Prepress Image Processing Effective Strategies

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.

The reason is that the prepress production process uses a low-grade platform scanner (about 20% or so), and the quality of scanning and separation is poor;

Second, due to the lack of physical and chemical concepts and the standardized operation methods of reverting and scanning color separation, operators rely only on screen colors for subjective adjustments, so that the 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. They cannot perform correct parameter settings and appropriate adjustments according to the characteristics of various types of manuscripts. 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 level information;

The second is that the image file input from the scanner is usually in the RGB format. Before the output of the color separation film, the file needs to be converted into the CMYK format, that is, the color space conversion. This step is the key to color separation 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 photos have a maximum density of 2.50 and can effectively exhibit a range of 0.20-2.20. The maximum density of the color reversal film is 3.50, and the effective density range 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 coating 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 main focus of the adjustment and the standard of contrast of the density should be set at a high level, and the highlights of the required level should be set as the basic white, ie, the offsets should be C5%, M3%, and Y3%; the OPP film should be gravure 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 main picture is dark tone, and the color density is deep. The second highest light can be netted to highlight the brightness of the highlight and reduce the lightness (lightening).

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:

(1) The density contrast standard image, to restore the copy, the black field setting should select the picture dark tone pure black parts, transmissive document 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) If the density contrast is small, the picture with light tone should be deepened, and the black field setting should select the black part of the picture. The density of the transmitted manuscript is set to 2.30-2.50. The color photo density is set to 1.50-1.60 and the basic black is set.

(3) High density contrast, heavy tone, to reduce light (brightening) copy, select the darkest part of the picture, and increase the set density value, the density of the transmitted manuscript is set to 3.00-3.50, the color photo density is set Set it as 2.00-2.30 and set the basic black.

4. The key to dubbing the success or failure of duplication lies in the correct black edition setting. Currently, there are widespread problems. First, the operator does not attach importance to the setting and adjustment of black movies; second, most of the dark shading of the pictures is often found. To neutral black, affect the accuracy of the black version setting; Third, the black version is the use of short-term adjustment skeleton curve, that is, in the C version of 50% at a small point, resulting in the middle level of the picture is not enough, three-dimensional sense is not strong . Practice has proved that the effective use of the black version can enhance the contrast of pictures and enhance the dark and middle levels. The essentials are:

(1) The black site selection should be selected as far as possible in the neutral black part of the black object to ensure the accuracy of the black version of the network value setting.

(2) Black Edition dot value setting: The offset printing process is preferably 75%-80%. The OPP film gravure printing process is 65%-70%.

(3) The effective use of the black edition is not only to add black editions to the three primary colors, but to match with the UCR, and the amount of tricolor inks should be reduced in the black portion in order to make room for increasing the uniform black version. The dark version of the dark tone level, to enhance the photographic contrast. General removal of 20%, C78%, M68%, Y68%, K90%.

(4) The starting point of the black version is extended to 35% to 40% of the C version. Small points can be added to the picture, and the darkness and depth of the picture can be increased. Of course, the black tone will be extended and the color part will be deepened. Pay attention to lighten accordingly.

Handle white and black in color pictures

The white and black colors in the color picture are not only the important colors in the picture, but also have a black and white tone, which makes the color appear pure and pure. Colorists believe that the world is not absolutely white, nor absolutely black. No matter how bright "white" is, or how dark the "black" is, it has a certain color tendency, but on the concrete picture, there may be exaggerated white and exaggerated black. Therefore, we must pay attention to the black-and-white tones in the color maps when copying. This is an important part of improving the image arts.

At present, most operators only pay attention to the adjustment of color, but ignore the black and white treatment. Therefore, white objects are often treated as white, completely losing the layered texture of white objects; the black objects are handled in darkness, which does not highlight the hierarchical texture of black objects. The solution to this problem is:

1. Improve the understanding of the importance of white and black tones in color pictures

Everyone knows that black and white photographs are more visually pleasing than color photographs because the difference in reflectance between black and white is much larger than any color. According to scientists' determination, the contrast between colors, even the most intense contrast between primary colors or complementary colors, is much weaker than the contrast between black and white. For example, red and green juxtapositions are comparable to black and white juxtapositions, and the former can only reach the latter. 40%, compared with black and white, black and white, the former can only reach 23% of the latter. Compared with black and white, the former is only 19% of the latter. This is why black and white works are sometimes more colorful than black and white. Works come quickly. Therefore, the white and black colors in the color map are processed during the scan, which not only vividly displays the texture of various layers of white and black objects, but also highlights the contrast of light and dark in the visual sense.

2. The white color of the white object is called the extreme color in the color science. The white color in the color picture can not only show the texture characteristics of various white objects, but also make the white eye-catching and bright in the light.

(1) Various white objects in color photography images are extremely rich in color tone due to the influence of light source color. Before processing, it is necessary to fully understand the texture characteristics of various white objects, and then adjust the processing according to different texture characteristics, such as: white Paper, white clouds, white flowers, white silk, white jade, white porcelain, etc., have different texture characteristics and should be different when setting adjustments. The white paper can be handled lightly, while the white porcelain and the white porcelain must show a jade texture and a porcelain texture. For example, some monochromatic porcelain surfaces such as shadow, azure and white glaze are smooth and exquisite. With the texture, the gray level of the highlight area should be maintained during the scanning setup. However, in some of these porcelain albums, the porcelain colors of various white porcelain bowls, white porcelain vases, etc., which are mostly bright and medium-sized, are printed in white, and they are pale like white paper, with only 3% of bright highlights. -5% of the C version, M, Y version is a net, there are several even C version of the net, completely lost the sense of porcelain.

(2) The white art of fine arts and graphic arts is more characteristic. For example, the oil tanker, gouache, and white are represented by the pigment covering it, and the white is white and has texture. Therefore, when setting, it is necessary to use a three-primary color to form a neutral white, and it is only possible to make the local extremely high light net. In watercolor, white is reflected by the white color of the water color itself. At least one piece of each watercolor is pure white, so that the picture can be bright and clear. Therefore, the local highlight white can be eliminated when setting. Leonardo da Vinci once said: "The whitest part of a painting is as valuable as a precious stone." This means that the white place is very rare in the painting and is a special effect pursued by the painter. Therefore, when setting up the scan, be sure to show it carefully.

(3) The white objects of many current prints often exhibit color cast and dew, because some operators lack consciousness of gray balance. For high-gloss white objects that require a level, set the proportion of gray balance points, such as C5%-6%, M3%-4%, and Y3%-4%. Such as white warm, three-color version of the dot value can also be close to the ratio, but must not lack a color version, otherwise it will be color cast, dew.

Reprinted from: Chen Yi