Correction of pictures with poor quality in the pr

  • Detail

Correction of poor original quality pictures in production

in the field of books and periodicals and packaging printing, color printing has become the mainstream of printing, and the quality of printed matter is also improving day by day. However, due to subjective and objective factors, there are not many high-quality products that can be compared with foreign high-quality products. We know that the quality of color prints is restricted by many factors in the reproduction process, such as the tone level of the color original image, point enlargement value, color balance, etc., and the quality of prepress image production directly determines the quality of color prints. Although some deficiencies can be partially corrected in plate making and printing, there is basically nothing we can do to print high-quality products. Therefore, when making images before printing, if the quality of the original is found to be insufficient, we should find ways to correct it. Next, we will briefly introduce some processing technologies and methods

I Requirements for originals

(1) original density

the difference (i.e. contrast) between the maximum and minimum density in the original. Now, the maximum density of general printed matter is 1.8, the maximum density of printing paper image is 1.7, and the ink density of the support before and after the black movement of the trimmed original is 1.8. Generally speaking, all the density of the original can only be reproduced in the range of 0~1.8 on the white paper. Therefore, for the original, there is a density range suitable for plate making and printing in terms of the title, standard, accuracy level, type, manufacturer, factory number and factory date of the tensile machine on the nameplate. When the density range of the original is too large, the sensitivity of the scanner and the electric extension to the part beyond the density range decreases, so the color separation plate level is flat. According to practice, the manuscript is 0.3~2.1, that is, when the contrast is 1.8, it is the most appropriate. The density difference of color negative originals shall be controlled within 2.4; If the contrast of the original is less than 2.5, the effect of reasonable compression during copying is also ideal. If the contrast of the original is greater than 2.5, even if the gradient compression is performed during copying, too many layers will be lost and the effect will be poor

(2) original color deviation

generally divided into overall color deviation, low-key color deviation, high-key color deviation and high-low-key color deviation (i.e. cross color deviation). Therefore, when correcting, we should take a comprehensive view and consider it as a whole. We can't deal with one color immediately when we see one color deviation, otherwise it will cause other color deviations

(3) manuscript level

there are three main indicators to measure the quality of replicas: level, color and clarity, and level is the most important. The level of a normal manuscript should have the characteristics that the picture is not too bright or too dark, high, medium and low-key, many levels of density change, rich levels and so on

II Original color problems and treatment methods

no □ the true state of the original tone can be observed directly from the measurement of the contrast of the original trichromatic density and the description of the trichromatic density curve. Color printing reproduction divides the density range of the original and the copy into four segments, that is, the density range between the highlight and the dark point is divided into bright tone, intermediate tone and dark tone, and the area less than the highlight density is called extreme highlight

in the analysis of the original color quality, if the three color density is basically the same as the contrast, it means that it is not biased, and the tone is basically normal; If the gradation 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 manuscript, and its color reproduction can be corrected according to the normal color error data, with emphasis on the gradation reproduction

if the three color density is different, it indicates that it is biased towards a certain color, but the contrast of the three color density is basically the same, which is a balanced color bias. It is better to design a hierarchical reproduction curve to adjust the balance of three colors and gray according to the three color density curve, so that the hue of the full tone can be uniformly corrected

if the light trichromatic density difference of the original is large, and the trichromatic density at the dark end is close to the same, the color deviation is mainly manifested in the high-key area, which is unfavorable to the high-key level reproduction of the image. It is best to deduce and design the hierarchical replication curve of the trichromatic plate according to the trichromatic density curve, change the recording setting of the highlight trichromatic points of the color separation picture, and accurately correct the high-profile color deviation

if the highlight trichromatic density of the original is basically the same, only the trichromatic density at the dark end is different, and the density contrast is also different. In the medium to dark tones, there is obvious color deviation, the dark tone level is slightly lost, and the medium to high tone level is normal. The color separation chart can be changed, and multiple sets of equipment can be purchased at the same time. The recording setting of dark tone three color points of production films can be set, or the full tone base color of a certain color can be removed (only effective for mixed multicolor). However, when dealing with the original color deviation, we should pay attention to some artistic effects created by the author intentionally, such as backlight photos

in printing, there are generally three different concepts of color reproduction. One is color reproduction in the physical sense, which requires that the spectral distribution of the original color on each color point is exactly the same. The second is the reproduction in the sense of colorimetry, which makes the printed reproduction image consistent or close to the original color point in terms of colorimetry, that is, the effect of different spectrum and same color, which is the measurement standard for realistic and objective evaluation of color reproduction. The third is color reproduction in psychological sense, that is, the color reproduced by printing may be a little different from the original color in chromaticity, but it may meet the satisfaction of visual psychology in color effect

The fundamental principle of color reproduction is to restore the gray in the original. To measure the reproduction of gray balance printing, use a color densitometer to measure the three color neutral gray scale attached to the print, or measure the three color optical density that should be in the neutral gray level in the picture. If observation is used instead of judgment, it is necessary to use standard neutral gray blocks to compare with the gray levels in the printed picture under the condition that the light source and the environmental color temperature are very standard

III Manuscript level density problem and processing method

for originals with different level density and quality, the processing method varies from manuscript to manuscript

(1) the exposure is normal, and the main parts of the density contrast standard

are in bright tone and middle note, which are directly proportional to the lightness, and are at level 4~6 of the new lightness standard system

