I. Introduction The printed matter is the result of graphic reproduction. The combined effect of the original manuscript, materials, equipment, imaging and transfer process and other factors form a printed image. The vector attributes of graphics and text are very important for page description, typesetting, and RIP interpretation. Especially when outputting, two types of objects of different nature, such as graphics, text, and images, must be treated differently to obtain the most reasonable results. However, the vector attributes of graphics and text have become insignificant to the printing results, because these objects lose their vector characteristics once transferred to the surface of the paper. Therefore, it is not necessary to distinguish between the dot matrix description and the vector description feature of the object when evaluating the print quality. It is entirely reasonable to think that the print quality evaluation is equivalent to the print image quality evaluation (hereinafter referred to as image quality evaluation). The research and application of image quality evaluation technology by human beings have never stopped. Many improvements have been made due to the continuous development of image quality improvement activities in the field of imaging technology. For example, image quality evaluation technologies in the fields of traditional printing, silver halide photography, color TV, and electrophotographic digital printing, many of which have shown good correlation with physical characteristics and subjective evaluation results, have also been developed for Image quality evaluation system in different fields. However, it is difficult to be universally accepted to use subjective evaluation of only one scale. 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 unstable and very difficult task. Therefore, it is very important how to objectively evaluate and fully apply technical processes to achieve the quality control of color printing images. Print image quality evaluation refers to the use of data to express the correlation between the product and the original. Use certain data and indicators to control the allowable range and quality decline of product quality errors, and ensure the repeatability and quality stability of image reproduction. There are three key factors in the quality of the printed image: ①The clarity of the image, that is, whether the density change of the image edge is consistent with the human visual sensitivity, mainly depends on the screen printing and the expansion of the dot in the printing. ② The level of the image, that is, the visually distinguishable density level in the image, mainly depends on the contrast expansion and contraction before and after printing. ③ The color of the image, that is, the reproduction accuracy and neutral gray balance of each color of the image, mainly depends on the color separation mechanism, the quality of the photosensitive material, paper, ink, etc. Second, the main methods of traditional printed image quality evaluation Regardless of the method adopted, the evaluation method of printed image quality will ultimately come down to the three aspects of image clarity, gradation and color. Evaluation methods are mainly divided into subjective evaluation and objective evaluation and a comprehensive method based on visual evaluation. 1. Subjective evaluation The subjective evaluation of printed images is a method of evaluating the quality of images based on experience. The subjective evaluation methods commonly used are visual evaluation methods and qualitative index evaluation methods. The visual evaluation method refers to the observation of manuscripts and printed materials by multiple experienced managers, technicians and users under the same evaluation environment conditions (such as consistent light source and illuminance), and then based on each person ’s experience, emotions and hobbies as According to the basis, according to the excellent, good, medium, and differential grades of each printed product, and the frequency of each grade is counted, those who get consistent praise are excellent and good, and vice versa. The qualitative index evaluation method refers to a certain qualitative index and lists the important factors of each index's impact on quality. It is scored by multiple experienced raters. The higher the total score, the better the quality, and the lower the poor. ①Multi-dimensional scaling method: Multidimensional scaling is a scaling technique based on mathematical statistics. When comparing the differences between the samples in pairs or determining the degree of satisfaction with the samples, the multi-dimensional scaling method can be used to analyze and identify the main parameters used by people in the assessment. When judging the printing samples in this way, the relative importance of the main parameters of the printing quality can be determined; the value obtained by the judgment can make the subjective evaluation and the objective evaluation or the inherent correlation between the paper properties; also can obtain the quality of each print Information about 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. The multi-dimensional scaling technique was proposed by Togo Sen, and its content is: if there is a difference between the two elements, then the difference can be expressed by a geometric distance. If this difference is recorded on a linear scale, the scale value on the scale shows this distance, and then the distance can be used to build more than one-dimensional geometric model that reflects the relationship between samples. An important feature of the multi-dimensional scaling technique is that multi-dimensional scaling can be performed for the weighted condition of the subjective psychological factors of the judges, and the role of each parameter in an evaluation can be expressed by an expectation vector. ② Paired comparison method: When people judge the quality of printing, they have subjective characteristics, and different people will make completely different conclusions. Such objective inconsistencies cannot be regarded as deviations or randomness and ignored. Even if there is a reference object that can be used as a benchmark during the evaluation, there will be inconsistencies in the evaluation results. The inconsistencies in the subjective evaluation can also not be regarded as deviations and randomness and ignored. In many cases, there is no reference that can be used as a benchmark. At this time, only