In the field of packaging and printing, plate color printing has become the mainstream of printing. The quality of prints is also increasing. A picture with beautiful printing quality will give people a comfortable and natural feeling, making people feel like being there. 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, but overall, there are not many high-quality products, and they can not be compared with foreign high-quality products. There are objective factors such as equipment and materials, but the quality and technical deficiencies of the operating personnel are also important factors. We know that the quality of color printing products is restricted by all the factors involved in the copying process, for example, the tone level, color tone and clarity quality of the color manuscript image; the tone level, color and clarity of the color-separated screen image Emphasize the accuracy of the data such as copy and adjustment; the amount of change and stability of the dot conversion in intermediate processes such as copying and printing; the density of the ink layer, the expansion value of the dot or the relative contrast value of the printing (or) proof and the stability of the control Overprint efficiency, tone evolution, color balance and gray balance reproduction of various ink layers; performance suitability and operation suitability of printing paper, chromaticity characteristics and operation suitability of ink; equipment, equipment and materials for each operation process Printability, quality of process operation and adjustment data of copied objects, etc.
Among the above influencing factors, the quality of pre-press image production directly determines the quality of color printing products. Therefore, when creating images before printing, if you find that the quality of the original is not good, you should make every effort to correct and improve it to ensure the high quality of the printed matter. Most of the manuscripts provided for printing today have some shortcomings. If the pre-press production personnel do not have a high technical level and a good artistic aesthetics, it is difficult to produce high-quality products. Briefly introduce some techniques and methods to deal with the original quality defects in prepress image production.
How to deal with the gradation problem of manuscript
Reflective manuscripts such as color drawings, prints, and photographs have similar gradation density contrasts. However, the contrast between the overall density and the gradation of the color transmission manuscript is relatively large. Generally speaking, the density contrast is between 1.8 and 2.4, which is a medium-normal manuscript, greater than 2.4 is a high-contrast manuscript, and less than 1.8 is a low-contrast manuscript. The initial density is from 0.3 to 0.5 as a normal manuscript; if it is higher than 0.5, regardless of the contrast size, it is caused by insufficient exposure (or development), which is a dull manuscript; the contrast is lower than 1.6, and the initial density is lower than 0.3 is caused by overexposure (or development) and is a thin manuscript.
The overall density and contrast of the color transmission manuscript are closely related to the lighting conditions, exposure and development processing of the original manuscript. The position of the image on the photosensitive characteristic curve is different, and the contrast of the density of each layer also changes. The distribution of its levels varies. For photographic art manuscripts, the aesthetic adjustment is mainly based on the impression color and visual effects, emphasizing the contrast effect of the picture, highlighting the sense of light, emphasizing the color saturation, and exaggerating and rendering the important colors in the picture to highlight the color visual impact. It is necessary to emphasize to make up for some shortcomings in the process of shooting and rinsing, adjust and handle the color light and shadow, light and shade, cold and warm, space discoloration in the art of photography, and highlight the styling effects of the color, atmosphere, light and shadow, texture and other photographic pictures. Make the original production with a strong sense of light, bright highlights, strong dark tone, giving people the beauty of light and shadow; strong color saturation, bright color, rich color changes, giving people the artistic appeal of color; very high The clarity, the sense of scenery material are clear, the particles are delicate, and the subtle layers are rich, giving people a sense of authenticity. In actual production, the dark tones of the photographic pictures are easy to shift color, which results in black objects that are not pure. Therefore, the UCR / UCA function is used to correct the deviation, so that the three-primary color version achieves the effect of neutral black, and then the black version is mainly used, so as to achieve the pure black effect, and better show the texture level of black objects.
It is impossible to copy the originals faithfully, because the density contrast of most of the originals is greater than the density contrast of the printed products, so the copy must compress the density. Therefore, it is necessary to lose some of the subtle levels. In 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 parts on the reflective or transmissive manuscript should be adjusted. Compression is carried out so that the levels of main parts are emphasized and characterized. [next]
Under the premise of normal contrast of color manuscripts, adjust the gradation curve correctly according to different types of manuscripts. But we must pay attention to the balance of the gradation curve of the color separation film. The neutral gray balance should be appropriate. The incorrect gradation curve will distort the replica. The color cast of the original can be corrected by adjusting the gradation curve. For manuscripts with different gradation density and quality, when compressing the tone contrast of the manuscript, the following methods are often adopted.
