How to master scanning color separation technology and image processing (I)

Although some of China's printing products can reach or reach the advanced level of the country, the current overall quality of book printing is still unsatisfactory. There are generally poor quality, many reworks, and low efficiency. There are even conditions where some high-end books are printed as inferior products. This is mainly due to the fact that the technical level of the image production staff of many units, especially small and medium-sized enterprises, is still not very high, combined with other objective factors, resulting in the unsatisfactory quality of the production and printing of many color images. There is still a large gap between the printing levels of out-of-print-items, and at best we can only count as the medium-lower level.

In China, many American editors, photographers, and people in the printing industry are often heard to reflect the poor quality of color images printed on today's illustrated newspapers, albums, and periodicals. It is impossible to restore originals. Color distortion is serious, and some colors become The pattern color blocks, some of the light and shade, the level of inversion, and even some prepress production processes to color the characters, landscape and other issues on the manuscript made a serious cast color and other issues, all these drawbacks, in addition to a part of the manuscript itself is too poor quality, Most of this is due to improper scanning and color separation techniques.

There are mainly the following three aspects. First, pre-press production processes use low-grade platform scanners (about 20% or so). The quality of scanning and separation is poor. Second, because operators lack physical and chemical concepts and standardized operation methods to restore color separation, they mainly want to rely on post-processing. Depending on the subjective and arbitrary adjustment of the screen colors, the high-end networking advanced technology and powerful functions of high-end professional scanners and electric extensions are not fully utilized. Third, the operators lack the aesthetic level and the correct analysis capabilities of the original images. The correct parameter settings and proper adjustments cannot be made according to the characteristics of various types of manuscripts.
#2

Scanning color separation technology is the key to the quality of color printing. Because on the one hand, it is the current computer system - the version - the input of image art such as analog color photo, reversal film, etc., must first be scanned and converted into a digital image in order to be transmitted in the digital process flow of the prepress system. There are two key technologies that need to be mastered during the scanning and conversion process. The first is how to use the scanner to get the best color gradation information from the original when scanning from the beginning. This has a great impact on quality. Second, the image file input from the scanner is usually in RGB format, and the output color separation film Before, this document needs to be converted into CMYK format, which is the conversion of the color space. This step is the key to color separation processing. It depends on the printability of ink, paper, etc. to set the dot gain value, gray balance parameters, and black-versions. - Tone adjustment, and whether to select the GCR or UCR separation mode, it is possible to output high quality color separations only if these parameters are correctly set.

On the other hand, the scanning separation is technical and artistic. It is necessary to set and adjust the parameters such as gray balance, tone contrast, color correction, and sharpness from the technical aspects in accordance with different types of originals. The aesthetic aspects of art deal with the tone of color, saturation, spatial distance, light and shade, and changes in light and shadow. The level of the operator in these two areas greatly influences the product quality. Therefore, to improve the image quality of current color prints, we must resolve to do a good job of the key technology of scanning and color separation and improve the quality of operators.

Pay attention to the problems of tone contrast, gray balance, and color correction. Establish a standardized restore file for scanning color separation, make full use of electrical extensions or scanners to obtain better image information directly from the original, and ensure good image color restoration. Whether it is the production of high-end products or the production of middle- and low-end products, it is the first work, especially the production of art-painting-painting, etc., to reproduce the original beauty of the picture, the depth of artistic conception, and the uniqueness of painters and photographers. Temperament, style, and characteristics are particularly important, because the artistic conception in the work cannot be tuned out later.

1, the tone contrast

Some customer reactions. Photographic prints have poor contrast handling, and some highlights do not shine, dark tone is not dark, photographic images of the luminosity is not strong, the contrast effect is not good; some high light a white, dark tune a mass of black. There is too much loss at both ends. Here are the objective reasons for copying the innate lack of outlets, as well as the subjective factors caused by the setting and adjustment of the tone during scanning and color separation.

The standard reduction of pre-scan color separation refers to the use of standard transmissive, reflective grayscale and color standard testing tools for replication experiments based on the manuscript conditions. Because of the current photographic creation, various pre- and late-stage processing are used to emphasize the contrast of tone and color, and different shots and techniques are used to obtain an exaggerated image.

At present, high-quality photos have a maximum density of 2.50, and can effectively exhibit a range of 0.20 to 2.20. The maximum density of the color reversal film is 3.50, and the effective density range of normal exposure is 0.20 to 3.20. The color reproduction is limited by the printability of paper and ink, and the maximum density of the tone range that can be reproduced by advanced coated paper prints is only 1.8 to 1.9.

For this reason, the gradation range of the manuscript must be compressed when copying, and some levels will be lost after compression, so it is unlikely that the printed matter will reproduce the contrast effect of high quality photographic pictures, especially the color reversal film. Copying can only make full use of the whiteness of the paper and the maximum density of the four-color ink superimposed in this color space to achieve maximum print density contrast and visual contrast between brightness and darkness. This aspect should be explained to the customer and get their understanding.