(2) medium contrast manuscript

the low density is 0.3~0.5, and the high density is 2.4~2.8, which is close to the standard manuscript. While compressing the tone contrast of the manuscript, it can achieve good replication effect by adjusting the level according to the reproduction curve of the tone level close to the standard

(3) high contrast originals

originals with a high optical density of no more than 0.4 and a maximum density of more than 3.0 are mostly originals with too large contrast between the light and shade of the original scene. Generally, the intermediate levels are rich and complete, and the ratio of the brightest and most convenient dark levels starting from a production line is small. For this kind of manuscript, when making highlight and dark tone selection, it is generally possible to omit the secondary levels at both ends to reduce the range of tone reproduction

(4) low contrast manuscript

tone contrast is less than 1.7, and high optical density is less than 0.3. The tone of the thin manuscript, the main part of which is lighter and thinner, has high lightness, and is at level 7 and 8 of the new lightness standard system. The lightness level is rich, but the density level difference is very small, and accounts for a large proportion of the area. When making, you should deepen the copy to reduce the lightness and approach the standard lightness. For this kind of manuscript, it is necessary to strengthen the level reproduction and redistribution of bright middle note. The black-and-white field calibration density value should be smaller. The level curve of bright and middle note should be slightly deepened to achieve better level contrast and visual brightness effect. According to the shape of its hierarchical distribution curve, its main tone should be distinguished from the bright tone or middle note density range. At the same time, it also needs to use deepened and lengthened black tone as a supplement

(5) thick dull manuscript

thick dull manuscript with a high optical density of more than 0.5 or even more than 1.0. Due to insufficient shooting exposure or development, most of the image shading levels fall on the toes of the negative image characteristic curve and the lower half of the straight line. The main part of this kind of manuscript is dark and deep, which is at level 2 and 3 of the new lightness standard system. There are few levels of lightness, rich levels of medium and dark tones, and the level difference is flat and soft, and the area ratio is large. The main tone also falls within the range of middle note density. When making, it is necessary to reduce the lightness and improve the copy, so as to improve the lightness and approach the standard lightness. The calibration density value of the black-and-white field should be larger, and the level curve of the medium and dark tones should be slightly shallower, so that the medium and bright tone levels, including the main tone, should be reproduced brightly, so as to achieve better tone contrast and visual brightness effect

(6) originals with different intermediate tone levels

for some originals with medium and high density contrast, the middle note level is less, the density level difference is larger, and the high and low tone level is more and gentle, such as the color film taken in backlight. When making, we should find a way to make the level of the main tone of this kind of image coordinate, which can achieve a better reproduction effect

the size of the image hierarchy contrast, which can be compressed and adjusted for density range during copying. The initial density can also be changed by adjusting the highlight density setting and recording setting. The contrast range of the transmitted originals is very different, but the color printed matter has to be reproduced as a picture with basically the same density range, which is mostly lower than that of the color originals. During color separation and plate making, it is necessary to compress and adjust

the density range of printed matter is large, which can not only reproduce some layers of the original manuscript or the original scenery, but also make the contrast between the layers at all levels more open, and the three-dimensional sense is strong. In particular, it can enhance the fine level contrast and enhance the clarity of the image level. On the other hand, it also makes the color of the picture open the level. The deep is thick and saturated, and the shallow is bright. However, the increase of printing ink layer density is in contradiction with point increase, and it is impossible to achieve the high density of color transparent film. Only under the excellent conditions of paper, ink, equipment and printing machine precision, can there be low point increase value and high ink layer density. The objective technical standard to determine this quality index is to use a color reflection densimeter to measure the printing field density and 75% point printing density of each printing ink layer, and calculate its printing K value. When this value is high (yellow version 0.3~0.4, magenta and cyan version 0.4~0.55, black version 0.5~0.6), their respective field density is high, and the overprint efficiency of each color ink layer is close to 90%, the printing tone is also better. In order to save ink, some people advocate reducing the density of printing ink layer to reduce the point increase, but this will reduce the merging phenomenon of image dark end layer. The overall tone contrast is not the highest, nor can it achieve the best reproduction of printing tone level

because the printing process gradient is nonlinear transmission, compensation adjustment should be made before printing. In order to adapt to the printing process, the three dot jump areas of printing must be adjusted, that is, 3%, 50% and 80% dot areas. Because 3% dots are easy to lose, which will affect the color level of the highlight area, and 50% dots are easy to increase, which will affect the middle tone, especially the change of the skin color of the character. 80% of the dot area is easy to be merged, which will affect the dark tone level, especially the merging of the main color of the dark tone skin color and the deep primary color, making it difficult for printing to give consideration to both. Therefore, the important colors and parts of the original should be adjusted to avoid these three point jumping areas$ Page break $

IV The sharpness of the original and its processing methods

(1) the reality of the hierarchical contour of the image

must first be based on the reality (i.e. sharpness) of the hierarchical boundary of the original. In the whole process of reproduction, the transition width of the image level boundary should be gradually widened, especially enlarged reproduction. The level boundary realness can no longer be restored. The clarity of the printed matter's hierarchical boundaries only seems to be emphasized by color separation reproduction

(2) the clarity of the contrast change of light and shade between two adjacent layers of the image

if the density difference between adjacent layers is large, the visual experience is clear. You can emphasize the contrast of the level boundary during color separation, resulting in the illusion of improvement, while the subtle level

Copyright © 2011 JIN SHI