At present, most printing units are equipped with desktop systems for image production. However, due to the fact that many units, especially SMEs, have poor technical skills and are subject to other objective factors, many of the production and printing quality of color pictures are not Satisfactory, compared with the printing level of international publications, there is still a certain distance, can only be regarded as the lower level. Not only did many American editors, photographers, etc. complain that the current graphic arts, catalogues, periodicals, and magazines are of poor quality. The originals cannot be restored, and the colors are seriously distorted. Some colors become pattern blocks, and some are dark and dark. Even some of the prepress production processes have cast a serious cast of colors on the characters and landscapes of the manuscripts. In addition to the poor quality of some manuscripts, most of them are caused by improper scanning and color separation techniques. This is a pre-press production process using a low-grade platform scanner ( about 20% or so ) , the poor quality of the scanning and separation; the second is the operator's lack of physical and chemical concepts and standardization of the operation method to restore color separation, only rely on the screen color For subjective adjustments, advanced technologies and powerful functions of high-end professional scanners and electric extensions are not fully utilized. Third, operators lack the aesthetic level and correct analysis capabilities of the original drawings, and cannot be correctly performed according to the characteristics of various manuscripts. Parameter settings and proper adjustments. Of course, there are also very few cases where the customer does not know what to do and what the commander is doing.
Practice has proved that color image copy printing, scanning color separation technology is the key to quality, because on the one hand is the current computer plate to put the simulation of the color photo, reversal film and other graphic arts input, first of all have to be converted by scanning For the digital image, it can be transmitted in the digital process flow of the prepress system. In this scanning and conversion process, there are two key technologies that need to be mastered. One is how to apply the scanner, and the original is scanned from the beginning. Get the best color gradation information; Second, the image file input from the scanner is usually RGB format. Before outputting the color separation film, the file needs to be converted into CMYK format, which is the color space conversion. This step is color separation. The key to processing. It should be based on the printability of ink, paper, etc., set dot gain values, gray balance parameters, black plate tones, and select the GCR or UCR color separation mode. Only when these parameters are set correctly can high quality output be possible. Color film. On the other hand, the scanning separation is technical and artistic. It is necessary to rationally 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 manuscripts, and also from aesthetic aspects. Deal with the color tone, saturation, spatial distance, light and shade, light and shadow changes. The level of these two aspects of the operator has a great influence on product quality. Therefore, in order to improve the image quality of current color prints, it is necessary to resolve the key technology of scanning separation and improve the quality level of operators. The following is a brief introduction to the major issues and solutions in the current color, level, and tone processing in computer platemaking for reference.
Handling the Contrast of Photographic Images
Some customers responded that the contrast between photographic prints was not good, some highlights were not bright, and dark tone was not dark. The photo-luminosity of the pictures was not strong, and the contrast was not good; some of the highlights were white, and the tone was dark. There are too many losses. There are objective reasons for the innate lack of copying in the outlets, as well as subjective factors in the setting and adjustment of tone adjustments in scanning and color separation. Because of the current photographic creation, various pre- and post-processes are used to emphasize the contrast of tone and color, and images are exaggerated with different lenses and techniques. At present, high-quality photographs have a maximum density of 2.50 and can effectively exhibit a range of 0.20-2.20 . The maximum density of color reversal film is 3.50 , and the normal density of normal exposure is 0.20-3.20 . The color reproduction is limited by the printability of paper, ink, etc. The maximum density of the tone range that can be reproduced by advanced coated paper prints is only 1.8-1.9 . For this reason, the gradation range of the manuscript must be compressed when copying, and it is inevitable that some of the gradation levels will be lost after being compressed. Therefore, the print product is unlikely to reproduce the contrast effect of high quality photographic images, 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 to achieve the maximum print density contrast and visual contrast between light and dark. This should be explained to the customer, so that they understand.
In addition, it can also be improved from the tone setting and adjustment of the scanning color separation. The difficulty here is that a range of original tonal adjustments that are ever-changing are to be compressed and adjusted within the range of tone that the printed matter can reproduce; and secondly, each adjustment is compressed. In the manuscript, it is necessary to be able to determine what part of the layer is to be compressed, what part of the layer should be highlighted so as 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. The general solution is as follows:
1. Many current high-quality photographic images have a wide range of densities and cannot be fully integrated into the linear region of the assembly. Therefore, first determine the brightest ( whitest ) and darkest ( black portion ) as the starting and ending points of the tone setting according to the density contrast and the tone thickness of each photographic image , and try to match the original density range. Set the maximum dot value of the color separation sheet so that the print contrast is as close as possible to the contrast of the picture.
2. The white field setting should not be too peaceful. Can not be too embarrassing, otherwise it will make high-light level loss, and signal noise, easy to produce coarse, its essentials are:
( 1 ) The image with the emphasis on the medium tone and the standard of density contrast, with extremely high light setting and netting, and the highlight of the required level is set as the basic white, ie offset printing is C5% , M3% , Y3% ; OPP film gravure is C7% , M5% , Y5% .
( 2 ) For photos with light highlights and low color density, the white field setting values ​​may be slightly larger, and the basic white may be set at the brightest and whitest points to deepen the tone level of the highlights.
(C) In the main dark tone, deep color density images, can be given the next highest absolute optical networks to high brightness projection light and lightening (brightens) in tune.
3. The black market setting should not be too shallow or too shallow, which will make the dark tone of the printed product not strong enough and not strong enough; it can not be too deep, too deep will make the print dark tone dark, a level upgrade. The essentials are:
( a ) 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 is set to 1.80-1.90 . The basic black offset printing process is C95%-98% , M85-88% , Y85-88% , K75%-80% . The gravure printing process of OPP film is C90-95% , M80-85% , Y80-85% , K65-70% .
( 2 ) For images with low density contrast and dark colors, deeper copying is required. For the black field setting, the black part of the picture should be selected . The density of the transmitted manuscript is set to 2.30-2.50 . Color photo density is set to 1.50-1.60 , setting the basic black
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