The main method of scanner color correction

It is well-known that the basic elements of print media products are graphics and graphics, which are most important for color printing product images. Therefore, in the printing process, scanning and color separation of pictures is a key link in prepress work. The scanner is the input of prepress images. Important tool. Currently used scanners have both high-end networking of electronic color separation machines, high-grade roller scanners that have evolved from electrical extensions, and a variety of flatbed scanners. The color correction process involves many processes, controls, and Operations are also often complex, and only the main methods of color correction are described here.

First, scan the original

Due to the leap forward in color management technology, current high-end scanning devices, such as ICG370HS, SCREEN8060PII or high-end networked extensions, will have satisfactory scanning quality as long as they are handled properly. As long as the operators of high-end scanning equipment follow certain principles and deeply understand the subject matter reflected in the pictures, they can obtain good image effects. In fact, the quality bottlenecks reflected in the prepress scan mainly focus on the quality of the scanned original. The quality of the scanned original document directly affects the quality of the final print, but the different scanning methods must be taken for different originals. The main subject reflected in various manuscript photographs, prints, Chinese paintings, watercolors, and oil paintings is different. Therefore, according to the characteristics of the manuscripts, it is necessary to summarize the rules and handling points in the scanning and copying process. After many times of practice, the characteristics of different manuscripts are established. Scan the curve to improve the quality of the scanning separation. When the original is being authenticated, correction or machining is required for non-applicable originals, and for originals that cannot be copied, originals are replaced so as not to affect the overall quality of the final printed product.

Second, the scanner's benchmark settings and adjustments

Baseline corrections for the scanner include focus adjustment, brightness, contrast, white balance, and color adjustments. The calibration of the scanner reference is to ensure that the image input, image gray balance, network removal, color shift, size, and resolution meet the set control requirements. The function of the white balance correction is to adjust the maximum output working voltage of the three-channel (R, G, B) photomultiplier tubes of the scanning head and balance the signals of the three channels (R, G, B). Different types of originals have different white balance settings. The white balance point of the transmitted document is in the clean area of ​​the roller, and the white balance point of the reflected document is in the white area of ​​the original or on the white coated paper.

The setting of the resolution 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 is coarse, the image edges are jagged, and the quality is poor; if the resolution is set too high, unnecessary details in the original will be made, such as on the screen. Spots, wrinkles, and other backgrounds around the image are highlighted, and the scanned image storage space is too large, affecting the scanning speed. Therefore, the resolution of the scanned image must be set correctly to obtain a clear image. Because the scan resolution = number of screen lines × magnification × quality factor, when the scanner is determined, selecting the appropriate quality factor (Quality Factor) is critical to the output image quality. In order to ensure the output image quality, the quality factor is often 2.0.

Third, the color integrity correction

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

The correct white point and black point selection and setting of the manuscript is the key to reproducing the color and level of the manuscript, and is the basis for effective color and level adjustment. The correct white-field and black-field selection and setting should take full advantage of the whiteness of the paper and the maximum density of the four-color ink superposition to achieve the best visual contrast effect according to the characteristics of various types of originals and printability conditions. Since the printing of photographic manuscripts is a process of contrast compression, the selection and setting of white fields should make full use of the whiteness of the paper while taking into account the level of high-profile; the selection and setting of the black field should fully represent the darkness of the image. Adjust levels to meet visual habits and psychological requirements. The white field / black field is set to expand the tone of the original body 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 tone. That is, after the pre-scanned crop image is set to the black and white field, the data is adjusted in the tonal area where the image needs to be corrected so that the gray value reaches a normal value. This is helpful for color separation scanning of color-originated originals.

Gray balance refers to the neutral ash generated by the three color versions of yellow, magenta, and cyan on the basis of numerical ratios of different outlets. Neutral gray characteristics depend on the characteristics of the ink, and the 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 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 entire channel. Not only can corrections be made to 1/4, middle, and 3/4 tones, but they can also correct other gradations in the same way by repeating the selection. The correction of gray balance must fully understand the original color in the real environment, otherwise it will cause the gray axis reflected in the picture to have an undue displacement, which will seriously affect the color quality of the overall image.

Fourth, color correction

The print copy process of a color manuscript is a combination of two processes of color decomposition and color reproduction of image information. In the actual copying process, due to the unsatisfactory conditions, the presence of color differences is inevitable. The color difference comes from three aspects:

1. The color shift caused by the color shift caused by the color shift (color attenuation) caused by the original itself due to the photographic process and materials.
2. Color shift caused by errors such as light source, lens, color filter, and photomultiplier tube in the color decomposition process.
3. Color shift caused by paper and ink errors during the color reduction process.

Therefore, color correction must be performed. The color correction function eliminates the above-mentioned chromatic aberration that is objectively present in the color reproduction process, and also satisfies the color correction when the original color reproduction is particularly demanded. The color correction is usually based on six colors such as Cyan, Magenta, Yellow, Red, Green, and Violet. For a picture with a certain color as the basic color, the basic color can be properly exaggerated and sufficient color tone can be given to make the printing effect more prominent. The purity of the basic color can be enhanced by appropriately reducing the amount of the opposite color. On the basis of not affecting the overall grey balance of the image, the local subject color can be corrected using the region processing method. For some images with gray as the main body, proper enhancement of 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 workmanship. Only by mastering its laws can the manuscript and the user's actual needs be best represented.