.2.1 Printing system requirements
Effective interpretation of the Contrast–Resolution test chart depends upon printing a set of sample prints
of Contrast_Resolution test chart with minimal mis-registration between the different colorants that may
be utilized in the printing process. This requirement is conventionally met by either printing the single
colorant version of the Contrast–Resolution test chart using a single separation colorant, or by using the
well controlled registration of a modern, well maintained, colour managed process colour printing system to
print one of the process colour versions of the Contrast-Resolution test chart. Refer to Annex B and Annex C
for details.
4.2.2 Scanning system requirements
In the evaluation method specified in this document, a high resolution scanning system is utilized as a
surrogate for a human observer. This imposes strict requirements on the setup, calibration and performance
of the scanning system used in the evaluation process. In simplest form, the measurement device shall be
calibrated and able to accurately capture luminance factor (CIE Y) or lightness information (CIE L*) at a high
optical resolution. A scanning system with a useful optical resolution of 1 200 ppi in both the horizontal
and vertical direction is required. If the Contrast–Resolution test chart has been printed on a structured
substrate, multi-directional illumination is required to minimize shadowing. A scanning system shall
conform to the specifications defined in Annex D.
4.3 Procedure
4.3.1 Test chart
The exact procedure specified in this document for evaluating perceived printing system resolution
depends upon the structure of the printing system workflow that is being assessed and the objective of the
assessment. Evaluation intents are specified in this document to assess the different workflow structures
and to meet the objectives of printing application and engineering assessments. Each of these specified
evaluation intents utilize one of three colour encoding variants of the Contrast-Resolution test chart to
1) The Contrast-Resolution test chart files are provided in the URN: https:// standards .iso .org/ iso/ ts/ 18621/ -31/ ed -2/
en/ .
properly assess the resolution capability of a printing system. The three colour encoding variants of the
2)
Contrast-Resolution test charts are defined in Annex A and are summarized here:
a) ISO_ConRes20_SepK.pdf which is intended for single colorant (black) resolution capability evaluation.
b) ISO_ConRes20_sRGB.pdf which is intended for process colour resolution capability evaluation.
c) ISO_ConRes20_Lab.pdf which is intended for resolution capability evaluation utilizing the optional
linearization process defined in Annex C.
4.3.2 Evaluation intent
Six different evaluation intents are specified in this document for the evaluation of the perceived resolution
of different printing system workflows and assessment objectives; four are further specified in Annex B
and two are further specified in Annex C. Each of these different evaluation intents defines a process that
specifies the appropriate colour encoding variants of the Contrast–Resolution test chart, defined in Annex A,
required to properly assess the resolution capability of the printing system workflow. The discussion here
and the summary of evaluation intents provided in Table 1 are intended to aid in selecting the appropriate
intent.
1) Device-specific evaluation of a printing system. This evaluation process is primarily for engineering
evaluation of the resolution capability of a single printing system in different printing setup conditions,
using a single conforming reflection scanning system. Repeatability is dictated by the printing system
stability. Device-specific evaluation may use any of the Contrast-Resolution test chart colour encoding
variants.
2) Single colorant evaluation of inherent resolution capability, where the printing system workflow is
3)
set up to provide a rendition of the single colorant ISO_ConRes20_SepK.pdf version of the Contrast-
Resolution test chart using a single colorant, usually only the black colorant. This evaluation process
avoids any resolution degradation due to the mis-register of multiple colorants and aligns with the
perceived resolution capability of a printing system when printing text content or black and white
graphics. Further discussion is found in Annex B.
3) Process color evaluation of practical resolution capability, where the printing system workflow is
4)
set up to provide a colour managed rendition of the ISO_ConRes20_sRGB.pdf Contrast-Resolution test
chart that is printed with multiple process colorants, usually with a "rich" black. This setup closely
aligns with the perceived printing system resolution capability when printing image content. Further
discussion is found in Annex B.
