Prepress quality image production techniques

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 picture with beautiful print quality will give people a comfortable and natural feeling, 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 greatly improved over the past, on the whole, there are not many quality-grade products, and they cannot be compared with foreign quality products. These include objective factors such as equipment and materials, but the quality of operators and technical deficiencies are also important factors. We know that the quality of color printing products is constrained by all the factors involved in the copying process, such as the tone levels, color tone, and sharpness quality of the color original image; the color separation screening picture tone levels, colors, and sharpness Emphasizes the accuracy of the data such as copying and adjustment; the amount of change in the intermediate process dot-changes such as copying and printing, and the degree of stability; the ink layer density, dot gain value, or printing relative contrast data of the printing (or proofing), and the degree of control stability; Overprinting efficiency of each color ink layer, tone evolution, color balance and gray balance reproduction; performance applicability and operation adaptability of printing paper, color characteristics and operational applicability of ink; equipment, equipment and materials for each operation process The printability, the quality of the process operation, and the adjustment of the data for copying the object are many aspects.

Among the above-mentioned influencing factors, the quality of pre-press image production directly determines the quality of color printing products. Therefore, when making images before printing, if the quality of the originals is found to be poor, corrective measures should be taken to improve the quality of the printed matter. As most of the original manuscripts are now available, most of them have some deficiencies. If pre-press production personnel do not have a high level of technology and do not have a good aesthetic aesthetic, it is very difficult to produce high-quality products. Briefly introduce some techniques and methods for processing defects in the quality of originals in prepress image production.

The method of dealing with the problem of the original tone level

Reflective manuscripts, such as color drawings, prints, and photographs, have relatively close contrasts in tone density. However, the overall density and contrast of color-transmitted texts are relatively large. In general, the density contrast is between 1.8 and 2.4, which is the original manuscript. It is higher than 2.4 for the high contrast manuscript and less than 1.8 for the low contrast manuscript. The initial density of 0.3 to 0.5 is the normal draft; those above 0.5, regardless of the size of the contrast, are due to insufficient exposure (or development), and are boring originals; 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 color-transmitting manuscripts are closely related to the illumination conditions of the manuscript, the exposure, and the development process. The segmentation of the image on the photosensitivity curve is different, and the contrast in the density of the layers is also changed. Its level of distribution varies. For the photographic art manuscripts, the aesthetic adjustment is based on impressions and visual effects, emphasizing the contrast effect of the pictures, highlighting the sense of light, emphasizing color saturation, doing some exaggeration and rendering on the important colors in the picture, highlighting the color visual impact. It is necessary to emphasize some of the deficiencies in making up for the shooting and the rinsing process, and to adjust and deal with the relationship between color, light, shadow, light, darkness and space in the photographic art, highlighting the styling effects such as color tone, atmosphere, light and shadow, and texture of photographic images. Make the manuscript has a strong sense of light, bright light, dark tone with intensity, give people the feeling of light and shadow beauty; have a strong color saturation, bright color, rich color changes, give people a color art appeal; have a high Sharpness, scenery, clear material, delicate particles, rich levels, giving a sense of reality. In actual production, the black color of the object is not pure due to the dark color of the photographic image. Therefore, the UCR/UCA function is used to correct the bias, so that the trichromatic version achieves a neutral black effect, and then the black version is dominant, so as to achieve a pure black effect and a better expression of the texture level of the black object.

It is impossible to faithfully copy manuscripts because most of the manuscript density contrast is greater than the density contrast of the printed matter, so the copying of the printed matter must inevitably compress the density. Therefore, it is necessary to lose some of the finer levels. In practice, the reasonable level curve should be adjusted according to the contrast of the original and the specific content of the tone, that is to say, whether it is a reflection draft or the level of the minor part above the transmission document. Compression is made to emphasize the emphasis and specialization of the main site hierarchy.

Under the premise of normal contrast of the color draft, the level curve is correctly adjusted according to different types of originals. However, we must pay attention to the balance of the color separation layer curve, the neutral gray balance should be appropriate, and the incorrect level curve will distort the reproduction. For the entire color cast of the original, it can generally be corrected by adjusting the level curve. For originals with different levels of density and quality, the following methods are often used when compressing the contrast of originals.

(1) Normal exposure and density contrast standards. The main parts are in bright and medium tone, and it is proportional to the brightness. It belongs to the best visual brightness range and is in the 4th to 6th level of the new system of brightness. This kind of manuscript black and white calibration is based on standard density values. As a result, all the information on the original and middle notes of the original can be reproduced on the scanning color separation sheet. Normal manuscripts have moderate contrasts, and they should make the low-profile levels even. In particular, when it is necessary to emphasize the middle tone and the tone level, it is more appropriate.