(1) Normal exposure and standard density contrast. The main part is in the bright tone and middle tone, which is proportional to the brightness. It belongs to the best visual brightness range and is in the 4-6 level of the new standard brightness system. This type of original black and white field calibration is set according to the standard density value. If it is fixed, all the information of bright and middle tone on the original can be reproduced and appeared on the scanned color separation film. Normal manuscripts with moderate contrast should compress the low-key gradations flat, especially when it is necessary to emphasize the mid-tone and bright-tone gradations.
(2) Medium contrast originals. Low density is 0.3 ～ 0.5, high density is 2.4 ～ 2.8, contrast is normal, close to standard manuscript, high and low to medium gradation of the image can be shown in the linear part of the photosensitive characteristic curve, rich midtone gradation, main body gradation is bright Tone, the difference in gradation density level is proportional to the original scene, and the color tone is generally more normal. While compressing the contrast of the original tone, adjusting the gradation according to the gradation reproduction curve close to the standard gradation can achieve a good reproduction effect. Due to the different distribution states of the tone levels of the originals, which are also medium-contrast originals, the reproduction curves of the tone levels should also be different. If it is a clear manuscript, the main tone is within the range of bright and middle tone, and the copy should be slightly deepened to meet the visual response. If it is a thick original, the main tone is usually within the mid-range density range, and it should not be copied deeper. The gradation reproduction curve of reflective drafts (except photos) can be between the above two. The contrast of the original is basically suitable for printing and copying. The photos with mainly midtones have a small area of ​​dark tones. The dark tone levels can be compressed to emphasize the middle tone levels.
(3) High-contrast originals. The manuscripts with high optical density not 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 intermediate tone levels are rich and complete. The ratio of the brightest to the darkest area is not large, nor is it generally. The main level is just a sudden jump in high and low-density densities, increasing the contrast. 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. Its level reproduction curve can be adjusted and changed according to the image level distribution curve.
(4) Low-contrast originals. The tone contrast is below 1.7, and the light density is less than 0.3. The light-colored originals are mostly caused by overexposure or excessive development. The main part is brighter and thinner, and the brightness is high. It is at the 7th and 8th levels of the new standard brightness system. Most of the mid-range brightness falls on the shoulder of the negative characteristic curve. The brightness level is rich, but the density difference is very small. , And accounts for a large proportion of the area. When making, it is necessary to deepen the copy to reduce the brightness and move closer to the standard brightness. This type of manuscript needs 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, and 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, it is also necessary to use deepened and lengthened black tone to supplement, so that the middle tone level of the reproduction screen is deeper than the original level distribution, to meet the artistic requirements of visual psychology on the picture. If the color photo original is thin and the contrast is small, you can compress the bright and dark tone levels to make the contrast of the middle tone level larger.
(5) Thick manuscript. Thick manuscripts with high optical density above 0.5, or even over 1.0, are mostly caused by insufficient exposure or development, so that the dark tones of the image mostly fall on the toes of the negative characteristic curve and the lower half of the straight line. The contrast of the original is thick and dull. The entire screen is large in low-key areas, and the bright tones appear flat. If the emphasis is on the dark tones and mid-tone levels, the mid-tone levels will appear dull. For the middle tone level of the original is relatively flat, the contrast is small, and the bright tone level can be compressed to make the contrast of the middle tone level larger.
The main part of the thick manuscript is darker and deeper, which is in the second and third levels of the new standard system of lightness. There are few levels of bright tone, rich levels of mid-dark tone and soft level difference, 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. For copying of this type of original, the black and white field calibration density value is larger, and 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. It makes the reproduced picture clear, and also emphasizes the level difference of medium and dark tones to achieve better tone contrast and visual brightness effect.
