Mitsubishi 3F offset press ghost failure
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It is well known that the basic elements of printed media products are graphic texts, which are most important for images with color printed products. Therefore, in the printing process, the scanning color separation of pictures is a very important part of prepress work. The scanner is the prepress image input. Important tool. The scanners currently used include high-end networking of electronic color separation machines, high-end roller scanners evolved from electrical extensions, and various flatbed scanners. The color correction process involves many links, control and The operation is also often complicated, and only the main methods of color correction are introduced here.
First, scan the original
Due to the leap in color management technology, today's high-end scanning equipment, such as ICG370HS, SCREEN8060PII or high-end networked extensions, will be able to get satisfactory scanning quality as long as they are properly operated. The operator of the high-end scanning device can obtain a good image effect by following a certain principle and deeply understanding the subject matter reflected in the picture. In fact, the quality bottleneck reflected by prepress scanning is mainly focused on the quality of scanned originals. The quality of the scanned original directly affects the quality of the final print, but different originals must take a different scanning method. Various original photographs, prints, Chinese paintings, watercolors, and oil paintings reflect different themes. Therefore, it is necessary to summarize the rules and processing points in the scanning and copying process according to the characteristics of the original. After many practices, the characteristics of different originals are established. Scan the curve to improve the quality of the scanned color separation. In the case of original authentication, the non-applicable original should be corrected or processed, and the original should be replaced for the original that cannot be copied, so as not to affect the overall quality of the final printed matter.
Second, the scanner's benchmark settings and adjustments
The scanner's baseline corrections include focus adjustment, brightness, contrast, white balance, and color adjustment. Tuning the scanner reference is to ensure that the image input, image gray balance, descreening, color cast, size and sharpness meet the set control requirements. The function of the white balance correction is to adjust the maximum output operating voltage of the three-channel (R, G, B) photomultiplier tube of the scan head and balance the signals of the three channels (R, G, B). Different types of original white balance are selected differently. The white balance of the transmission draft is selected at the cleanliness of the drum, and the white balance of the reflective draft is selected in the white area of the original or on the white coated paper.
The resolution setting has a great influence on the quality of the scanned image. When the image is scanned, if the resolution setting is too low, the scanned image is rough, the edges of the image are jagged, and the quality is poor; if the resolution is set too high, unnecessary details in the original, such as on the screen, are made. Spots, wrinkles, and other backgrounds around the image stand out, and the storage space of the scanned image is too large, affecting the scanning speed. Therefore, the resolution of the scanned image must be set correctly to obtain a clear image. Since the scanning resolution = the number of screen lines × the magnification × the quality factor, when the scanner is determined, selecting the appropriate quality factor is critical to the quality of the output image. To ensure the quality of the output image, the quality factor is often taken as 2.0.
Third, the color integrity correction
1, correct selection and setting of original white space/black field
The correct white field and black field selection and setting of the original are the key to reproduce the original color and level, and are 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 originals and printing suitability conditions, and make full use of the whiteness of the paper and the maximum density of the four-color ink superimposed to achieve the best visual contrast effect. Since the printing of the photographic manuscript is a contrast compression process, the white point selection and setting should make full use of the whiteness of the paper, and at the same time take into account the high-profile level; the black field selection and setting should fully express the darkness of the image. Levels are adjusted to meet visual habits and psychological requirements. The setting of the white field/black field should extend the tone of the original body as much as possible, and increase the contrast intensity.
2, automatic gray setting and gray balance correction
Automatic gray correction is a fast correction method for gray color cast in 1/4, middle, 3/4 tone. That is, after the pre-sweep crop image is set to the white and black field, the data is adjusted in the tone region where the image needs to be corrected, so that the gray reaches the normal value. This is useful for color separation scanning of color cast originals.
Gray balance refers to the neutral gray produced by the three color versions of yellow, magenta and cyan on the printed matter according to the proportion of different dot values. Neutral gray characteristics depend on the characteristics of the ink, and gray balance is extremely important for color reproduction. Gray balance is the basis for any color separation method to achieve correct color reproduction. If gray balance cannot be achieved when color separation, the gray in the original after copying is not gray, then other colors will inevitably produce color shift. When the gray balance is corrected, the CMY single channel can be corrected simultaneously with the overall channel. Not only can 1/4, middle and 3/4 tone be corrected, but by repeating the selection, the other tones can be corrected in the same way. The correction of the gray balance must fully understand the color of the original in the real environment, otherwise the gray axis reflected by the picture should not be offset, which seriously affects the color quality of the overall image.
Fourth, color correction
The print copying process of a color original is a combination of two processes of color decomposition and color reduction of image information. In the actual copying process, the existence of chromatic aberration is inevitable due to various conditions. The color difference comes from three aspects:
1. The color shift caused by the color shift of the original itself due to the photographic process and materials and the color change (color decay) of the color medium.
2. Color shift caused by errors such as light source, lens, color filter and photomultiplier tube in color decomposition process.
3. Color shift caused by paper and ink errors during color reduction.
Therefore, color correction must be performed. The color correction function not only eliminates the above-mentioned chromatic aberration which is objectively present in the color reproduction process, but also satisfies the color correction when the original color reproduction special requirements are met. Generally, color correction is based on six colors of Cyan, Magenta, Yellow, Red, Green, and Violet. For a picture with a certain color as the basic color, the basic color can be appropriately exaggerated and the color tone can be made to make the 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 local subject color can be corrected by the method of region processing. For some gray-based pictures, appropriately increasing 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, manuscripts, crafts and workers in color reproduction. Only by mastering the rules can the original needs of the manuscript and the user be best represented.