(2) Medium contrast originals. The low density is 0.3-0.5, the high density is 2.4-2.8, and the contrast is common. Near the standard manuscript, the high and low profile levels of the image can be expressed in the straight part of the photosensitive characteristic curve, with rich layers of intermediate tone, and the main level is in the middle light. Adjustment, leveling density level difference and the original scene is proportional, the tone is generally more normal, while the original contrast tone compression, according to the tone level of the tone reproduction curve close to the standard level adjustment, can achieve a good copy effect. Due to the different distribution of levels of the original tonal hierarchy, the same is the original in contrast, and the reproduction curve of the tone level should also be different. If the manuscript is clear, the subject is adjusted within the bright midtones, and the copy should be slightly deepened to satisfy the visual response. If it is a thick-tune manuscript, the subject tone is often within the medium-density range and should not be deepened. The reflection draft (except for photos) level reproduction curve can be between the above two. For the contrast of the manuscript, which is basically suitable for printing and copying, the photograph with the main tone of the middle tone has a darker tone area, and the tone level can be compressed to emphasize the level of the intermediate tone.

(3) High contrast originals. High optical density of not more than 0.4, but the highest density of more than 3.0 manuscripts, mostly the contrast of the original contrast of light and shade is too large, the general middle tone level rich and complete, the ratio of the brightest and darkest areas is not large, generally not At the main level, only high- and low-key density jumps and increases the contrast. This type of manuscript, when selected for highlights and shadows, can generally be rounded off at both ends of the secondary level, in order to reduce the scope of its tone reproduction. Its level reproduction and reproduction curve can be adjusted according to different levels of the image distribution curve.

(4) Low contrast originals. The gradation contrast is below 1.7, and the low light density original document with a high optical density of 0.3 or less is often caused by overexposure or excessive development. Its main part is bright and thin, with high lightness. It is at the 7th and 8th grades of the new system of lightness. Most of the lighter midtones fall on the shoulder of the negative image characteristic curve. The brightness adjustment level is rich, but the density difference is small. , And a large proportion of the area. Make a deeper copy when making, to reduce the brightness, and approach the standard brightness. This kind of manuscript needs to strengthen the reproduction and redistribution of levels in the bright mid-tones, and the black and white field calibration density values ​​should be smaller. The light curves of the middle and middle tone levels should be slightly deepened to achieve better levels of contrast and visual brightness. The shape of the curve should be distributed according to its level, and its main body should be adjusted to be in bright or medium density range. At the same time, it is also necessary to use deepened and lengthened black edition to complement, so that the intermediate level of reproduction and reproduction is deeper than the level of the manuscript and meets the artistic requirements of visual psychology. If the original color photo is thin, the contrast is small, and the contrast between the middle tone level can be made larger by compressing the light and dark tone levels.

(5) thick stuffy manuscript. High-density light sources with a density of 0.5 or more, or even more than 1.0, are often caused by insufficient exposure or underdeveloped images. Most of the image darkness levels fall on the toes of the negative image characteristic curve and the lower half of the straight line. The manuscript is thick and boring. The whole picture has a large low-key area and the bright tone appears to be flat. If emphasis is placed on the level of tone and intermediate tone, the intermediate tone level will be dull accordingly. For the middle level of the manuscript, the level is relatively flat and the contrast is small. The level of the tone can be compressed and the contrast between the middle tone levels is greater.

The thick main part of the manuscript is darker and darker, and it is located in Levels 2 and 3 of the Shinsei Shinbiao System. There are few bright tone levels, and the dark tone level is rich and the level difference is soft, and the area ratio is large. In the mid-range density range, the reproduction should be reduced by increasing the lightness in order to brighten the brightness and approach the standard lightness. For copying of this type of manuscript, the density of the black and white field calibration standard should be larger, and the level curve of the medium and dark tone should be slightly reduced, and the level of the medium brightness tone including the subject tone should be reproduced more brightly. Make the reproduction of the picture clear, but also stressed the dark tone of the secondary differences, in order to achieve better tone contrast and visual brightness effect.

(6) Different manuscripts are distributed at the midtone level. Some manuscripts with medium-to-dense contrasts have fewer intermediate tone levels, a greater density difference, and high and low tone levels, such as color films shot against the light, should be designed to adjust the level of darkness of this type of image subject. Good copy reproduction effect can be achieved.

