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On the evaluation of print mottle
KTH, School of Computer Science and Communication (CSC), Numerical Analysis and Computer Science, NADA.
2005 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

Print Mottle is perhaps one of the most disturbing factors influencing overall Print Quality. Mottle has traditionally been evaluated by estimating the reflectance variation in the print. Although the amplitude of the reflectance variation is probably the most important aspect of print mottle, other aspects may also influence the perceptibility of mottle. Since the human visual system is optimised to fit the conditions prevailing in its surroundings, it is also important to consider aspects such as mean reflectance factor level, spatial frequency content, structure of the mottle, and colour variations.

In this thesis, a new evaluation model for the estimation of print mottle is proposed. The model is best explained as a six-step chain. First, a digital RGB image of the print is acquired with a scanner. The digital RGB image is then calibrated and transformed into the L*a*b* colour space. Next, the three colour components are transformed into the frequency domain by a Fourier transform and the power spectra are calculated. The power spectra are thereafter filtered with respect to the contrast sensitivity functions representing the human eye’s sensitivity to spatial variations in the three colour channels. To account for systematic variations in the sample, the spectra are filtered a second time with texture enhancement filters, which are based on local calculations of chi-square measures in the power spectra. The energy within the visually detectable area of the filtered power spectra is then integrated to obtain a single measure of the variation for each colour component. A single mottle estimate is obtained as the square root of the sum of the squared variation measures for the three components. To acknowledge the influence of mean lightness level on perceived print mottle in a way that agrees with the results presented in Paper I, the mottle estimate obtained is finally multiplied by the sixth root of the mean reflectance factor level.

The theoretical foundations of the model are consecutively developed through the first five papers of the thesis. The first paper considers the influence of the mean reflectance level on perceived print mottle. The second and third papers describe the contrast sensitivity filter and the texture enhancement filter applied. The fourth paper compares the new model with other models for print mottle evaluation. The fifth paper extends the grey-scale version of the model into colour. The sixth paper presents the unified model that takes all the mentioned factors into account.

To test the model, samples from both simulated sets of prints with various degrees of colour and/or systematic mottle and sets of real prints from various conventional presses were analysed a) visually, b) with traditional print mottle evaluation models, and c) with the new model. Results obtained using the different evaluation models were compared with visual assessments of the sets of prints. In each one of the evaluations the new model was found to be as good as or superior to the traditional print mottle evaluation models in its agreement with visual assessment. The new model is particularly promising in cases where the evaluated prints show colour and/or systematic disturbances.

Place, publisher, year, edition, pages
Stockholm: KTH , 2005. , XV, 82 p.
Series
Trita-NA, ISSN 0348-2952 ; 0540
Keyword [en]
Mottle, Print quality, Texture, Image analysis, Perception
National Category
Computer Science
Identifiers
URN: urn:nbn:se:kth:diva-533ISBN: 91-7178-205-2 (print)OAI: oai:DiVA.org:kth-533DiVA: diva2:14329
Public defence
2005-12-09, STFI salen, STFI Packforsk, 10:00
Opponent
Supervisors
Note
QC 20101012Available from: 2005-12-02 Created: 2005-12-02 Last updated: 2010-10-12Bibliographically approved
List of papers
1. The Influence of Mean Reflectance on Perceived Print Mottle
Open this publication in new window or tab >>The Influence of Mean Reflectance on Perceived Print Mottle
2003 (English)In: Journal of Imaging Science And Technology, ISSN 8750-9237, Vol. 47, no 1, 54-59 p.Article in journal (Refereed) Published
Abstract [en]

The perceived magnitude of a stimulus detected by the human visual system is, according to Fechner's Law, proportional to the logarithm of its physical intensity. This suggests that the perceived mottle in a printed image depends not only on the reflectance variation of the image, but also on its mean reflectance level. One possible approach is therefore to use the reflectance variation divided by the mean reflectance level, dR/R, as an estimate of the perceived mottle in an image. However, practical use of this method indicates that perceived mottle may be overestimated in dark and underestimated in light images. The present study therefore attempts to establish a better relationship between perceived mottle and mean reflectance level. A set of halftone patches was constructed by digital simulation, and then visually evaluated. Results suggest that the measured variation should be divided by a power function, with an exponent less than 1, of the mean reflectance to obtain the best correlation between visual and instrumental evaluation. The highest correlation was attained using the inverse square root, i.e., an exponent equal to 0.5.

Keyword
Computational complexity, Computer simulation, Correlation methods, Functions, Imaging techniques, Light reflection, Fechner law
National Category
Computer and Information Science
Identifiers
urn:nbn:se:kth:diva-8855 (URN)000181676900010 ()
Note
QC 20101011Available from: 2005-12-02 Created: 2005-12-02 Last updated: 2010-10-11Bibliographically approved
2. Evaluating Systematic Print Mottle
Open this publication in new window or tab >>Evaluating Systematic Print Mottle
2003 (English)In: Journal of Graphic Technology, ISSN 1544-9599, Vol. 1, no 2, 19-28 p.Article in journal (Other academic) Published
Keyword
systematic mottle, ordered noise, texture, simulation, visual print quality evaluation
National Category
Computer and Information Science
Identifiers
urn:nbn:se:kth:diva-8856 (URN)
Note
QC 20101011. Presented at the IPGAC 2002, Bordeaux.Available from: 2005-12-02 Created: 2005-12-02 Last updated: 2010-10-11Bibliographically approved
3. Perceptual Assessment of Simulated Print Noise with Random and Periodic Structure
Open this publication in new window or tab >>Perceptual Assessment of Simulated Print Noise with Random and Periodic Structure
2005 (English)In: Journal of Visual Communication and Image Representation, ISSN 1047-3203, E-ISSN 1095-9076, Vol. 16, no 3, 271-287 p.Article in journal (Refereed) Published
Abstract [en]

