As we all know, the basic element of print media products is graphics, which is most important for color printing product images. Therefore, in the printing process, the color separation of the scan of the picture is a very critical part of the pre-press work. The scanner is the input of the pre-press image. Important tool. At present, the scanners used are not only high-end networking of electronic color separation machines, but also high-end drum scanners evolved from electric extension machines, as well as various flatbed scanners. The color correction process involves more links, control and The operation is often complicated, and only the main methods of color correction are introduced here.
1. Scan the original
Due to the rapid progress of color management technology, current high-end scanning equipment, such as ICG370HS, SCREEN8060PII or high-end networked electrical extensions, as long as they are operated properly, they will definitely get satisfactory scanning quality. As long as the operators of high-end scanning equipment follow certain principles and have a deep understanding of the thrust of the pictures, they can get good image effects. In fact, the quality bottlenecks reflected in pre-press scanning mainly focus on the quality of scanned originals. The quality of the scanned manuscript directly affects the quality of the final printed matter, but different manuscripts must adopt different scanning processing methods. Various manuscripts, prints, Chinese paintings, watercolors, and oil paintings reflect different themes. Therefore, according to the characteristics of the manuscript, it is necessary to summarize its rules and processing points in the scanning and copying process. Scanning curve to improve the quality of color separation. In the original identification, the non-applicable originals should be corrected or processed, and the originals that cannot be copied should be replaced to avoid affecting the overall quality of the final printed matter.
Second, the reference setting and adjustment of the scanner
The reference calibration of the scanner includes focus adjustment, brightness, contrast, white balance and color adjustment. The calibration of the scanner is to ensure that the image input, image gray balance, descreening, color shift, size and clarity 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 to balance the signals of the three-channel (R, G, B). Different types of manuscripts have different white balance selection points. The white balance selection point of the transmission manuscript is at the clean place of the roller, and the white balance selection point of the reflection manuscript is the white area of ​​the original manuscript or white coated paper.
The resolution setting has a great influence on the quality of the scanned image. When scanning an image, if the resolution setting is too low, the scanned image will have coarse grain, jagged edges, and poor quality; if the resolution setting is too high, it will cause unnecessary details in the original, such as on the screen Spots, wrinkles, and other backgrounds around the image are highlighted, 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 × magnification × quality factor, after the scanner is determined, selecting the appropriate quality factor (Quality Factor) is essential to the quality of the output image. To ensure the quality of the output image, the quality factor is often taken as 2.0.
Three, color integrity correction
1. Correct selection and setting of original white / black
The selection and setting of the correct white and black points of the original document is the key to reproducing the color and gradation of the original document, and is the basis for the effective adjustment of color and gradation. The correct selection and setting of the white and black fields should be based on the characteristics of various types of manuscripts and printability conditions, making 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 photographic manuscripts 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 gradation; Adjust the level to meet the visual habits and psychological requirements. The setting of white point / black point should expand the tone of the main body of the original as much as possible, and increase the contrast.
2. Automatic gray setting and gray balance correction
Automatic gray correction is a quick correction method for gray cast in 1/4, middle and 3/4 steps. That is, after setting the white and black fields in the pre-scan cropped image, adjust the data in the tone area where the image needs to be corrected so that the gray reaches the normal value. This is very helpful for color-separated scanning of color cast originals.
Gray balance refers to the neutral gray generated 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 during color separation, the gray in the copied original is not gray, and other colors will inevitably produce color shift. During the gray balance correction, the CMY single channel can be corrected simultaneously with the whole channel. Not only can the 1/4, middle and 3/4 tones be corrected, but by repeatedly selecting points, the other tones can also be corrected in the same way. The correction of gray balance must fully understand the color of the original in the real environment, otherwise it will cause the gray axis reflected in the picture to be undesirably shifted, which seriously affects the color quality of the overall image.
Fourth, color correction
The process of printing and copying color manuscripts is a combination of color decomposition and color restoration of image information. In the actual reproduction process, due to the unsatisfactory conditions, the existence of color difference is inevitable. The color difference comes from three aspects:
1. The color shift caused by the photographic process and the material of the original and the color shift caused by the discoloration (color attenuation) of the coloring medium.
2. Color deviation caused by errors such as light source, lens, color filter and photomultiplier tube during color decomposition.
3. Color deviation caused by paper and ink errors during color reduction.
Therefore, color correction must be performed. The color correction function should not only eliminate the above-mentioned color difference objectively existing in the color copy process, but also meet the color correction when the original color copy special requirements. Color correction is usually based on six colors such as cyan (Cyan), magenta (Magenta), yellow (Yellow), red (Red), green (Green), and blue (Violet). For a picture with a certain hue as the basic color, the basic color can be exaggerated appropriately and the foot color to make the printing effect more prominent. 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 regional processing can be used to correct the local subject color. For some pictures with gray as the main body, properly 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, craftsmanship and operating personnel in color reproduction. Only by mastering its laws can the manuscript and the actual needs of users be best expressed.
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