4) Standardized process colour evaluation of the resolution capability of a printing system set up to
simulate a standardized printing application, e.g. commercial printing, packaging or newsprint. This
evaluation process relies on the standard printing application ICC profiles available from www .ECI
.org to provide the defined color reproduction aims of a standardized printing system workflow of
commercial interest. Standardized process colour evaluation uses colour managed interpretation of the
sRGB encoded version of the Contrast-Resolution test chart, ISO_ConRes20_sRGB.pdf, and is particularly
useful in the comparison of different printing systems
5) Linearized single colorant evaluation of printing system resolution capability. Linearization
maximizes the information content of the test chart through the printing process and the analysis
of the resolution capability of a printing system but utilizes a tone-scale that is not representative of
normal printing operations. This evaluation process utilizes the single colorant version of the Contrast
2)
Resolution test chart, ISO_ConRes20_SepK.pdf , and is aimed at engineering assessment of the ultimate
resolution capability of a single colorant in a printing system. Further discussion is found in Annex C.
2) The Contrast-Resolution test chart files are provided in the URN: https:// standards .iso .org/ iso/ ts/ 18621/ -31/ ed -2/
en/ .
3) The corresponding files are provided in the URN: https:// standards .iso .org/ iso/ ts/ 18621/ -31/ ed -2/ en/ .
4) This PDF file is provided in the URN: https:// standards .iso .org/ iso/ ts/ 18621/ -31/ ed -2/ en/ .
6) Linearized process colour evaluation of printing system resolution capability. Linearization
maximizes the information content of the test chart through the printing process and the analysis of the
resolution capability of a printing system but utilizes a tone-scale that is not representative of normal
printing operations. This evaluation process utilizes the CIELab encoded Contrast Resolution test chart,
5)
ISO_ConRes20_Lab.pdf , and is aimed at engineering assessment of the ultimate resolution capability of
a process colour printing system. Further discussion is found in Annex C.
Table 1 — Comparison of evaluation intents
Evaluation Single Printing
SepK sRGB Lab Process color Linearization Engineering Purpose
intent colorant applications
Compare printer
configurations.
(1) Device-
X X X X X N/A X X Restricted to a
specific
single printing
system.
Single colorant
printing.
(2) Single-
X X No X Text, Line-art and
colorant
B&W
reproduction.
Normal operation
of a printing
(3) Process- system.
X X No X
colour Photographic
reproduction
market.
(4) Standard-
Compare dif-
ized
X Standardized N/A X ferent printing
process-
systems.
colour
Maximum
(5) Linearized resolution
single- X X External X capability of
colorant single colorant
printing.
Maximum
(6) Linearized resolution
process- X X Inherent X capability of
colour process-color
printing.
Users of this document shall take careful note of the image structure in printed Contrast-Resolution test
charts to verify that the test charts are printed according to the desired evaluation intent.
4.3.3 Printing and scanning
Print a set of at least two sample prints of the appropriate Contrast–Resolution test chart, defined in
Annex A, for the appropriate evaluation intent according to the printing system requirements specified in
Annex B and Annex C. Discard the first, warm-up, print. Ensure that no geometric scaling is applied (the box
around the test chart should be square with sides of 109,7 mm.). For example, "Scale to page" should be set
to "No Scaling" and "Magnification" should be set to "100 %". In general, any spatial enhancement applied in
the printing of the assessment test chart will distort the evaluation of printing system resolution and should
be avoided. For example, "Sharpen" or "Unsharp Mask" should be set to "None" in the workflow used to print
the Contrast-Resolution test chart. For an accurate evaluation of a printing system and its workflow, the
printing system should be configured as it is intended to be used.
Scan the sample prints of the Contrast–Resolution test chart produced in the printing process using a
scanning system that meets the requirements of Annex D. Ensure that the scanning system can accurately
capture the high resolution lightness information of the printed test charts. Ensure that the print is
unrotated and aligned with the scanner platen. Colour enhancement and spatial filtration in the scanning
system data path shall be disabled. Colour calibration of the scanning system is required to provide CIE
5) The Contrast-Resolution test chart files are provided in the URN: https:// standards .iso .org/ iso/ ts/ 18621/ -31/ ed -2/
en/ .