On the other hand, to improve the tone setting and adjustment of the scanning separation, the difficulty here lies in the following: First, the face of the ever-changing original tone range must be compressed and adjusted to the range of tone that the print can reproduce; When each manuscript is compressed and adjusted, it is necessary to be able to determine what part of the gradation is the compression, what part of the gradation should be highlighted in order to emphasize the level of the main part, and it can be determined to what extent it is emphasized and compressed to what extent. This requires both an analysis of the manuscript and a correct method of operation.

Because many current high-quality photographic images have a wide range of densities, it is not possible to place the linear region of the element completely. Therefore, first, according to the density contrast and tone thickness of each photographic image, determine the highest (whitest) and darkest (black portion) as the starting and ending points of the tone setting, and try to correspond to the original density range. The maximum dot value of the color separation sheet is set so that the contrast of the printed matter is as close as possible to the contrast of the picture.

The white field setting cannot be too peaceful. Taiping will cause the high-gloss of the printed product to be grayed out. Can not be too embarrassing, too ambiguous will cause high light level loss, and signal noise, easy to produce coarse particles. It is generally necessary to master such things as: (1) The image with the emphasis on the medium tone and the standard of density contrast, with extremely high light setting, and the high light white with the required level set as the basic white, ie, offset printing as “C5%, M3%, Y3” %". The OPP film gravure is "C7%, M5%, Y5%". (2) For the picture with light highlights and low color density, the white point setting can be slightly larger. The basic white color is set at the brightest and whitest point to deepen the bright color tone. (3) The dark color tone of the main picture, the deep color density of the picture, the second high light can be net, in order to highlight the brightest lightening and lightening (lightening) in the tone.

The black field setting should not be too shallow or too shallow, which will result in darkness in the prints, which is not thick enough and not strong enough. It cannot be too deep or too deep, which will cause the dark dark tone of the printed products to rise to a level. The mastery is: (1) The density contrast standard picture, to restore the copy, the black field settings should select the picture dark tone pure black parts, transmissive manuscript density is set to 2.70 ~ 3.00, the color photo density setting It is 1.80 to 1.90. The basic black offset printing process is C95% to 98%. M85%~88%. Y85% to 88%, K75% to 80%. The gravure printing process of OPP film is C90%~95%, M80%~85%, Y80%~85%, K65%~70%. (2) The density contrast is small, and the picture with light tone should be deepened. For the black field setting, the black part of the picture should be selected. The density of the transmitted manuscript is set to 2.30 to 2.50. The color photo density is set to 1.50 to 1.60 and the basic black is set. (3) The density contrast is large and the hue is heavy. To reduce (lighten up) the copy, the darkest and darkest part of the image can be selected, and the set density value can be consciously increased. The density of the transmitted manuscript is set to 3.00 to 3. 50. The color photo density is set to 2.00 to 2.30 and the basic black is set.

The key to dubbing the success or failure of duplication is the correct black-version-setting. Currently, there are common problems: First, the operator does not pay attention to the setting and adjustment of black-version; second, most of the dark shading of the picture, often Can not find neutral black, affect the black - version - set the accuracy; three is black - version - are used to adjust the skeletal curve of the short turn, that is, in the C-version -50% at a small point, resulting in the middle of the picture The level of adjustment is not enough and the sense of three-dimensionality is not strong. Practice has proved that the effective use of black-printing can not only enhance the contrast of pictures, but also enhance the dark and medium tone levels. The essentials are: (1) The black site selection should be selected as far as possible on the neutral black part of the black object to ensure the accuracy of the black-version-network value setting. (2) Black - Edition - outlet value setting: offset printing process is 75% ~ 80% is better. The OPP film gravure printing process is 65% to 70%. (3) The effective use of the black-print version is not simply to add black-versions to the three primary colors. The technical method is to match the UCR, and the amount of ink of the three primary colors should be appropriately reduced in the black portion so as to increase uniformity. Real black - version - make room, with black - version - unveiled the tone level, to enhance the photographic contrast. General removal of 20%, C78%, M68%, Y68%, K90%. (4) Black - version - the starting point of the extension to the C - version of the 35% to 40% at a small point, you can increase the picture of the middle and dark tone levels and three-dimensional, of course, black - version - tone extension, color Some will also be deepened, and attention should be given to reducing it accordingly.

The tone curve of the tone is the abscissa of the standard gray scale, and the gradation of the dot of the color separation film is the relation curve of the ordinate. Through the scanning and out-printing experiments, a standard curve file corresponding to the density values ​​of the gray scales at all levels and the tone points of the dichroic patches at various levels was established, which was used as a standard density manuscript application. At the same time change the midtone curve, establish a few preset curves, respectively +5%, +10%, a 5%, a 10%, etc., to be stored for the selection of different density contrast manuscripts.