In the field of packaging and printing, color printing of plates has become the mainstream of printing. The quality of printed matter is also increasing. A beautifully printed picture will give people a feeling of comfort and naturalness, making people feel like they are in the real world. However, due to the influence of subjective and objective factors, although the quality of color printing products has been greatly improved compared with the past, in general, there are not many quality grades, and it is not comparable with foreign quality products. There are objective factors such as equipment and materials, but the quality of the operators and technical deficiencies are also important factors. We know that the quality of color printing products is subject to all factors involved in the reproduction process, such as the tone level of color original images, color tone and definition quality; color separation screen tone level, color and definition Emphasize the accuracy of the data such as copying adjustment; the amount of change in the dot density of the intermediate process such as copying and printing; and the degree of stability; the density of the ink layer printed, (or) proofed, the dot gain value or the printed relative contrast data and its control stability. Overprinting efficiency, tone evolution, color balance and gray balance reproduction of each color ink layer; performance suitability and operational suitability of printing paper, chromaticity characteristics of ink and its suitability; equipment, equipment and materials for each operation process Printability, process quality, and copying object adjustment data.
Among the above influencing factors, the quality of prepress image production directly determines the quality of color printing products. Therefore, when making an image before printing, if the quality of the original is found to be bad, it should be tried and corrected to ensure the high quality of the printed matter. Since most of the manuscripts currently produced are insufficiency, prepress producers do not have a high level of skill and have no good artistic aesthetics. It is very difficult to produce high quality products. A brief introduction to some techniques and methods for processing defects in the quality of the original in prepress image production.
Handling method of manuscript tone level problem
Reflexes such as color drawings, prints, and photographs have similar gradation density contrasts. The overall density of the color transmission is quite different from the horizontal contrast. In general, the density contrast is between 1.8 and 2.4, which is a medium-original manuscript, with a high-contrast manuscript of more than 2.4 and a low-contrast manuscript of less than 1.8. The initial density is 0.3-0.5 as the normal draft; above 0.5, regardless of the contrast size, it is caused by insufficient exposure (or development), which is a boring original; the contrast is lower than 1.6, and the initial density is lower than 0.3, which is caused by excessive exposure (or development), is a thin original.
The overall density and contrast of the color transmission draft are closely related to the illumination conditions, exposure amount and development processing of the original image. The position of the image on the photosensitive characteristic curve is different, and the contrast difference of each layer is also changed. The distribution of its levels varies. For the photographic art manuscript, the aesthetic adjustment is mainly based on the impression color and visual effect, emphasizing the contrast effect of the picture, highlighting the light sensation, emphasizing the color saturation, exaggerating and rendering the important colors in the picture, highlighting the color visual impact. It is necessary to emphasize some of the deficiencies in the process of shooting and rinsing, adjust and handle the color, light, shadow, light and warm, and spatial discoloration in the art of photography, and highlight the color, atmosphere, light and shadow, texture and other styling effects of photographic pictures. Make the original manuscript have a strong sense of light, high brightness, dark tone and intensity, giving people the feeling of light and shadow; strong color saturation, bright color, rich color change, giving people the power of color art; very high Clearness, clear sense of matter, fine particles, and subtle layers give a sense of realism. In actual production, since the darkness of the photographic picture is easy to be color cast, the object black is not pure. Therefore, the UCR/UCA function is applied to correct the deviation, so that the three primary color versions achieve the effect of neutral black, and then the black version is mainly used, thereby achieving the black pure effect and better expressing the texture level of the black object.
It is impossible to faithfully copy the original, because most of the original density contrast is greater than the density contrast of the printed matter, so the copy of the print must be compressed. Therefore, it is necessary to lose the subtle level of the part. In the actual operation, the reasonable level curve should be adjusted according to the contrast of the original and the specific content of the tone, that is, the level of the secondary part on the reflective draft or the transmissive draft. Compression, focusing on the main parts of the hierarchy, highlighting the characteristics. [next]
Under the premise of the normal contrast of the color draft, the gradation curve is correctly adjusted according to different types of originals. But be sure to pay attention to the balance of the color separation curve, the neutral gray balance should be appropriate, the incorrect level curve will make the replica distortion. For the entire color cast of the original, it can generally be corrected by adjusting the gradation curve. For originals with different gradation densities, when compressing the original gradation contrast, the following methods are often used.