Lightness variation in a supposedly uniform printed surface is referred to as "print mottle." This phenomenon is one of the most detrimental to overall print quality. A fundamental problem in the evaluation of human sensitivity to a print quality variable such as mottle is the difficulty in controlling external variables, such as variations in color or in average print density, that arise in the printing process. These variables can influence the rating of quality. To analyze the impact of a systematic mottle pattern compared to that of the common case of a random pattern, a digital simulation technique was used to create gray test samples with various amounts of stochastic and systematic noise of different characters. The samples were printed using a high quality inkjet printer and evaluated by a panel of judges. Two different evaluation methods were used. Observers rated dissimilarity and preference in a pairwise comparison task, and also by positioning samples in the horizontal and vertical directions of a digitizing tablet. The results show that individuals rate the samples in a very consistent way and that systematic noise is perceived to be more annoying than random noise of a similar physical magnitude. Furthermore, the consistency between the two different evaluation methods is very good, which suggests that two-dimensional scaling on a digitizing tablet is a viable method for grouping samples in a plane. The results also show that digital simulation of print artifacts is a powerful too] for creating samples with controlled disturbances.

Keyword
print mottle, systematic disturbances, ordered noise, simulation, multidimensional scaling, two-dimensional scaling
National Category
Computer and Information Science
Identifiers
urn:nbn:se:kth:diva-8857 (URN)10.1016/j.jvcir.2004.11.002 (DOI)000229591500003 ()
Note
QC 20101011Available from: 2005-12-02 Created: 2005-12-02 Last updated: 2010-10-11Bibliographically approved
4. A Comparison of Different Print Mottle Evaluation Models
Open this publication in new window or tab >>A Comparison of Different Print Mottle Evaluation Models
2006 (English)In: TAGA Journal, ISSN 1748-0337, E-ISSN 1748-0345, Vol. 2Article in journal (Refereed) Published
Keyword
Print, Mottle, Quality, Standardization
National Category
Computer and Information Science
Identifiers
urn:nbn:se:kth:diva-8858 (URN)
Note

QC 20101012. Uppdaterad från accepted till published (20101011).

Available from: 2005-12-02 Created: 2005-12-02 Last updated: 2016-12-12Bibliographically approved
5. Evaluating colour print mottle
Open this publication in new window or tab >>Evaluating colour print mottle
2007 (English)In: ADVANCES IN PRINTING AND MEDIA TECHNOLOGY: VOL XXXIII / [ed] Enlund, N.; Lovrecek, M., 2007, Vol. 33, 329-339 p.Conference paper, Published paper (Refereed)
Abstract [en]

Among the most detrimental features of print quality are unintentional spatial reflectance variations in prints, frequently referred to as print mottle. The ability of the human visual system to discriminate between colours makes it sensitive not only to lightness variations, but also to chromatic variations. Previously, several models have been put forward to estimate the amount of lightness variations in prints, but few to measure chromatic variations. In this paper, a new evaluation model, based on a previous achromatic model but generalized to colour, is presented. Printed samples from two sets of simulated prints with various degrees of colour variations and two sets of colour prints from various conventional presses were analysed to evaluate the performance of the new model. The new model gave a result which correlated considerably better with visual evaluation than the achromatic model when the variation was chromatic in character. In practical printing situations, however, the variation does not seem to be mainly chromatic in character and a colour mottle evaluation model may therefore often be redundant.

Series
ADVANCES IN PRINTING SCIENCE AND TECHNOLOGY, 33
Keyword
Chromatic variation, colour print mottle, simulation, print quality, visual assessment
National Category
Computer and Information Science
Identifiers
urn:nbn:se:kth:diva-25171 (URN)000267935100031 ()978-953-7292-03-4 (ISBN)
Conference
33rd International Research Conference of Iarigai/International-Association-of-Research-Organization-for-the-Information-Media-and-Graphic-Arts-Industries, Leipzig, GERMANY, SEP, 2006
Note
QC 20101012Available from: 2010-10-12 Created: 2010-10-12 Last updated: 2010-10-12Bibliographically approved
6. Print Mottle Evaluation
Open this publication in new window or tab >>Print Mottle Evaluation
2006 (English)In: International conference on Printing Technology: St. Petersburg, Russia, 2006, 2006Conference paper, Published paper (Other academic)
Keyword
print mottle, image analysis, systematic disturbances, colour variations, visual evaluation
National Category
Computer and Information Science
Identifiers
urn:nbn:se:kth:diva-8860 (URN)
Note
QC 20101012Available from: 2005-12-02 Created: 2005-12-02 Last updated: 2010-10-12Bibliographically approved
7. Print Mottle Evaluation of Flexographic Prints: Using a Scanner-based Measurement System
Open this publication in new window or tab >>Print Mottle Evaluation of Flexographic Prints: Using a Scanner-based Measurement System
2004 (English)In: Flexo Magazine, no October, 14-16 p.Article in journal (Other academic) Published
Keyword
Flexography, Print Quality, Print Mottle, Visual Assessment, STFI Mottling Analysis System
National Category
Computer and Information Science
Identifiers
urn:nbn:se:kth:diva-8861 (URN)
Note
QC 20101012Available from: 2005-12-02 Created: 2005-12-02 Last updated: 2010-10-12Bibliographically approved

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