internal comparison can be made between the judged objects. There are two common methods: one is to arrange the judged samples in a certain order; the other is to compare each sample in a group of judged samples with 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. Pay attention to the implementation of the pairwise comparison method: ensure that when the judges compare two samples at the same time, there should be no unrelated factors that interfere with this comparison. The printed materials to be compared should be under the same standard lighting conditions, the background should be neutral, and the room used for comparison should ensure that the judges can concentrate their efforts and cannot interrupt the judges 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 will not be biased. The judges must be explained clearly and clearly on what basis to judge, and no ambiguous words are allowed. Such a subjective comparison experiment is only a comparison between samples, and the comparison result is only related to the samples being compared. Whether the result of the pairwise comparison is reliable can be known by examining the judges' "reliability of judgment", which is measured by calculating the reliability coefficient. 2. Objective Evaluation Method ①Evaluation of reproduction of tone (level): The contrast range of transmission manuscripts is very different, but color prints are to be reproduced as a screen with a substantially uniform density range. This density range is mostly lower than the density range of color manuscripts. In the color separation and plate making, compression adjustment must be done. What kind of redistribution adjustment is adopted for each tone level depends on the distribution of tone levels of the original manuscript, and it is also related to human visual experience. Among them, there are both the Munsell lightness factor of visual response and the subjective factors of people's visual psychological requirements. To adjust the copy of the original tone level, we need to summarize the visual psychological needs of people, add the physical value of the visual response, and then consider the distribution status of the original level to obtain the density tone level reproduction curve of the printed picture, and then include the copy With the evolution data in the reproduction process, the tone reproduction curve of the specific manuscript is designed. For the evaluation of the reproduction of the tone of the printed image, from the perspective of printing quality inspection, it is: measuring the printing solid density of each color ink layer; measuring and calculating the overprint rate of each color ink layer; measuring the increase of the printing dot tone or Measure and calculate the printing K value; check the transfer quality of the ink dots; measure and plot the density gradation reproduction curve of the printed product to the original. Through the measurement of these objective technical data, and then compared with the quality specification standards formulated by this department, the quality grade of specific color printing products can be determined. ②Evaluation of color reproduction: There are three different concepts of color reproduction. First, the color reproduction in the physical sense requires that the spectral distribution of the reproduced color and the original color at each color point are exactly the same. The printed matter is for visual viewing, and it is difficult to achieve the same color reproduction in the physical sense, and it is not necessary. The second is the reproduction in the sense of chromatics, which makes the printed reproduced image and the original color point consistent or close in chromaticity, that is, the different spectrum and the same color effect, which is the measurement standard for realistic and objective evaluation of color reproduction. The third is the color reproduction in the psychological sense, that is, the color reproduced by printing may be slightly different from the original color, but the color effect may reach the visual psychological satisfaction. The subjective evaluation factors are added here. Due to the lack of chroma expression characteristics of printed materials (paper, ink, etc.), the imperfect performance of color separation copying means and equipment, and the lack of color representation of the print reproduction method, realistic printing technology cannot yet faithfully restore the original or All the colors of the original scene, even its reproducible parts, cannot be faithfully restored, and can only be relatively close. This brings certain difficulties to the objective evaluation of the color reproduction quality of the printed matter. [next] The results of chromaticity measurement can only be compared from the color closeness of the printed matter to the original or the original scene. Between similarity and dissimilarity, the comprehensive and comprehensive evaluation of the color reproduction of printed matter can be made by incorporating people's psychological requirements for color vision, that is, the degree of psychological reproduction. If an objective technical measurement standard is set for the closeness of the printing color to the reproduction of the original color, it should include: the measurement test of the printing ink color reproduction range, the printing gray balance reproduction measurement test, and the absolute accuracy of the printing color to the original color reproduction. Measurement inspection, and measurement of relative reproducibility. ③ Evaluation of clarity reproduction: The clarity of color prints is an important quality indicator for image reproduction. Except for the special concept of expressing images, there should always be a part of the level (subject or background) of each picture that is clear. There are also three aspects of the evaluation of the clarity of the printed picture: the solidity of the outline of the image layer; the clarity of the contrast between the two adjacent layers of the image, that is, the slight contrast; the resolution of the original or printed picture layer, that is, its subtlety The level of subtlety is to express the essential appearance of the material composed of the objective scenery, that is, the so-called texture. 