(1) Normal exposure, density contrast standard. The main part is in bright and medium tone. It is directly proportional to the brightness. It belongs to the best visual brightness range. It is in the 4~6 level of the new brightness system. The black and white field calibration of this kind is set according to the standard density value. If it is fixed, all the information on the original and the middle of the original can be reproduced on the scanning color separation sheet. The normal manuscript, the contrast is moderate, should make the low-key level flattened, especially when it is necessary to emphasize the middle tone and the brightness level.
(2) Medium contrast originals. The low density is 0.3~0.5, the high density is 2.4~2.8, the contrast is often, close to the standard manuscript, the high, medium and low level of the image can be expressed in the linear part of the photosensitive characteristic curve, the middle adjustment level is rich, the main level is bright in the middle. The tone density level difference is proportional to the original scene, and the color tone is generally normal. At the same time, the original tone is contrast-compressed, and the curve is adjusted according to the standard tone level reproduction curve, which can achieve a good copying effect. Since the original tone level distribution state is different, and the same is the middle contrast original, the tone level reproduction curve should also be different. If it is a clear original, the subject is adjusted within the range of light and middle, and the copy should be slightly deepened to satisfy the visual response. If the original is thick, the main body tone is usually within the medium density range and should not be deepened. The reflective draft (except photo) hierarchical rendering curve can be between the two. For the original contrast is basically suitable for printing and reproduction, the photo with the middle tone as the main, the dark adjustment area is small, and the dark tone level can be compressed to emphasize the middle adjustment level.
(3) High contrast originals. The high optical density is not higher than 0.4, and the highest density is more than 3.0. Most of the originals are too bright and contrast contrast. Generally, the intermediate level is rich and complete, and the ratio of the brightest and darkest areas is not large, generally not The main level is only the high and low-key density jump, which increases the contrast. Such manuscripts, when selected for highlights and shadows, generally have the secondary levels at both ends reduced to reduce the range of their tone reproduction. The layer replicates the reproduction curve, which can be adjusted according to the image hierarchy curve.
(4) Low contrast originals. The tone contrast is below 1.7, and the low-density manuscript with a high optical density of 0.3 or less is mostly caused by overexposure or overexposure. The main part is bright and thin, and the brightness is high. It is in the 7th and 8th grades of the new brightness system. Most of the light and middle adjustments fall on the shoulder of the negative characteristic curve. The brightness level is rich, but the density difference is very small. And occupy a large proportion of the area. Deepen the copying during production to reduce the brightness and approach the standard brightness. For such manuscripts, it is necessary to strengthen the horizontal and medium-level copying and redistribution. The black and white field calibration density value should be smaller, and the bright and medium-level hierarchical curves should be slightly deepened to achieve better horizontal contrast and visual brightness effects. According to the shape of the hierarchical distribution curve, the main body should be adjusted in the range of bright or medium density. At the same time, it is necessary to use the deepening and lengthening black tone to supplement, so that the intermediate level of the reproduction and reproduction picture is deeper than the hierarchical distribution of the original, satisfying the artistic requirements of the visual psychology for the picture. If you encounter a thin color photo original, the contrast is small, you can make the contrast of the middle adjustment level larger by compressing the light and dark levels.
(5) Thick and boring manuscript. Thick and boring originals with a high optical density of 0.5 or more and even more than 1.0 are mostly caused by insufficient exposure or development, so that the dark level of the image mostly falls on the toe and the lower half of the negative characteristic curve. The contrast of the original is thick and boring. The whole picture has a large low-key area and the brightness is flat. If the emphasis is on the dark and middle tone levels, the middle adjustment level is flat. For the manuscript intermediate level is relatively flat, the contrast is small, you can compress the bright level, so that the middle adjustment level contrast is larger.
The main part of the thick manuscript is dark and dark, and it is in the 2nd and 3rd levels of the new standard system of brightness. The level of brightness is small, the level of middle and dark is rich and the level is soft, and the area ratio is very large. In the middle density range, the production should be lightened to improve the reproduction to brighten the brightness and approach the standard brightness. For the reproduction of such manuscripts, the black-and-white field calibration density value should be larger, and the medium-and-dark tone level curve should be slightly reduced. It should be reproduced in a brighter tone including the main body tone. The picture of the reproduction is made clear, and the layer secondary difference of the middle and the dark tone is also emphasized to achieve a better tone contrast and visual brightness effect.
