Main methods and steps of prepress color correctio

2022-08-24
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As we all know, the basic element of printing media products is graphics and text, which is the most important to the image of color printing products. Therefore, in the printing process, the scanning and color separation of pictures is a very key link in the prepress work, and the scanner is an important tool for prepress image input. At present, the scanners used include not only the high-end combination of electronic color separators, but also the high-end roller scanners evolved from electronic color separators, as well as various flat-panel scanners. The process of color correction involves many links, and the control and operation are often complex. Here, only the main methods of color correction are introduced. 1、 Scanning originals

due to the rapid progress of color management technology, the current high-end scanning equipment, such as icg370hs, screen8060pii or high-end electrical extension, as long as it is operated properly, it will get satisfactory scanning quality. As long as the operators of high-end scanning equipment follow certain principles and deeply understand the theme reflected in the picture, they can get good image effects. In fact, the quality bottleneck reflected by prepress scanning mainly focuses on the quality of scanned originals. The quality of scanned originals directly affects the quality of the final printed matter, but different originals must adopt different scanning methods. Various original photos, prints, traditional Chinese paintings, watercolors and oil paintings reflect different themes, so it is necessary to summarize the rules and processing points in the process of scanning and copying according to the characteristics of the original, and establish characteristic scanning curves for different originals after many times of practice to improve the quality of scanning and color separation. In the identification of originals, the non applicable originals should be corrected or processed, and the originals that cannot be copied should be replaced, so as not to affect the overall quality of the final printed matter

II. Scanner's benchmark setting and adjustment. At the same time, the elastic modulus of chemically reduced single-layer graphene is 0.25tpa.

scanner's benchmark correction includes focal length adjustment, brightness, contrast, white balance and color adjustment. The calibration of the scanner benchmark is to ensure that the image input, image gray balance, removal, color deviation, size and sharpness meet the set control requirements

the function of white balance correction is to adjust the maximum output working voltage of the three channel (R, G, b) photomultiplier tube of the scanning head and balance the signals of the three channels (R, G, b). Different types of originals have different selection points for white balance. The selection points for white balance of transmission draft are at the clean place of the drum, and the selection points for white balance of reflection draft are in the white area of the original or on white coated paper

the setting of resolution has a great impact on the quality of scanned images. When scanning an image, if the resolution setting is too low, the scanned image particles are rough, the image edges are jagged, and the quality is very poor; If the resolution is set too high, unnecessary details in the original, such as spots, folds on the screen and other backgrounds around the image, will be highlighted, and the storage space of the scanned image will be too large, affecting the scanning speed. Therefore, the resolution of the scanned image must be set correctly to get a clear image. Since the scanning resolution = the quality factor of the magnification of the number of lines added, when the scanner is determined, selecting the appropriate quality factor is very important to the quality of the output image. In order to ensure the quality of the output image, the quality factor is often taken as 2.0

the focal length adjustment depends on the orientation of the film surface of the original on the surface of the drum, the type of the original, the size of the optical hole, the magnification, etc. When processing the printed manuscript, the focusing time circle should be slightly larger and appropriately faint, so as to eliminate moire. Now high-end scanners automatically control the adjustment of focal length. When virtual halo is needed, they also adjust the focal length through the setting of color management software to achieve the purpose of eliminating moire, such as icg370hs

III. color integrity correction

1. The correct selection and setting of the white field/black field of the original

the selection and setting of the correct white field and black field of the original is the key to reproduce the color and level of the original, and is the basis for effective color and level adjustment. The correct selection and setting of white field and black field should make full use of the whiteness of paper and the maximum density of four-color ink superposition to achieve the best visual contrast effect according to the characteristics of various types of originals and printing suitability conditions. As the printing of photographic originals is a process of contrast compression, the selection and setting of white field should make full use of the whiteness of the paper, while taking into account the high-profile level; The selection and setting of the black field should fully reflect the dark tone level of the image to meet the visual habits and psychological requirements. The setting of white field/black field should expand the tone of the main body of the original as much as possible, increase the intensity of contrast, and at the same time, on the basis of understanding the theme of the original, know how to abandon the level of bright and dark tones, so as to highlight the theme of the original

2. Automatic gray setting and gray balance correction

automatic gray correction is a fast correction method for gray color deviation in 1/4, middle and 3/4 tones. That is, after setting the white and black fields in the pre scanning and cutting image, adjust the data in the gradient area of the image that needs to be corrected to make the gray reach the normal value. This is very helpful for color separation and scanning of color biased originals

establish a macro-control and market conditioning mechanism. Grey balance refers to the neutral grey generated by the yellow, magenta and cyan color plates on the printed matter according to different numerical proportions. Neutral gray characteristics depend on the characteristics of ink, and gray balance is extremely important for color reproduction. Gray balance is the basis for any color separation method to achieve the correct reproduction of color. If gray balance cannot be achieved during color separation, the gray in the copied original is not gray, and other colors will inevitably produce color deviation. During gray balance correction, CMY can be corrected by a single channel or by an overall channel at the same time. It can not only correct 1/4, middle and 3/4 steps, but also correct other steps in the same way that this technology can provide good machinability, ductility, spinnability and uniformity for melt blown fibers by repeatedly selecting points. The correction of gray balance must fully understand the color of the original in the real environment, otherwise the gray axis reflected in the picture will be shifted unnecessarily, which will seriously affect the color quality of the overall image

IV. color correction

the printing and reproduction process of color originals is a combination of color decomposition and color restoration of image information. In the actual replication process, due to various conditions are not ideal, the existence of color difference is inevitable. Its color difference comes from three aspects:

1. The color deviation caused by the photography process and materials of the original and the color deviation caused by the discoloration (color attenuation) of the color medium

2. Color deviation caused by errors of light source, lens, color filter and photomultiplier tube during color decomposition

3. Color deviation caused by paper and ink errors in the process of color reduction

therefore, color correction must be carried out. The color correction function should not only eliminate the above color differences objectively existing in the process of color reproduction, but also meet the special requirements of original color reproduction. Usually, color correction is based on six colors such as cyan, magenta, yellow, red, green and blue. For pictures with a certain tone as the basic color, the basic color can be exaggerated appropriately and the full color tone can be given to make the printing effect more prominent, and the amount of opposite colors can be appropriately reduced to enhance the purity of the basic color. On the basis of not affecting the overall gray balance of the image, the regional processing method can be used to correct the local subject color. For some pictures with gray as the main body, appropriately improve the color purity. 2. Material change. The routine maintenance of the experimental machine is before, after and during the operation of the experimental machine. The cleaning and inspection will enhance the overall color visual effect of the image

in a word, color correction is a comprehensive reflection of the quality of equipment, originals, processes and operators in color reproduction. Only by mastering its rules can we best perform the actual needs of originals and users

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