Print image quality analysis and evaluation

I. Introduction

Prints are the result of photocopying, and the combined effects of the originals, materials, equipment, imaging, and transfer processes result in printed images. The vector attributes of graphics and text are crucial for page description, layout, and interpretation of RIP. In particular, when outputting, it is necessary to treat graphics, text, and images in two different types of objects in order to obtain the most reasonable results. However, the vector properties of graphics and text have become meaningless for print results because these objects lose their vector characteristics once they have been transferred to the paper surface. Therefore, there is no need to distinguish the dot matrix description and the vector description feature of the object when evaluating the print quality, and it is considered that the print quality evaluation is equivalent to the print image quality evaluation (hereinafter referred to as the image quality evaluation).

The research and application of human image quality evaluation technology has never stopped, and many advances have been made due to the continuous development of image quality improvement activities in the imaging technology field. For example, image quality evaluation techniques in the fields of traditional printing, silver halide photography, color television, and electrophotographic digital printing, among which many evaluation methods show a good correlation with physical characteristics and subjective evaluation results, and have also been developed Image quality evaluation system in different fields. However, it is difficult to universally accept only a subjective evaluation of a standard.

Color offset printing is the main means of modern graphic printing. For a long time, the evaluation of the quality of printed images is judged by the customer's visual and subjective impressions. This is a very poor and very difficult job. Therefore, how to objectively evaluate and fully apply technical process means to achieve the control of the color printing image quality is very important. Print image quality assessment refers to the use of data to represent the relationship between a product and a manuscript. With certain data and indicators, the allowable range of product quality error and quality degradation are controlled to ensure the repeatability and quality stability of image replication.

There are three key factors in the quality of printed images: (1) The sharpness of the image, ie, whether the change in edge density of the image is consistent with the visual sensitivity of the person, mainly depends on screen printing in the screen printing and dot enlargement. 2 The level of the image, that is, the visually distinguishable density level in the image, mainly depends on the expansion and contraction of the contrast between the prepress and the print. 3 The color of the image, that is, the reproduction accuracy of various colors of the image and neutral gray balance, mainly depends on the color separation mechanism, the quality of the photosensitive material, paper, ink, and the like.

Second, the main methods of traditional print image quality evaluation

Any method used to evaluate the quality of printed images will ultimately be attributed to the clarity, level and color of the image. The evaluation methods are mainly divided into subjective evaluation and objective evaluation and a comprehensive method based on visual evaluation.

1. Subjective evaluation method

The subjective evaluation of printed images is a method of evaluating the quality of images based on experience. Subjective assessment methods commonly used are visual evaluation methods and qualitative index evaluation methods. The visual assessment method refers to the observation of originals and printed materials by multiple experienced managers, technicians, and users under the same evaluation environment conditions (such as uniform light source and illumination), and then takes each person’s experience, emotions, and hobbies as the According to the results, each print was rated as excellent, good, medium, and differential, and the frequency of each grading was counted. Those who received favorable reviews were rated as excellent and good, and vice versa. Qualitative index evaluation method refers to a certain qualitative index, and lists the important factors of the impact of each index on quality. It is scored by multiple experienced assessors, and the total score is higher, and the lower is worse.

1 Multidimensional scaling method:

The multidimensional scale is a scale technique based on mathematical statistics. In the case of pair-wise comparison of differences between samples or determination of sample satisfaction, multidimensional scaling methods can be used to analyze and identify the main parameters used by people for assessment. When judging a printed sample in this way, the relative importance of the main parameters of the printing quality can be determined; the value obtained from the evaluation can make the subjective evaluation and the objective evaluation or intrinsically related to the nature of the paper; the quality of each print can also be obtained. The reliability of the evaluation, the consistency of each evaluation staff (such as printing, papermaking experts, readers, advertising staff, etc.) and the evaluation team's evaluation and other information.

Multidimensional scaling technology was proposed by Tokosson. Its content is: If there is a perceived difference between two elements, then the difference can be expressed in a geometric distance. If this difference is recorded on a scale, the scale on the scale shows this distance, and then the distance can be used to create more than one-dimensional geometric models that reflect the relationships between the samples.

An important feature of multidimensional scaling technology is that it can be used to multidimensionally scale the subjective psychological factors of the judges. The role of each parameter in an evaluation can be represented by a desired vector.

2 pair comparison method:

People have subjective characteristics in judging the quality of printed matter. Different people will make completely different conclusions. This objective inconsistency cannot be regarded as bias or randomness and ignored. Even if there is a reference that can be used as a benchmark for comparison, there will be inconsistencies in the evaluation results, and the inconsistencies in the subjective evaluations cannot be regarded as deviations and randomness, either.

In many cases, there is no reference that can be used as a benchmark. At this time, only internal comparisons can be made between the objects being evaluated. There are two common methods: one is to arrange the samples to be judged in a certain order, and the other is to put each sample in a group of judged samples The other judged samples are compared one by one, scored on the basis of comparison, and judged according to points. This is the pairwise comparison method. It belongs to a subjective evaluation method.

When implementing pairwise comparison methods, it is important to ensure that when assessors simultaneously compare two samples, there should be no irrelevant factors that interfere with such comparisons. The printed products to be compared should be in exactly the same standard lighting conditions. The background should be neutral. The room used for comparison should ensure that the judges can focus on their work and cannot interrupt the judgement or interfere with the judges. The samples to be compared should be provided to the judges in a random order so that the comparison process can be conducted without any prejudice. The judges must be simply and clearly explained on what basis they should be judged and ambiguous.

Such subjective comparison experiments are only internal comparisons between samples. The comparison results are only relevant to the samples being compared. Whether the results of the paired comparisons are reliable can be determined by examining the "reliability of the judgement" of the judges, that is, by calculating the reliability coefficient.

2. Objective evaluation method

1 tone (hierarchical) reproduction evaluation:

The contrast range of transmission originals varies greatly, and color prints have to be reproduced as a screen with a substantially uniform density range. This density range is mostly lower than the density range of color originals. In the separation and printing process, compression adjustments must be made. What kind of redistributive adjustments are taken at each tone level depends, on the one hand, on the distribution of the tonality levels of the manuscript, and at the same time it relates to human visual perception. Among them, both the Munsell brightness value of the visual response and the subjective factors of people's visual psychological requirements. To adjust the copying of original tonal hierarchies, it is necessary to sum up people's visual psychological needs, add the physical values ​​of visual response, and then consider the distribution of the original level to be able to obtain the reproduction curve of the density tone of the print screen, and then include the copy. With the evolution data in the reproduction process, a tone reproduction curve of a specific manuscript is designed.

The evaluation of the reproduction of printed image tone, if only from the perspective of print quality inspection, is to measure the density of printed solids in each color ink layer; to measure and calculate the overprint rate of each ink layer; to measure the increase in printed dot pitch or Measure and calculate the printing K value; Check the printing quality of the ink dot; Measure and draw the density reproduction curve of the print to the original. Through the measurement of these objective technical data and comparison with the quality specification standards formulated by this department, the quality level of a specific color printing product can be determined.