(6) Originals with different levels of intermediate adjustment. Some medium-and high-density contrast originals have fewer mid-levels and larger density differences, while higher and lower-key levels are more and more flat. For example, color films taken in backlighting should be designed to make the horizontal and dark coordination of such image subjects. A better copy reproduction effect can be obtained. [next]
The contrast of the image gradation level can be adjusted for the density range during copying. The starting density can also be changed by the high optical density setting and the recording setting adjustment. However, whether the distribution state of the original image level can meet the artistic requirements, it is necessary to make the hierarchical contrast distribution compression and redistribute the distribution. The depiction and analysis of the distribution of manuscripts is crucial to the redistribution of hierarchical reproduction and is the primary basis for hierarchical reproduction. When the print is restored, the tone is usually adjusted. When the density contrast of the original is greater than the highest recording density of the printed matter, in order to make the original tone of the original appear on the printed matter, it is necessary to restore the tone. Appropriate adjustments. Secondly, in order to adapt to the copying requirements of some special manuscripts (such as bright-based photos, dark-key photos), certain tones of the original must be appropriately emphasized, and other tones must be compressed accordingly. The main feature of linear compression is the equal scaling of the various grading segments. Linear compression does not represent true visual proportional compression, because the D-Y curve is also non-linear. After this compression, the bright focus is on the human eye. It is not easy to distinguish, resulting in a "grey level" phenomenon. The reproduction of the tone refers to the relationship between the original density and the percentage of duplicate dots. To make all the tones and colors on the original appear on the printed matter, first select the correct level curve. The level is the basis of the tone and color change. If the level of the picture is not rich, the performance of the color must be monotonous and boring. Only when the level is realistic, can the vivid color change.
The difference of the contrast range of the natural color positive film of the transmission draft is relatively large (about 3.0). Under normal circumstances, the compression contrast is required to achieve the requirement of copying the printed matter; while the contrast of the original of the reflective manuscript is small, the original is boring, and can be different according to different The original features are brightly copied. For example, the copying point of a portrait is the color of the character, and the two colors of M and Y are the key; the focus of the landscape is the natural scenery, and the two colors of C and Y are the main points. Flexible use of the level knob, adjust out a reasonable level curve, correct the deficiencies of the original, maintain the main part, the corresponding loss of the secondary part, boldly choose, try to replicate the better results. However, if the adjustment of the level curve is unreasonable, it may cause the image to be reversed first, the level is unclear, and the hue is inaccurate. Since the color prints are reproduced as a picture having a substantially uniform density range, the density range is much lower than the density range of the color original, and compression adjustment is inevitable in the color separation plate making. What kind of redistribution adjustment is adopted for each tone level depends on the hierarchical distribution of the manuscript and the visual perception of the person. Among them, there are both the Mengsel brightness value factor of visual response and the subjective factor of people's visual psychological requirements. To copy and adjust the manuscript gradation level, it is necessary to comprehensively summarize the visual psychological needs of people, add the physical quantity of the visual response, and then combine the state of the original layer distribution to consider the density gradation level reproduction curve of the printed picture. The evolution data in the process of copying and reproducing, and designing the tone layer copy curve of the specific manuscript.
The density of printed matter is large, which can reproduce some originals or the level of the original scene, and also make the contrast between the levels of each level open, and the stereoscopic effect is strong. In particular, it enhances the subtle contrast and enhances the sharpness of the image hierarchy. On the other hand, the color of the picture is opened, which makes the deeper saturated and shallow, and enhances the three-dimensional sense of color. However, the increase in the density of the printing ink layer is inconsistent with the increase in the dot size, and it is impossible to achieve a high density as a color transparent sheet. Only under the excellent conditions of paper, ink, equipment adaptability and printing machine precision, can we have lower dot gain and higher ink layer density.
Therefore, the objective technical standard for determining this quality index is to use a color reflection densitometer to measure the printed solid density of each printing ink layer and the printing density of 75% dot printing, and calculate the printing K value. When it has a high printing K value (yellow plate 0.3-0.4, magenta plate and cyan plate 0.4-0.55, black plate 0.5-0.6), its respective solid density is higher, and the overprinting efficiency of each color ink layer is close to 90. When % is used, the tone of the print is better. Such prints must be clearly structured, thick and full, and have a strong three-dimensional image. In order to save the printing ink, it is advocated to reduce the density of the printing ink layer to reduce the dot gain. Thus, although the printing dot dot tone is lessened, the leveling phenomenon of the image darkening end layer is reduced, but the overall tone contrast is not the highest. It also does not achieve the best reproduction of the print tone level.
