How to correct poor quality images in pre-press production

In the field of books, periodicals and packaging printing, color printing has become the mainstream of printing, and the quality of printed matter is also increasing. However, due to the influence of subjective and objective factors, not many of them have reached the quality level, and they still cannot be compared with foreign high-quality products. We know that the quality of color prints is restricted by many factors in the copying process, such as the gradation level of the color original image, the dot expansion value, and the color balance. Among them, the quality of the pre-press image production directly determines the color print Although the quality can be partially corrected in plate-making and printing, it is basically impossible to print quality products. Therefore, when creating images before printing, if you find that the quality of the original is not enough, you should try to correct it. Below, a brief introduction to some processing techniques and methods.

1. Requirements for manuscripts

(1) Original density

The difference between the maximum and minimum density in the original (that is, the contrast), the maximum density value that can be achieved in ordinary prints today is 1.8, the image of the printing paper can reach 1.7, and the black ink density of the trimmed original is 1.8. In general, all the density of the original can only be reproduced in the range of 0 to 1.8 on white paper. Therefore, for the original, there is a density range suitable for plate-making printing. When the density range of the original is too large, the sensitivity of the scanner and the electrical extension unit to the part that exceeds the density range decreases, so the level of the color separation plate is relatively flat. According to practice, the manuscript is 0.3 to 2.1, which is the most suitable when the contrast is 1.8. The density difference of color negative manuscripts should be controlled within 2.4; if the original contrast is less than 2.5, reasonable compression will be achieved when copying, and the effect will be ideal. Poor.

(2) Color cast of original

Generally divided into overall color cast, low-key cast color, high-key cast color and high-low cast color (ie cross color cast). Therefore, when correcting, it is necessary to look at the overall consideration. If you cannot see a color cast, immediately process a color, otherwise it will cause other color casts.

(3) Original level

There are three main indicators to measure the quality of replicas: layer, color and clarity, and layer is the most important. The level of the normal manuscript should have the characteristics of no bright or dark picture, high, medium and low key, many levels of density change and rich tone.

2. Manuscript color problems and processing methods

The true condition of the original color tone can be directly observed from the measurement of the original three-color density contrast and the drawing of the three-color density curve. Color printing and copying decomposes the density range of the original and the replica into four segments, that is, divides the density range between highlight and dark points into bright tones, middle tones, and dark tones, and the area less than the density of highlight points is called extreme highlight.

In the analysis of the color quality of the original, if the three-color density is basically the same as the contrast, it means that it is not color-shifted and the color tone is basically normal; if the tone is complete, the level is rich, and the distribution is reasonable, even if the total contrast is slightly different, it can be regarded as a normal original. The color reproduction can be corrected according to the normal color error data, and the emphasis is on the reproduction of tone levels.

If the three-color density is different, it means that it is biased to a certain color, but the contrast of the three-color density is basically equal, which is a balanced color cast. This color cast is best based on the three-color density curve to design a gradation replication curve that adjusts the three-color gray balance, so that the full-tone tones are uniformly corrected.

If there is a large gap between the light three-color density of the original, the three-color density at the dark end is close to the same, and the color cast is mainly manifested at the end of the high-tone area, which is not conducive to the high-level gradation reproduction of the image. It is better to design a three-color version of the layered copy curve based on the three-color density curve derivation, change the record setting of the highlight three-color dots of the color separation picture, and accurately correct the high-key color cast.

If the high-color trichromatic density of the manuscript is basically the same, only the three-color density at the dark end is different, and the density contrast is not equal.

You can change the setting of the dark tone tri-color dot record of the color-separated picture, or remove the full-tone background color of a certain color (only for mixed complex color). However, when dealing with the color cast of the original, it should be noted that some of the artistic effects are intentionally created by the author, and the backlight photo is an example.

In printing, there are generally three different concepts of color reproduction. One is color reproduction in the physical sense, which requires the spectral distribution of each color point to be exactly the same as the original color. The second one is the reproduction in the sense of chromatics, which makes the printed reproduced image consistent with or close to the color point of the original in terms of chromaticity, that is, the effect of different spectrum and same color. The third is the color reproduction in the psychological sense, that is, the color reproduced by printing may have a gap with the original color in chroma, but it may achieve visual psychological satisfaction in color effects.

The basic principle of color reproduction is to restore the gray in the original. To measure the reproduction of gray balance printing, a color density meter can be used to measure the three-color neutral gray ladder attached to the printed matter, or the three-color optical density of the neutral gray level in the measurement screen. If observation is used instead of judgment, a standard neutral gray block should be used to compare with the gray level in the printed picture under the condition that the light source and ambient color temperature are very standard.

Three. The original layer density problem and processing method

The processing methods for manuscripts with different gradation density and quality vary from manuscript to manuscript.

(1) Normal exposure, density contrast standard

The main parts are in bright tone and middle tone, which are proportional to the brightness, and are in the 4-6 level of the new standard system of brightness.

(2) Medium-contrast manuscript

Low density is 0.3 ～ 0.5, high density is 2.4 ～ 2.8, close to the standard manuscript, while compressing the original tone contrast, according to the standard tone gradation reproduction curve to adjust the gradation, can achieve a good copy effect. [next]

(3) High-contrast manuscript

The manuscripts with high optical density no higher than 0.4 and the highest density above 3.0 are mostly the contrast between the original scene and the dark contrast. Generally, the middle tone levels are rich and complete, and the ratio of the brightest to the darkest areas is not large. In this type of manuscript, when highlight and dark tone are selected, the secondary layers at both ends can be discarded to reduce the range of tone reproduction.

