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Prepress Correction Of Unusual Manuscripts In Prepress Production
May 31, 2018

The quality of color printing is constrained by many factors, among which the quality of color manuscripts, such as image tone and color balance, directly determines the quality of printed products. Therefore, when prepress production, if it is found that the quality of the original is not good, it should be resolved as soon as possible. The following is a brief introduction to the prepress processing techniques and methods for some non-normal manuscripts.

Printing requirements for manuscripts

1. Original density range

Color printing usually divides the original density range into 4 segments, namely bright highlights, midtones, dark notes, and extremely high light areas below the density of high spots, and the difference between the maximum density and the minimum density is called the density contrast. Generally, the maximum density value of offset printing is 1.8. That is, all the density of the original is converted to white in the range of 0 to 1.8. In addition, the original input device has a limited density range, and the original contrast is too large. , Scanners and extensions have reduced sensitivity to out-of-range density responses, resulting in image layer loss. Therefore, for a manuscript used for printing, there should be a density range suitable for platemaking, that is, a contrast. Practice has shown that the density of reflective originals should be 0.3 to 2.1 with a contrast of 1.8; the original contrast of color negatives is best within 2.4. If the original contrast is less than 2.5, a reasonable compression is performed when copying, the effect is also ideal; if the original contrast is greater than 2.5, even if the copy compression is performed, the level will be lost too much, and the effect is not good.

2. Original color cast

The original color cast is generally divided into overall color cast, dark pitch color cast, bright tune cast color and cross cast color (dark tune, bright tune cast color different). Therefore, it is necessary to consider comprehensively when coloring, and you cannot correct one spot with a color cast so as not to cause other color casts.

3 original level

The main indicators of manuscript quality are level, color, and sharpness, and the most important is the level. Normal manuscripts should have features such as the picture is not bright, not dark, high, medium, dark tone, rich tone and other features.

Unusual manuscript processing methods

1. Method of processing color-originated originals

Whether the original hue is normal can be observed by measuring the contrast between the three-color density of the original and the three-color density curve. It is also possible to use a color densitometer to measure the neutral gray scale of the original, or to measure the three-color density that should be neutral gray in the screen. The following situations must be dealt with in a targeted manner.

(1) If the three-color density and contrast are basically the same, it means that the manuscript is not color cast, and the color tone is basically normal; if the tone is complete, the layers are rich, and the distribution is reasonable, even if the total contrast is slightly different, it can be regarded as a normal manuscript, according to the normal color difference. Data correction focuses on the reproduction of tone levels.

(2) The original three-color density is different, and the contrast of three-color density is basically the same, which means that the original is biased towards a certain color, and it is an equilibrium color cast. It is better to adjust the gray balance curve according to the three-color density curve in order to deal with such a color-changing manuscript, so that the tone of the full-tone tone can be uniformly corrected.

(3) The difference in the density of the three colors of the original highlight is relatively large, and the density of the three colors in the dark tone is basically the same, indicating that the color shift is mainly manifested in the high-profile area. According to the three-color density curve can be derived from the three-color version of the level of replication curve, change the highlights on the highlight part of the three-color network of recording settings, accurate correction of high-profile color cast.

(4) The three-color density of the original high-light part is basically the same, only the dark-colored three-color density contrasts, there is a clear color cast in the middle tone to the dark tone, the dark tone level is slightly lost, and the high-profile level is normal. Processing this kind of manuscript can change the three-tone dot setting of dark color tone, or add a full-color full tone color to remove. However, when dealing with the color cast of the manuscript, attention should be paid to the artist's intention to create artistic effects. Backlit photos are typical examples of this.