Since the printing process tone is a nonlinear transfer, compensation adjustment should be made before printing. The dot tone is limited by various conditions during the transfer process of printing and printing processes, and the transmission of the tone dots is nonlinear. In order to adapt to the printing process, compensation adjustments must be made. The key point is to adjust the three dot jump areas of the printing, also called the three key areas, namely 3%, 50%, and 80%. Because 3% small points are easy to lose, it will affect the color level of the highlight area. The 50% dot area is easy to increase, which will affect the middle tone. The color of the special character changes. 80% of the dot area is easy to level, which will affect the dark level. It is the level of the main color of the dark-toned skin color and the depth of the primary color, making it difficult to take care of both, and often feel at a loss. Therefore, the important colors and important parts of some originals are adjusted to avoid the three dot jump areas, thereby providing an ideal printing plate for printing, making it easy to print and printing good products. [next]
How to deal with the problem of original sharpness
The sharpness of color prints is an important quality indicator for image reproduction. Except for the special artistic conception of expressing images, at least a part of the level (subject or background) of each picture is clear. The requirements for clarity mainly include the following aspects.
(1) The realness of the image hierarchy outline. First, there must be a solidity (ie, sharpness) of the boundary of the original document as the basis. At the same time, it depends on the size of the aberration of the plate-making optics, the thickness of the electronic color separation analysis and the recording scan point, and the accuracy of the four-color image fit. In the whole process of copy reproduction, the transition width of the image hierarchy boundary is to be gradually widened, especially to enlarge the copy. The level of boundary solidity cannot be restored. The sharpness of the boundaries of the prints is only apparently highlighted by the separation of the separations.
(2) The clarity of the contrast between the two adjacent levels of the image, that is, the slight contrast. If the density difference between adjacent layers is large, the visual perception is clear. Conversely, if the density difference is small, the level is flat and the vision is not easy to distinguish. The fine-level contrast is mainly determined by the secondary difference of the original layer, and is also restricted by the copying and grading adjustment of the tone level. It is possible to carry out hierarchical boundary contrast emphasizing in the case of color separation, and to increase the illusion of visual contrast of the hierarchical boundary, and the contrast of the subtle level can be substantially strengthened.
(3) The resolution of the original or printed picture hierarchy, that is, the subtle level of its subtle level, is the essence of the material that represents the objective scene, that is, the so-called texture. The print is based on the subtle resolution of the original image, but it is compromised by the resolution, magnification, thickness of the wire and the four-color version of the plate. Through the plate-making process, the subtlety of the original level cannot be restored. In the color separation plate making, it is necessary to adopt measures such as virtual mask and electronic effect to add "edges" to the image boundary, thereby emphasizing the visual clarity of image layer reproduction. The "edges" added to the boundary of the copied image, including the two visual effects of its width and peak height. The peak positive and negative density is nonlinearly transmitted to the vision, and only enhances the Mach band effect of the vision, but is not easily visually decomposed. The width of the "trimming" is linearly transmitted to the vision. Too large (especially on the bright side of the boundary) is clearly distinguished by the visual, resulting in a sturdy embossing; too small, which is not conducive to the visual Mach band effect. Only when the "edge" width and the peak height are properly matched can the visual Mach band effect be clearly and unresolved, and at the same time enhance the visual illusion of subtle horizontal contrast and subtle horizontal resolution. [next]
Therefore, the effect of the visual Mach band effect produced by the “edge decoration†of the hierarchical boundary is the standard for measuring the quality of the hierarchical resolution. Because of the visual Mach band effect and resolution, it is based on a certain amount of visual physiological and physical quantities, and is basically universal. Therefore, it is a direct and simple objective evaluation by visually comparing the contrast between the printed image and the original, and making a clear copy of the hierarchy. For this observation, the printed matter should be placed under suitable illumination and at an appropriate viewing distance. The fineness, graininess, moiré, spots and unevenness of the printed image will affect the normal reproduction of the image clarity, and it can be directly visually and intuitively perceived. The fineness and granularity of the flat clothes are mainly determined by the fineness of the original document, and also by the magnification and the slight emphasis on the initial action value (adjustment of the particle strength). A slight emphasis is placed on the proper adjustment of the starting value, and the granularity can be improved, but it affects the emphasis of the level of clarity. Manual revision can also reduce the graininess and increase the smoothness of the flat clothes, but it is easy to leave traces of manual repair, and generally will not get perfect results. The dot shape, dot uniformity and print dot quality of the screen image affect the flatness of the printed image. Poor quality or unevenness of the printed dots will destroy the smoothness of the flat clothes. The moiré is caused by the incorrect angle of the cable. For the magenta version of the character picture, the green version of the landscape picture, and the black version of the Chinese picture, a 45° cable angle is used. These surface qualities can be verified by scanning density amplification or microdensity measurement of their reflection density, setting objective technical measurement standards. However, it is generally objective and simple to use visuals for visual evaluation.