Lightness for resolution-score processing (see Annex D). Evaluating multiple print samples permits better
statistical estimation of the true resolution-score in the presence of printing system and scanning system
noise.
4.3.4 Evaluation process
The process used to evaluate the digital information captured with scans of the printed Contrast–Resolution
test charts shall follow the steps specified in Clause 5. Resolution-score processing, Processing shall conform
to the requirements specified in Annex E.
5 Resolution-Score processing
5.1 General
The printing system requirements specified in Annex B and C and the scanning system requirements
specified in Annex D ensure that the scanned image of a Contrast-Resolution test chart print sample will
permit a meaningful analysis. These requirements are essential for analysis and should be checked prior to
any further processing.
5.2 Element identification
Supplied with the three versions of the printable Contrast–Resolution test chart, is a reference bitmap image
6)
of the Contrast–Resolution test chart, ISO_ConRes20_Reference_1200.tif , representing the output of a
perfect printer, scanned by a perfect scanner at 1 200 ppi. This is the ideal case to which 1 200 ppi scans
of real prints are compared. This reference bitmap image is utilized in the analysis of any version of the
printed Contrast-Resolution test chart.
The exact locations of the circularly symmetric elements in the scanned image of the Contrast–Resolution
test chart shall be identified, so that each of these elements may be compared with the corresponding
element in the reference bitmap image. Identical fiducial marks are provided in both the printable and
reference Contrast-Resolution test charts to aid in locating each region of interest (ROI). Significant errors
in identification of the geometric position of an element with respect to its corresponding element in the
reference bitmap image will degrade the analysis results.
5.3 Scanning signal interpretation
The scanning system shall deliver a calibrated lightness signal, CIE L*. CIE L* is a well-defined mathematical
transformation of the CIE luminance factor, Y and this CIE L* representation of the scanned data is used
in all calculations. The means to calibrate a scanning system to deliver this signal can be provided by a
suitable linearization of a monochrome scan or is provided by scanner ICC profile creation applications for
RGB scans. See Annex D for details. Analysis of the scanned Contrast–Resolution test chart print relies upon
the nearly uniform perceptual characteristics of differences in the CIE L* quantity.
5.4 Spatial filtering
A properly dimensioned Gaussian spatial filter is applied to the scanned bitmap image to mimic the spatial
response characteristics of the human visual system (HVS) at a standard viewing distance of 40 cm.
This filter slightly blurs the image and eliminates structure that a printing system may be able to render
that would not be visible to a human observer at this viewing distance. The same spatial filter is used in
processing the ideal reference bitmap image. The bitmap of a perfect scan of a perfect print, filtered in this
manner, will exactly match the similarly filtered reference bitmap image and represents the detail that a
human observer would be capable of seeing at the standard viewing distance of 40 cm. The Gaussian spatial
filter appropriate for a viewing distance of 40 cm. is specified in Annex F (normative), Spatial filter.
NOTE A Matlab example of the application of this Gaussian spatial filter to an input image to create afiltered
image is: FiltImage = conv2(double(Image), kernal, 'same')/sum(kernal(:));
6) The corresponding file is provided in the URN: https:// standards .iso .org/ iso/ ts/ 18621/ -31/ ed -2/ en/ .
5.5 Normalized 2-D cross-correlation
Following registration of the scanned image with the reference image and spatial filtering to minimize
structure in the scanned image that would not be visible to a human observer, each of the elements of
the scanned Contrast–Resolution test chart is compared with its corresponding element in the reference
bitmap. A normalized two-dimensional cross-correlation calculation is used to provide the measure of
similarity between each scanned element and its corresponding reference element. The cross-correlation
calculation utilizes a range of horizontal and vertical pixel offset values sufficient to accommodate residual
mis-registration between the scanned elements and the corresponding reference elements. The position of
the peak value in the cross-correlation coefficient matrix between scanned and reference elements defines
the horizontal and vertical pixel offsets that shall be applied to an element of the scanned imag
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