2. Hierarchy defective manuscript processing method

Theoretically speaking, the range of printing density is large, the level of originals can be better reproduced, and the contrast and three-dimensionality of the image can be increased. In particular, the contrast between subtle layers can be enhanced, and the sharpness of the image can be enhanced, so that the deeper prints can be saturated and the clearer. However, there is a contradiction between the increase in printing density and the increase in dots. Lower dot gain values and higher ink levels can only be achieved under the conditions of excellent printability of paper, ink, and various types of equipment, as well as excellent press accuracy. density. However, under the existing printing conditions, it is necessary to reduce dot gain by reducing the contrast of printing density. In order to adapt to the characteristics of this process, the original tone adjustment needs to be nonlinearly compressed before printing, that is, adjustment is made at the 3 dot jumping zones of the tone: 3%, 50%, and 80%. Because 3% of small outlets are easy to lose, affecting the level of high light areas; 50% of outlet areas are easy to increase, which will affect the midtones, especially the change of people's skin color; 80% of outlet areas are easy to merge, affecting the dark tone level, especially dark Tune the main color of the skin color and the level of the dark color. Therefore, important colors and important parts of the original document must be adjusted so as to avoid these three dot jump areas. Image level contrast can be adjusted by density range compression at the time of copying. The height of the initial density can also be adjusted by the high optical density calibration and recording density settings.

For manuscripts with different levels of defects, their handling varies from manuscript to manuscript.

(1) Medium contrast original

An original with a high optical density of 0.3 to 0.5 and a high density of 2.4 to 2.8, close to the standard, is compressed at the same time as the gradation of the original, and the gradation adjustment is performed on the tone curve close to the standard original so that a good copying effect can be achieved.

(2) High contrast originals

High-contrast manuscripts mostly refer to manuscripts with a high optical density of no more than 0.4 and a maximum density of more than 3.0. The contrast between the contrasts of light and shade is too large. The general characteristics are rich in intermediate tone levels, and the ratio between the brightest and darkest levels is small. In this kind of manuscript, when the highlights and dark tone are calibrated, the secondary levels at both ends can be generally discarded to shorten the tone reproduction range.

(3) Insufficient contrast originals

A manuscript with insufficient contrast is a manuscript with a contrast of 1.7 or less, a high optical density of less than 0.3, and a low color tone. The main part of the manuscript is bright and thin. Although there are various brightness adjustment levels, the proportion of the screen is large, and the contrast is small. Pre-press production requires deeper copying to reduce the brightness and approach the standard brightness. For example, strengthen the redistribution of light and intermediate levels, reduce the calibration density of black and white fields, and deepen the level curve of bright and midtones to improve the contrast of levels and the effect of visual brightness. However, during the adjustment, the scope of the subject's tone should be differentiated according to the shape of the level curve. At the same time, it is necessary to deepen and lengthen the black tone tone to supplement.

(4) thick stuffed manuscript

The thick stuffy manuscript refers to a manuscript with a high optical density of 0.5 or more, which is often caused by insufficient exposure or development, and causes the image darkness level to fall mostly in the lower half or toe of the straight line of the photosensitive material characteristic curve. The main part of this type of manuscript is darker and darker, with fewer bright tone levels, rich middle and dark tone levels, but uneven level differences, and it occupies a large proportion of the picture. The main tone is mostly within the midtone density range, and it needs to be reduced during production. Processing to enhance brightness, approaching the standard brightness. The black and white field calibration density value should be larger, and the level curve of medium and dark tone should be lightened to make the brightness level including the main tone higher to achieve better contrast and visual brightness.

(5) Distribution of different manuscripts at the intermediate tone level

Some manuscripts with medium-to-dense contrasts have fewer intermediate tone levels and have more contrast, while high-profile and dark tone levels are flat. For example, when photographs are backlit, they should think of ways to coordinate the shades of the main tone of the image and achieve better reproduction. Reproduction effect.

3. How to handle the sharpness of the original

(1) The sharpness of light and darkness in adjacent levels of an image

If the density of adjacent levels differs greatly, the visual experience is clear. It is possible to emphasize the contrast of the boundaries of the levels during the separation of colors, resulting in an illusion of increased clarity, so that the subtle levels of contrast can be substantially enhanced.

(2) Improve the resolution of the manuscript hierarchy

The level of resolution is the so-called texture. The print is based on the resolution of the subtle levels of the restored image, but it is affected by the resolution of the plate-making optics, the magnification, the thickness of the screen, and the accuracy of overprinting the four-color version. After the plate printing process, the fineness of the manuscript level cannot be easily improved.

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