(4) Low-contrast manuscript

The tone original with tone contrast below 1.7 and high optical density below 0.3, the main part is brighter and thinner, and the brightness is high. It is in the 7th and 8th level of the new standard brightness system. The brightness tone is rich, but the density difference is very It is small and has a large area ratio. When making, it is necessary to deepen the copy to reduce the brightness and move closer to the standard brightness. For this type of manuscript, it is necessary to strengthen the redistribution and redistribution of the gradation of the bright and middle tone. The black and white field calibration density value is smaller. The gradation curve of the bright and middle tone should be slightly deepened to achieve better gradation contrast and visual brightness effect. According to the shape of its layered distribution curve, it should be distinguished whether the main tone is in the bright tone or the middle tone density range. At the same time, you need to use the darkened and extended black tone to supplement.

(5) Thick manuscript

Thick manuscripts with high optical density above 0.5 and even over 1.0. Mostly due to insufficient exposure or development of the shooting, most of the dark tone levels of the image fall on the toes of the negative characteristic curve and the lower half of the straight line. The main part of this type of manuscript is darker and deeper, which is at the 2nd and 3rd levels of the new standard system of lightness. There are few levels of bright tone, rich levels of medium and dark tones and soft level differences, and the area ratio is large. Within the range of mid-density density, the production should be lightened to increase the copy to enhance the brightness and approach the standard brightness. The black and white field calibration density value is larger, the gradation curve of the middle and dark tones should be slightly lightened, and the gradation of the middle tones including the main tone should be brighter reproduced to achieve better tone contrast And visual brightness effects.

(6) Manuscripts with different mid-level distribution

Some medium-to-high density contrast manuscripts have fewer mid-tone levels and a larger density level difference, while the high and low-key levels are more and smooth. For example, color films shot in backlight should be made with a method to coordinate the level of the main tone of such images. Can get better copy reproduction effect.

The size of the image layer contrast can be adjusted for the density range compression when copying. The initial density can also be changed by adjusting the high optical density setting and the recording setting. The contrast range of transmission manuscripts is very different, but color prints are to be reproduced as a picture with a substantially uniform density range. This density range is mostly lower than that of color manuscripts. When color separation is made, it must be compressed and adjusted.

The density range of the printed matter is large, which can not only reproduce more levels of the original manuscript or the original scene, but also make the contrast between the levels of the various levels open, and the three-dimensional sense is strong. In particular, it can enhance the contrast of subtle levels and enhance the clarity of image layers. On the other hand, it also makes the color of the picture open, and the deeper ones are thicker and the lighter ones are brighter. However, the increase in the density of the printing ink layer contradicts the increase in the dots, and it is impossible to achieve the high density of the color transparent film. Lower dot gain value and higher ink layer density. The objective technical standard for determining this quality index is to use a color reflection densitometer to measure the printing solid density and 75% dot printing density of each printing ink layer, and calculate the printing K value. When this value is high (yellow version 0.3-0.4, magenta version and cyan version 0.4-0.55, black version 0.5-0.6), their respective solid density is higher, and the overprinting efficiency of each color ink layer is close to 90%, The tone of the printed matter is better. In order to save ink, some people advocate reducing the density of the printing ink layer to reduce the dot increase, but this will reduce the gradation of the dark end layer of the image. The overall tone contrast is not the highest, nor can it reach the best printing level. Tone level reproduction.

Since the tone of the printing process is non-linear transmission, compensation adjustment should be made before printing. In order to be suitable for the printing process, it is necessary to adjust the three dot jumping areas of printing, namely 3%, 50% and 80% dot areas. Because 3% of small dots are easy to lose, it will affect the color level of the highlight area, and 50% of the dot area is easy to increase, which will affect the middle tone, especially the change of the skin color of the character. The 80% dot area is easy to merge, which will affect the dark tone level, especially the combination of the main color of the dark tone skin color and the depth of the primary color, making it difficult to print both. Therefore, the important colors and important parts of the original should be adjusted to avoid these three dot jumping areas.

Four. Manuscript clarity problems and processing methods

(1) The solidity of the image layer outline

First of all, we must have the solidity (ie sharpness) of the original layer boundary as the basis. In the entire process of copying and reproduction, the transition width of the image layer boundary should be gradually widened, especially enlarged and copied. The solidity of the level boundary can no longer be restored. The clarity of the gradation boundary of the printed matter only seems to be emphasized by color separation and reproduction.

(2) The clarity of the contrast change between the two adjacent levels of the image

If the density difference between adjacent levels is large, the visual experience is clear. The contrast of the gradation boundary can be emphasized during color separation to create an illusion of improvement, while the subtle contrast can be substantially strengthened.

(3) The resolution of the original or printed screen level

The so-called texture. The printed matter is based on the fine-grained resolution of the original image, but is affected by the resolution of the plate-making optics, magnification, screen thickness, and four-color plate fit. After the plate-making printing process, the fineness of the original layer cannot be restored.

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