The level of image contrast can be used as a density range compression adjustment when copying. The height of the initial density can also be changed by setting the high optical density and adjusting the recording settings. However, whether the distribution of the original image hierarchy can meet the artistic requirements requires that the level distribution be compressed and the redistribution adjustment should be made. The depiction and analysis of manuscript level distribution is crucial to the redistribution adjustment of level reproduction and is the primary basis for level reproduction. When the printed matter is restored, the adjustment of the tone is generally required. When the density contrast of the original document is greater than the highest recording density of the printed matter, in order for the various tonality of the original document to be expressed in the printed matter, the tone must be restored. Appropriate adjustments. Second, in order to adapt to the reproduction requirements of certain special manuscripts (for example, photographs that are primarily highlighted, and photos that are primarily dark), some of the manuscripts must be properly emphasized, and other adjustments must be made. The main feature of linear compression is the adjustment of equal proportions of each tone segment. Linear compression does not represent true visually proportional compression because the D-Y curve is also nonlinear. After this compression, the luminance is adjusted to the human eye. It is not easy to identify and produce a "gray" phenomenon. The reproduction of tone refers to the relationship between the density of the original and the percentage of copying points. To make all the tone and color on the manuscript well represented in the printed matter, first select the correct level curve. The level is the basis of tone and color change. If the level is not rich on the screen, the color performance must be monotonous and boring, and only the realistic level can have vivid color changes.

The difference in the contrast range of the natural color film of the transmitted manuscript is relatively large (about 3.0 or so). Usually, the contrast is required to meet the requirements for the reproduction of the printed matter; while the manuscript of the reflected manuscript has less contrast, the manuscript is boring and can be based on different The original features are brightly copied. For example, the copy point of the portrait of a person is the color of the person's face, and the two colors M and Y are the key; the reproduction of the landscape picture is the natural scenery, and the two colors of C and Y are the main points. Flexible use of level knobs to adjust out a reasonable level of the curve, to correct the deficiencies of the original, to maintain the main part, the corresponding loss of the secondary part, boldly choose, try to make a good copy. However, if the adjustment of the level curve is unreasonable, it may result in the inversion of the primary and secondary images, the unclear level, and the inaccurate hue. Since color prints are to be reproduced as a picture with a consistent density range, this density range is mostly lower than the density range of color manuscripts. In the case of color separation and plate making, compression adjustment must be done. What kind of redistributive adjustments are taken at each tone level depends, on the one hand, on the distribution of the tonality levels of the manuscript, and at the same time it relates to human visual perception. Among them, both the Munsell brightness value of the visual response and the subjective factors of people's visual psychological requirements. To adjust and copy the original tonal hierarchies, it is necessary to sum up the people's visual psychological needs, add the physical values ​​of the visual response, and then consider the distribution of the original levels to obtain the density reproduction curve of the print screen. The evolutionary data in the process of copying and reproduction is designed to design a tone reproduction curve of a specific manuscript.

The density of printed matter is so large that it can not only reproduce the level of some manuscripts or original scenes, but also make it possible to open up some contrasts between levels and levels, and the sense of three-dimensionality is also strong. In particular, the subtle contrasts can be enhanced and the clarity of the image hierarchy can also be enhanced. On the other hand, it also makes the color of the picture open, which makes the deeper saturated, the shallower brighter, and enhances the three-dimensional color. However, the increase in the density of the printing ink layer is inconsistent with the dot gain, and it is impossible to achieve such a high density as a color transparency sheet. Only under the excellent conditions such as the adaptability of paper, ink, and equipment, and the precision of the printing press, can have lower dot gain value 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, and calculate the printing K value. When having a higher printing K value (yellow plate 0.3 to 0.4, magenta plate and plate plate 0.4 to 0.55, and black plate plate 0.5 to 0.6), the density of each of them is high, and the superimposition efficiency of each ink layer is close to 90 At %, the tone of the print is better. Such printed matter must be clearly structured, full and full of three-dimensional images. In order to save printing ink, some people advocate reducing the density of the printing ink layer to reduce the dot gain. In this way, although the number of printed dot adjustments is reduced, the leveling of the dark tone end of the image is reduced, but the overall tone contrast is not the highest. , also can not reach the best level of print tone reproduction.

