The technique and key points of tone reproduction (3)

Fifth, the white field (highlight) selection and printing network point setting law

1. The importance of white point selection and setting

The white-field setting is defined as the value and setting of the printing point value in the brightest and whitest part of the image.

The most important thing in color image scanning separation is the selection and setting of white field (high light). It is not only important to enhance the aesthetics of printed products, but also the key to the success of copying. Because it directly affects the brightness and subtle levels of reproduction of high light tones, it also influences the tone levels of light and midtones. This is the most important key area for copying.

The selection and setting of the high-light part of the original when scanning color separation is very scientific and strict, because it is both technical and artistic.

Why is Whitefield (highlight) selection and setting so important? because:

(1) Human vision is very sensitive to changes in brightness and subtle levels of highlights, such as tiny dust or traces on white, which are easily visible, and dust or traces of the same density are not easily found on black. .

(2) The high light and bright tone of most manuscripts, whether painting or photography, especially packaging design drafts, are the main part of the manuscript. They are the most exciting part of the picture light and color, and are the most exciting part of picture light and color. There are strong light beauty and rich color changes.

(3) The change of the tiny dot value of the high-light tone will affect the color tone of the bright tone and the middle tone, which will greatly change the printing result.

If the white field sample point is selected correctly and the dot value is set correctly, all information on highlights, highlights, and midtones on the original can be reproduced. On the contrary, it will result in high-gray or level talk and loss, losing the most exciting part of the picture. Therefore, random selection and setting are unscientific, even if the AI ​​of the scanner and its software is automatically set, but the dynamic setting or the manual setting is required to follow the rules.

2. White point selection, setting the impact on the quality of printed products.

Whether the whitefield selection and network value setting is correct or not, there will be three cases of print quality (Figure 5):

(1) If the highlight selection point and outlet values ​​are set correctly, and the minimum printing dot value is set correctly, all the information on the original and middle notes of the original document can be correctly reproduced on the printed matter without damage.

For example, the high optical density value on the reflective original is 0.19, and the high optical density value set on the desktop scanner is also 0.19, which means that the highlight information on the original can be reproduced ideally on printed matter. As shown in Figure 5B

(2) If the white field (high light) sampling point set the density value is too low, is 0.15, the network point value is set too large, the performance of the print is too dark, high light does not shine, giving a gray level, Dreary feeling. As shown in Figure 5A.

(3) If the density value set by the white field (highlight) is too high to be 0.23 and the dot value is set too small, the highlight color becomes lighter, so that the high-light part of the original that requires a level becomes 0% in the printed matter. , resulting in loss of high light levels. Figure C points:

3. The five bases for white point selection and setting.

(1) According to the density of the original document, the color tone may be bright or thin.

(2) It depends on which stage segment the subject of the manuscript is in, which is the focus of the copy.

(3) It depends on the product type, characteristics, and printability of the subsequent process.

(4) It depends on different processes, it is offset, gravure or flexo.

(5) It depends on the size of the original white material and the texture of the white material.

4. White field (highlight) points, set three elements.

(1) Select correct whitefield sampling point

A. First, according to the contrast of the original density, whether the color tone is bright or thick and where the original body is located, determine where the white field sampling point is selected, and whether it is selected in the highlight, the brightest part, or the second brightest part.

B. Correctly analyze which part of the brightest or whitest part of this picture is. As the brightest and whitest control point of the setting, there is no brightest white color, which can determine which part is the brightest. In short: A picture always has one of the brightest and loudest parts as a visual center. For example, one is extremely high light; the second is the most white part of the hierarchy; the third is the brightest part of the color.

C. After determining the brightest and whitest part of the manuscript, determine the dot value based on the texture characteristics of the original white matter. Such as white paper, white clouds, white cotton, white silk, white jade, white porcelain and so on. The second is based on the size of the white area, determine the setting of the outlet value. If the area is large and the texture of the hierarchy needs to be expressed, the small point can be set to 1-2% larger. If the area is small and the level is not important, the small point setting can be 1-2% smaller.

(2) Determine the basic white based on the printing process and printability conditions.

The specular white is defined as the minimum dot value that the printing can print, not the extreme highlight of the dot. That is, in the image, the brightest and whitest part of the hierarchy is required to set the minimum dot value that the printing can print out, which is called the basic white setting. The basic white dot value setting is basically a fixed value. At present, the minimum dot value of the offset printing process is generally 3%, and the best one is 1 to 2%. The basic white color is C7%, M3%, and Y3%. At present, the gravure carving process, the minimum dot value of the substrate OPP film can be transferred to 5%, the basic white is C7%, M5%, Y5%.

(3) Follow the gray balance dot value ratio.

a. Specular white is defined as a neutral white or a feature that needs to be white. Need a layer of high-light neutral white, according to the gray balance ratio C is greater than M, Y2-3%.

bC, M, Y three-color version of the outlet value can also be proportional. Such as 6,6,6;4,4,4;5,5,5, etc., then, high light white slightly warmer, it is also possible.

If the cC, M, Y color version is larger than C, the printed product is warm white.

d. Avoid a missing color plate. In the absence of a single color, then the highlight white color will be color cast, dew color, must pay attention.

One problem that currently exists in highlight settings is that operators do not have a sense of gray balance, high-light white is unbalanced, and color casts and dew are often present. Sometimes, although a certain color version has 1% or 2% of dots, it causes color casts and dew-colors when printed, proofed, and printed.