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If the manuscript itself is not well-tuned, and the scanner is directly connected to the computer without any modification, it will have 256 levels for each color channel. If Photoshop or other software is used to perform curve correction, the correction will be performed again. Loss levels, the final result is less than 256 levels / channels.
In newspaper printing, the dark tone level of the image is particularly easy to merge. If the darkness level can be better protected and enhanced by the curve adjustment of the scanner, the image reproduction is more advantageous. Fig. 5a does not perform dark level correction on the original during scanning, while Fig. 5b performs the darkening contrast and increases the luminance on scanning, and Fig. 5c illustrates darkening of Fig. 5a in Photoshop. The level of processing, but the effect is poor. The level of shadow tone in Figure 5 is mainly concentrated in the area of ​​the asphalt road, in which the level of shadow tone in Figure 5a is more serious, and the level of shadow tone in Figure 5b is reproduced more clearly. Although the level of shadow tone in Figure 5c is changed, it has already been lost. The dark tone level has little effect.
2. The color separation of color newspapers is based on the characteristics of print copying in newspapers, which limits the total dot area ratio of four-color printing. At the same time, a large proportion of gray component substitution (GCR) should also be adopted. In this way, printing failures caused by paper properties can be avoided.
In CMYK Settings in Photoshop, you can select the ink/paper combination used (Figure 6a). Under its "ink color" item, ink/paper combinations such as "SWOP (Newsprint)", "Euroscale (Newsprint)" and "AD-Litho (Newsprint)" can be selected. If you want the ink/paper to match the user's use, you can first print Cyan/Purple/Yellow/Black/Cyan+Purple+Yellow/Purple+Pink/Yellow/Cyan+Purple+Yellow/White, then Color The degree measuring instrument measures its chromaticity values ​​(x, y), Y, selects "custom", and inputs the measured data.
As mentioned earlier, "dot expansion" is set at 30% to 35%.
In the color separation setting, "Gray Component Substitution (GCR)" is usually selected. Among them, "Black Edition" can choose "More" or "Maximum". With this setting, the gray component of the image color and the gray portion of the image have less cyan/article/yellow ink, while the black ink has more ink. The "total ink amount limit" is set between 270% and 300%, "black ink limit" is set between 90% and 95%, and the "background color gain" is generally set at 0%. When the "Total ink amount limit" is set between 220% and 240%, "Background Gain" can be set between 10% and 15%.
3. The highlights and dark tone calibrations for newspaper copy images. For the characteristics of newspaper printing, it is a critical step to reasonably calibrate the image for highlight spots and dark spots.
In general, the so-called “highlight point†should be: The brightest point in the image that begins to appear at the bright tone level, but it is not the “brightest point†that is the brightest part of the image. "Dark tone point" is generally the end point of the image tone level, but it is not the darkest point.
In determining the highlights and dark spots, two factors should be considered.
1) Determination of area ratio of high-light spot/dark-dot dot network The minimum dot area rate that can be completely transmitted to newsprint is 6% to 8% according to the characteristics of newspapers. When selecting the highlight point, the area ratio of the highlight spot of the image should begin to appear brighter than this range, otherwise the transmission of the highlight spot will be incomplete or completely lost, so that the original tone of the image will be changed into a white Extreme light of paper.
The dot area ratio of dark dots is affected by the “Ink Total Limitâ€, “Black Edition Ink Limitâ€, and “GCR/Black Edition†settings in Photoshop software “CMYK Settingsâ€, and these three settings determine the darkest printable The dot area ratio of the dot, the corresponding data relationship is: R = G = B = 0, L = a = b = 0, CMYK data is subject to the above three settings, but generally in the setting of dark point data, Set by RGS=(10~15) data, not set to 0.
2) The selection of image scaling levels should be based on the tone level of the image itself.
If the image itself has a normal tone level, set a high light point at the brightest point of the level (non-extremely high light), and focus at the darkest point (unreal darkest point) having a dark level change. The tone level of the image can be reproduced normally.
If the image is bright and the contrast of the light-toning gradation is small, then a particularly bright area in the image, or even an extremely bright spot, can be calibrated as a highlight point, and the dark adjustment can be performed by normal calibration. After this calibration, the brightness of the image is no longer too bright, at the same time the turn-key points tend to be normal and the contrast of the image is opened.
If the image is dark, you can use the slightly darker highlight point in the image as the highlight point, and use the darkest point in the image as the dark point. The scale of the image after the scale will become normal. .
Adjusting the highlight/dark-tone calibration curve can also better handle the level of the newspaper image. When adjusting a curve, generally pulling the light level off makes the image darker, and pulling the dark level off makes the image brighter.
The calibration of the image can be performed in the "curve" adjustment. As shown in Fig. 7, in the curve interface, there are "Highlight Point", "Dark Point" and "Middle Point" calibration buttons. Double-click the button to display the color setting interface for inputting calibration data. When setting data, RGB, CMYK, Lab, HSB data are linked. According to a certain color mode input data, the data of other modes will also change accordingly. After the calibration data is set, you can use the mouse to click on the corresponding highlight and dark points on the image to complete the calibration.
Figure 8 shows the process of adjusting the grading of an image with highlights/dark scales and curves. The contrast of the original image is slightly insufficient. After the selection and calibration of the highlight point and the dark tone point, the contrast of the image is opened, and the intermediate tone is processed with the curve to make the image get better results.
4. Image color adjustment Because the newsprint can reproduce a smaller color gamut, it is difficult to faithfully reproduce color with high saturation. Therefore, for the color in the image, the hue should be as good as possible, and the brightness should be adjusted according to the tone. The overall relationship of levels. High-saturation color colors should be reproduced as much as possible, but for the small area, dotted, and particularly vivid colors appearing in the image, excessive pursuit of vivid color and loss of color change should be avoided. The red flag is made into a "red paper cut" with no layer change, and the green lawn is adjusted to a "plastic lawn" without any opposite color. In addition, when coloring, you should also pay attention to the color of the paper itself and give appropriate compensation.
5. Grayscale Image 1) Grayscale Image Settings Photoshop's Color Settings feature contains grayscale image settings. For print copying, "black ink" should be selected. Grayscale images can also be pre-compensated via Photoshop, and the compensation curve is the same as the “dot enlargement†setting.
2) How to convert a color image into a grayscale image In Photoshop, there are two ways to convert a color image into a grayscale image:
* Directly use the "mode" function for conversion, but the price adjustment level is not ideal;
* First convert the color image to Lab mode, then delete "a channel" and "b channel", and finally convert the image to "grayscale". In Chroma Lab mode conversion, the L channel retains more layer changes because of the brightness, so the conversion effect is good.
In short, in the prepress production of newspapers, only in accordance with the characteristics of newspaper printing, the corresponding image settings, corrections and compensations in the prepress stage can get better print reproduction effects. (End of the article)
Prepress Image Processing of Color Newspapers (below)
) Team-level processing on the scanner Adjustment of the tone-level curve on the scanner is less important than entering the Photoshop software for curve-level adjustment, and it is even more important for originals with abnormal tone levels. The reason for this is: When the scanner generates the initial digital signal of the image, it is performed with more than 8 bits/channel, such as 12-bit, 14-bit, or 16-bit/channel, that is, 4096, 16384, or 65536 per color channel. Levels. The initial digital image signal passes through the curve set on the scanner and is converted into an 8-bit (256-level)/channel image signal before being sent to image processing software such as Photoshop.