As the printing process tone is non-linear, compensation adjustments should be made before printing. The network tone adjustment is limited by various conditions during the process of printing and printing, and the transmission of the tone network is non-linear. In order to adapt to the printing process, compensation adjustments must be made. The key point is to make adjustments to the three outlets of the printing area, which are also called three key areas, namely 3%, 50%, and 80% outlets. Because 3% small points are easy to lose, it will affect the color level of the highlight area, 50% of the dot area will easily increase, affecting the midtones, changes in the color of the special characters, 80% of the dot area will be easy to merge, affecting the tone level, especially It is the dark color of the main color and the deep color of the same level, so that it is difficult to balance the two, often at a loss. Therefore, for some important colors and important parts of manuscripts, they are adjusted so as to avoid these three dot skip areas, thereby providing an ideal printing plate for printing, making it printable, and printing a good product.

Manuscript resolution problem

The clarity of color prints is an important quality indicator for image reproduction. Except for the special artistic conception of imagery, at least part of the hierarchy (main body or background) of each image is clear. The requirements for sharpness mainly include the following aspects.

(1) The solidity of the image level outline. First, it is necessary to base the factual (ie, sharpness) of the boundaries of the manuscript hierarchy. At the same time, it also depends on the size of the aberrations of the plate-making optics, the thickness of the electronic color separation analysis and the recorded scanning spot, and the accuracy of the four-color image fit. In the whole process of copying and reproduction, the transition width of the image level boundary is gradually widened, especially enlarged copying. The level boundary reality cannot be restored. The sharpness of the borders of the print hierarchy just seems to be emphasized by the separation of the colors.

(2) The clarity of contrast changes in the two adjacent levels of the image, namely the fine contrast. If the density of the adjacent levels differs greatly, the visual perception is clear. On the other hand, if the density difference is small, the level is flat and the vision is not easily discernible. The subtle contrasts are mainly determined by the secondary differences of the manuscript layer, and are also constrained by the duplication and compression of the tone levels and the distribution of the level adjustments. In the separation of color separations, the emphasis of the level boundary can be emphasized, and the visual contrast of the boundary of the level can be increased. The subtle levels of contrast can be substantially enhanced.

(3) The resolution of the manuscript or the print screen hierarchy, that is, the subtle degree of its subtlety, is the essence of the composition of the objective scenery objects, that is, the so-called texture. Prints are based on the fine resolution of the original image, but are compromised by the resolution of the plate-making optics, the magnification, the thickness of the screen, and the influence of the four-color version. After the plate printing process, the fineness of the original layer cannot be restored. In color separation and platemaking, measures such as false light masks and electronic effects should be adopted to add “trimmings” to the borders of the image levels, so as to emphasize the visual clarity of image layer duplication. The “trimmings” added to the boundaries of the copied image hierarchy include two visual effects produced by the width and height of the peak. The height of the peak positive and negative densities is non-linearly transmitted to vision, and only enhances the visual Mach band effect, but it is not easily visually decomposed. The size of the “trim” width is linear to the vision, and is too large (especially at the bright side of the border) to be visually discerned, giving rise to a stiff embossed sense; being too small is not conducive to producing a visual Mach band effect. Only with the appropriate combination of the width and peak height of the "trimming" can we produce visual Mach tapes that are clearly and unresolved, and at the same time enhance the visual illusion of subtle contrast and subtle levels of resolution.

Therefore, the effect of the visual Mach band produced by the border “trim” is the measure of the quality of the hierarchical clarity and reproduction. Because of the visual Mach band effect and resolving power, it is based on a certain visual physiological physical quantity, and it is basically universal. Therefore, through the visual comparison of printed images and manuscripts, it is also a straightforward and simple objective assessment to make a clear assessment of the level of reproduction. For this observation, the printed product should be placed under suitable lighting and at an appropriate viewing distance. The smoothness, graininess, moire, spots, and unevenness of the print screen will affect the normal reproduction of the image sharpness, and can be directly visually perceived. The smoothness and granularity of plain clothes are mainly determined by the granularity of the original, and are also conditioned by the magnification and the slight emphasis on the initial action value (particle strength adjustment). Subtle emphasis on proper adjustment of the starting value can improve graininess but affect the emphasis of hierarchical clarity. Manual revision can also reduce the granularity and increase the smoothness and delicateness of the clothes, but it is easy to leave traces of manual repair. Generally, it will not get perfect results. Dot shape, dot uniformity, and printing dot quality of screening images affect the flatness of printed images. The poor quality or uneven quality of printed dots will destroy their smoothness and delicateness. The moire is caused by the incorrect angle of the screen, and 45° screen angles are used for the magenta version of the character screen, the cyan version of the scenery screen, and the black version of the Chinese painting. The quality of these surfaces can be verified by scanning magnification or micro-density measurements of their reflection densities to establish objective technical measurement standards. However, it is generally intuitive to make visual evaluations. It is both objective and simple.