The main method of color calibration for Cigar Paper Boxes scanner

The main method of color calibration for Cigar Paper Boxes scanner

As we all know, the basic elements of printed media products are graphics, the most important for the image of color printing products, so in the printing process, the picture scanning color separation is a very key link in the pre-press work, the scanner is an important tool for the input of pre-press images. At present, the scanners used are both high-end networking of electronic color separators, and high-grade drum scanners evolved from electrical extension, as well as various flatbed scanners, the process of color correction involves more links, control and operation are often complex, here only the main methods of color correction are introduced.

1. Scan the original manuscript

Due to the leap in color management technology, today's high-end scanning equipment, such as ICG370HS, SCREEN8060PII or high-end networking extension, as long as it is properly operated, it will be satisfied with the scanning quality. As long as the operator of high-end scanning equipment follows certain principles and deeply understands the theme reflected in the picture, it can get a good image effect. In fact, the quality bottleneck reflected in prepress scanning is mainly focused on the quality of the scanned manuscript. The quality of the scanned manuscript directly affects the quality of the final printed matter, but different manuscripts must take different scanning processing methods. Various original photos, prints, 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 scanning curves for different original characteristics after many practices to improve the quality of scanning color separation. In the identification of the original, the non-applicable manuscript should be corrected or processed, and the original that cannot be copied should be replaced, so as not to affect the overall quality of the final print.

2. Benchmark setting and adjustment of scanner

The standard calibration of the scanner includes focal length adjustment, brightness, contrast, white balance and color adjustment. The calibration of the scanner is to ensure that the image input, image gray balance, mesh removal, color bias, size and sharpness meet the set control requirements. The role of white balance correction is to adjust the maximum output working voltage of the scanning head three channels (R, G, B) photomultiplier and balance the signal of the three channels (R, G, B). The selection points of white balance of different types of manuscripts are different, the white balance of transmission manuscripts is in the clean area of the roller, and the white balance of reflection manuscripts is in the white area of the manuscript or on the white coated paper.

The setting of resolution has a great influence on the quality of the scanned image. When scanning images, if the resolution setting is too low, the scanned image particles are rough, the image edges are jagged, and the quality is poor. If the resolution is set too high, it will make unnecessary details in the original manuscript, spots on the picturesque surface, wrinkles, and other background around the image stand out, and will make the storage space of the scanned image 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 = plus the number of network lines × magnification × Quality Factor, when the scanner is determined, choosing the appropriate quality factor is crucial to the output image quality, in order to ensure the output image quality, the quality factor is often 2.0.

Third, color integrity correction

1. Correct selection and setting of white/black field of the original manuscript

The correct selection and setting of white and black fields in the original manuscript is the key to reproduce the color and level of the original manuscript, and is the basis for effective color and level adjustment. The correct white field and black field selection and setting should be based on the characteristics of various types of manuscripts and printability conditions, and make full use of the whiteness of the paper and the maximum density of the four-color ink overlay to achieve the best visual contrast effect. Since the printing of the photographic manuscript is a process of contrast compression, the selection and setting of the 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 express the level of the dark tone 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 manuscript as much as possible and increase the intensity of contrast.

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 order tones. That is, after setting the white and black fields in the pre-sweeping image, adjust the data in the level area of the image that needs to be corrected, so that the gray value reaches the normal value. This is very helpful for color separation scanning of partial manuscript.

Gray balance refers to the neutral gray produced by yellow, magenta and green three color plates according to different dot values. The characteristics of neutral gray depend on the characteristics of the ink, and gray balance is extremely important for color reproduction. Gray balance is the basis of any color separation method to achieve correct color reproduction, if the gray balance can not be achieved when the color separation, then the gray in the copy of the original is not gray, then other colors will inevitably produce color bias. When gray balance is corrected, CMY can be corrected for a single channel or a whole channel at the same time. Not only can the 1/4, middle and 3/4 keys be corrected, but the other keys can also be corrected in the same way by repeating the selection of points. The correction of gray balance must fully understand the color of the original manuscript in the real environment, otherwise it will cause the gray axis reflected in the picture should not be offset, seriously affecting the color quality of the overall image.

Four, color correction

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

1. The color deviation of the original manuscript itself due to the photographic process and materials and the color deviation caused by the discoloration of the color medium (color attenuation).

2. Color deviation caused by errors of light source, lens, color filter and photomultiplier in the process of color decomposition.
3. Color deviation caused by paper and ink errors in the process of color reduction.

Therefore, color correction is necessary. The color correction function should not only eliminate the above color difference objectively existing in the process of color reproduction, but also meet the special requirements of color correction of the original manuscript. Usually color correction is based on six colors: Cyan, Magenta, Yellow, Red, Green and blue. For pictures with a certain hue as the basic color, the basic color can be properly exaggerated and given enough color to make its printing effect more prominent, and the amount of the opposite color 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 method of region processing can be used to correct the local subject color. For some pictures with gray as the main body, properly improving the color purity will enhance the overall color visual effect of the image.

In short, color correction is a comprehensive reflection of the quality of equipment, manuscript, process and operators in color reproduction, and only by mastering its laws can the actual needs of the manuscript and users be the best performance.