(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. [next]
The size of the image tone level contrast can be compressed and adjusted in the density range during copying. The initial density can also be changed by adjusting the high optical density setting and the recording setting. And whether the distribution state of the original image layer can meet the artistic requirements, it is necessary to make redistribution adjustment of the layer distribution while compressing the layer contrast in the copy. The description and analysis of the hierarchical distribution of manuscripts is crucial to the redistribution adjustment of hierarchical replication, and is the primary basis for hierarchical replication. When the tone of the printed matter is restored, it is generally necessary to adjust the tone. When the density contrast of the original is greater than the highest recording density of the printed matter, in order to make each tone of the original appear on the printed matter, it is necessary to adjust the tone. Appropriate adjustment. Secondly, in order to adapt to the copy requirements of some special manuscripts (such as photos with light tones and photos with dark tones), certain tones of the manuscript must be properly emphasized, and other tones must be compressed accordingly. The main feature of linear compression is the adjustment of equal proportions of each order. Linear compression does not represent true visual proportional compression, because the DY curve is also nonlinear. After this compression, the bright tone is in the human eye It is not easy to distinguish, resulting in "grey flat" phenomenon. Tonal duplication refers to the relationship between the density of the original and the percentage of dots copied. To make all the tone and color on the manuscript can be well expressed on the printed matter, first select the correct gradation curve. Hierarchy is the basis of tone and color changes. If the layers on the screen are not rich, the expression of colors must be monotonous and tasteless. Only when the layers are realistic, can vivid color changes.
The contrast range of the natural color positive film of the transmission manuscript is relatively large (about 3.0). Normally, it is necessary to compress the contrast to meet the requirements of printed copy; while the manuscript of the reflective manuscript has a small contrast and the manuscript is boring. The original features are copied brightly. For example, the main point of copying portrait portraits is the skin color of the person, the two colors of M and Y are the key; the focus of copying the landscape picture is the natural scenery, and the two colors of C and Y are the main points. Use the gradation knob flexibly to adjust the reasonable gradation curve, correct the shortcomings of the original, keep the main part, and lose the secondary part accordingly, make bold choices, and try to replicate better results. However, if the adjustment of the gradation curve is unreasonable, it may cause the effect of the picture being reversed, the gradation is unclear, and the hue is inaccurate. Because color prints must be reproduced as a picture with a substantially uniform density range, this density range is mostly lower than the density range of color manuscripts, and compression adjustment must be done during color separation and plate making. What kind of redistribution adjustment is adopted for each tone level depends on the distribution of tone levels of the original manuscript, and it is also related to human visual experience. Among them, there are both the Munsell lightness factor of visual response and the subjective factors of people's visual psychological requirements. For the adjustment of the original tone hierarchy, it is necessary to summarize the visual psychological needs of people, add the physical value of the visual response, and consider the distribution status of the original hierarchy to obtain the density tone hierarchy reproduction curve of the printed picture, and then include The evolution data in the process of copying and reproducing, and designing the gradation replication curve of the specific manuscript.
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 up, making the deeper and more saturated, and the lighter and brighter, enhancing the three-dimensional sense of color. However, the increase in the density of the printing ink layer contradicts the increase in the dots, and it is impossible to achieve the density as high as the color transparent sheet. Only under the excellent conditions of the adaptability of paper, ink, equipment and printing machine precision, can we have a lower dot increase value and a higher ink layer density.
Therefore, 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 the printing K 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 print is better. Such a print must be a layered, thick and full-bodied picture. In order to save printing ink, it is advocated to reduce the density of the printing ink layer to reduce the increase of dots. In this way, although the tone expansion of the printing dots is reduced and the gradation of the dark end of the image is reduced, the overall contrast is not the highest , Also can not achieve the best reproduction of the tone level.