3. Visual evaluation method After years of research on the human visual system (HVS), people have made significant progress in understanding the functions of the visual system in terms of neurophysiology and psychophysics. The important characteristics related to image quality are: Weber-Fechner's law, that is, the perceptible threshold of the stimulus signal is proportional to the background intensity; the modulation transfer function (MTF) is a band-pass characteristic, and the maximum response is between 2-5c / d; Area masking effect, that is, the visibility of the signal that appears on the background beyond the threshold is reduced; the multi-channel resolution of the area, frequency, and direction of the visual signal. An objective distortion measurement method usually related to vision is to use the early information processing stage of HVS, that is, to convert the error image into the "perceptual domain" from the retina through the optical nerve, the optic chiasm, and the lateral geniculate nucleus to the cerebral cortex. So that the errors of the same amplitude have the same visual perception at the end, the error is finally summed into a distortion scale with a suitable distance measurement. The complexity of the types of HVS and quality damage has brought many difficulties to the study of the problem. In the past, some scholars have used visual characteristics obtained from relatively simple psychovisual experiments, such as frequency selection and masking characteristics, to establish visual models for quality prediction. With the continuous development and requirements of coding technologies and applications, research on vision-based quality measurement has also continued to deepen, and there has been significant progress. S. Karunasekera & N. Kinsbery proposed a distortion measure for damage. This type of impairment is common in vector quantization and DCT encoding, and consists of vertical and horizontal edge distortions. First, subjective experiments were conducted on synthetic test images. By adjusting four parameters: edge amplitude, edge length, background brightness, and background activity, the visual sensitivity of edge damage was measured, and a visual model was established. It is worth mentioning that visual sensitivity is measured by the reaction time to stimuli, rather than the usual visual threshold method, and the model parameters are determined by the matching of constant sensitivity points (ConstantSensitivityPoint). The model for predicting the vertical edge error is (similar in the horizontal direction). The edge information is first extracted from the error image by a low-pass filter with a suitable bandwidth, and then the activity and regional masking effects are considered, where the activity mask is N × around the edge The weighted energy and determination of the coefficients of the Fourier transform of the N-box windowing; then a nonlinear transformation is performed to adapt to the nonlinear characteristics of the HVS, and finally the total error (absolute average) is calculated. In the inspection of actual natural images, the authors designed some images with the same MSE but different square structure distribution. The model prediction is in good agreement with the subjective test, and it also shows that MSE is not a good distortion measure. ①Image quality evaluation method based on visual perception: The effect of image degradation on human vision is determined by the sensitivity of the human visual system, and the visual sensitivity is determined by the human visual cells. In addition, the sensitivity of the human visual system is also affected by the local spatial frequency of the image. A large number of experimental results prove that the factor that affects the visibility of the pixel error is the local area environment around the error, not the background environment of the entire image. According to the above visual characteristics, people have established various HVS models to evaluate the quality of the image. The most typical HVS model is shown in Figure 1, which simulates three significant characteristics of visual perception, namely: visual nonlinearity (Weber's law) , Visual sensitivity bandpass and visual multi-channel and masking effect. ②Image quality evaluation method based on visual interest: Encoding technology based on image content has inspired people's research on image quality evaluation methods based on visual interest. From the perspective of visual psychology, vision is a positive feeling behavior, not only related to physiological factors, but also to a large extent depends on psychological factors. When observing and understanding images, people tend to be unconsciously interested in some of these regions. These regions are called "regions of interest (ROI)." The visual quality of the entire image often depends on the quality of the ROI, and the degradation of uninteresting areas is sometimes difficult to detect. In real life, due to the influence of cultural background, surrounding environment, and emotions, the evaluation of the same image will produce large deviations, but the areas of concern in the image are common, and they collectively convey most of the entire image to be expressed. Objective information. Measurement methods based on visual interest have opened up a new way for image quality evaluation, but these methods are still in the initial research stage, and there are still many problems to be studied in depth, for example, how to determine the region of interest in the image; If there are multiple interest areas, how to determine the interest weights of these areas. 3. Conclusion The correct evaluation of image quality is very important for the development of the overall image information engineering. It is believed that with the rapid development of multimedia information technology, the research on image quality evaluation will be paid more and more attention. In the traditional fusion image quality evaluation methods, there are defects such as the subjective evaluation method is too cumbersome and not repeatable, the objective evaluation result does not coincide with the actual image quality or even contradicts, and the existing evaluation methods are mostly suitable for strong correlation The image fusion evaluation is not suitable for fusion image evaluation in the field of entertainment photography. Therefore, the introduction of the HVS feature in the fusion image measurement method and the organic combination of the traditional objective evaluation method and the subjective evaluation method are an effective way to solve this problem and also the development direction of fusion image quality evaluation.
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