Since the tone of the printing process is non-linear transmission, compensation adjustment should be made before printing. In the transfer process of printing and printing processes, the dot tone is restricted by various conditions, and the transfer of the tone dot is non-linear. In order to adapt to the printing process, compensation adjustments must be made. The key point is to adjust the three dot jumping areas of printing, also known as the three key areas, namely 3%, 50%, and 80% dot areas. Because 3% dots are easy to lose, it will affect the color level of the highlight area, 50% dot area is easy to increase, it will affect the middle tone, especially the change of the skin color of the character, 80% dot area is easy to grade, and will affect the dark tone level It is the combination of the main color of dark skin tone and the depth of the primary color, which makes it difficult for printing to balance both, and often is at a loss. Therefore, the important colors and important parts of some manuscripts can be adjusted to avoid these three dot jump areas, so as to provide the ideal printing plate for printing, make it easy to print, and print a good product. [next]
How to deal with the problem of manuscript clarity
The clarity of color prints is an important quality indicator for image reproduction. Except for the special concept of expressing images, at least a part of the level (subject or background) of each picture must be clear. The requirements for clarity mainly include the following aspects.
(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. At the same time, it also depends on the size of the aberration of the plate-making optics, the thickness of the electronic separation analysis and the recording scanning point, and the accuracy of the four-color image registration. 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 sharpness of the contrast change between the two adjacent levels of the image, that is, the slight contrast. If the density difference between adjacent levels is large, the visual experience is clear. On the contrary, if the density difference is small, the level is flat and the vision is difficult to distinguish. The subtle level contrast is mainly determined by the level difference of the manuscript, and is also restricted by the gradation level copy compression and level adjustment distribution. The contrast of the gradation boundary can be emphasized during color separation and replication, which causes the illusion of visual contrast at the gradation boundary, and the subtle contrast can be substantially strengthened.
(3) The resolution of the level of the original or printed picture, that is, the fineness of its subtle level, is to express the essential appearance of the material constituting the objective scene, that is, the so-called texture. The printed matter is based on the fine-grained resolution of the original image, but it is damaged 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. In the process of color separation and plate making, measures such as virtual light mask and electronic effect should be adopted to add "trimming" to the image layer boundary, so as to emphasize the visual clarity of the image layer reproduction. The "trimming" added to the border of the copied image layer includes two visual effects produced by its width and peak height. The height of the peak positive and negative density is non-linearly transmitted to the vision. It only enhances the Mach band effect of vision, but it is not easy to be decomposed by vision. The width of the "trimming" is linearly transmitted to the vision. Too large (especially on the bright side of the layer boundary) is clearly distinguished by the vision, resulting in a sturdy relief; too small is not conducive to the visual Mach band effect. Only when the width of the "trimming" and the peak height are properly matched, can the obvious and unresolved visual Mach band effect be produced, and at the same time enhance the visual illusion of subtle contrast and subtle resolution. [next]
Therefore, the quality of the visual Mach band produced by the "trimming" of the boundary of the hierarchy is the standard for measuring the emphasis on the quality of the reproduction of the hierarchy. Because the visual Mach band effect and resolution ability are based on a certain amount of visual physiology and physical quantity, they are basically generally consistent. Therefore, by visually observing and comparing the printed pictures and the manuscript, making a judgement of the level of clear reproduction is also a direct and simple objective evaluation. For such observation, the printed matter should be placed under appropriate lighting and at a suitable viewing distance. The fineness, graininess, moire, spots and unevenness of the printed pictures will affect the normal reproduction of the image clarity and can be directly visually perceived. The fineness and granularity of plain clothes are mainly determined by the fineness of the original particles, and are also restricted by the magnification and the initial value of fine emphasis (adjustment of granularity). Finely emphasizing the initial value is adjusted properly, the granularity can be improved, but the emphasis effect on the clarity of the layer is affected. Manual revision can also reduce the granularity and increase the fineness of plain clothes, but it is easy to leave traces of manual repair and generally will not get a perfect effect. The dot shape, dot uniformity and quality of the printed dots of the screen-printed pictures affect the flatness of the printed pictures. The poor quality or unevenness of printed dots will destroy the fineness of plain clothes. The moire pattern is caused by the incorrect angle of the screen. For the magenta version of the character screen, the cyan version of the landscape screen, the black version of the Chinese painting, etc., use a 45 ° screen angle. These surface qualities can be verified by scanning magnification or micro-density measurement of their reflection density to establish objective technical measurement standards. However, visual assessment is generally used, which is both objective and convenient.
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