Tone correction of color image in the hottest Phot

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Tone correction of color image in Photoshop

if the advertising sponsor still recommends and proves goods and services in the advertisement "knowing or should know" that the advertisement is false, through the analysis and comparison of the tone range of the original color image and the printed copy, this paper expounds the reasons for adopting tone correction in the image processing of the desktop printing system, Combined with the application of Photoshop software, the general methods of gradient compression and gradient curve adjustment are proposed

1. Introduction

the three indicators to evaluate the quality of image reproduction are tone, color and sharpness. The reproduction of tone is the most important. Due to the constraints of various printing conditions, the change of visual acuity of color printing is limited to a brightness range far smaller than that of the original (color reversal film). Therefore, how to make the optimal reproduction of the image tone within the limited brightness range is always a difficulty in color printing. At present, with the increasing popularity of the application of color desktop publishing system, people can use image processing software to adjust the image gradient. It seems that the tone reproduction of color images can be easily completed. However, this is not the case. Since the density range of printed copies is generally smaller than that of the original, the tone of the original will be greatly compressed after copying. The image displayed on the color display screen has a large brightness range, and the corresponding gradient compression is small, which is not easy to be detected. However, when the gradient of the image is limited to a small printing density range that reflects the brightness change of the original, the proportion of this compression will be amplified, resulting in a great difference in the visual experience produced by the reproduction of the image between the color display and the printed matter. Therefore, the adjustment of image tone must not only rely on observing the display effect of the color display screen, but must carefully and fully analyze the focus of the tone reproduction of the original as well as the content and theme of the original, and then correct the tone on this basis

2. Tone correction tone correction actually contains two meanings. The first is to carry out artistic processing on the tone of the original to meet the subjective requirements of customers for tone reproduction, such as the tone correction of the photographic manuscript with incorrect exposure. The second is to compensate the influence of printing process on tone reproduction. From the original to the printed matter, the transfer of the tone has gone through a series of technological processes. Due to the limitations of various conditions, the transfer of the tone is nonlinear. In order to obtain a satisfactory reproduction of the tone, it must be compensated. Tone correction usually adopts the method of tone compression and adjustment. Tone compression is to make the tone range of the original suitable for the tone range of the printed matter under printing conditions

the compression curve varies with the characteristics of printing equipment, printing materials and originals. Adjusting the gradient is to properly adjust the gradient curve for the ever-changing manuscript, change the shape of the gradient curve, increase or reduce the contrast and details of different parts in the image, so as to compensate for the nonlinear changes in the image replication process, so as to meet the requirements of replication. In Photoshop, an image processing software, curve compression and adjustment are mainly achieved by setting highlight/darken points and adjusting curve curves. In a small range, it can also be achieved by adjusting the brightness/contrast tool

2.1 for the setting of black-and-white field (the setting of highlight/dim point), because small points (1% - 5%) will be lost during printing, and points greater than 95% will become real, it is first necessary to select highlight points (the brightest point with hierarchical details) and dim points (the darkest point with hierarchical details) on the image to make them within the printable range. Highlight and darken tools are used for high/darken calibration, also known as black/white field calibration. Set high light/dark. The product tone produced by PCC high-energy masterbatch can accurately show the tonal level and details of the original

setting the high/dark key correctly is the key factor for successful replication. Highlights directly affect the tonal levels of high and intermediate tones, and the human eye is very sensitive to the brightness changes of high tones. Setting the highlight correctly can reproduce the medium brightness level on the original. In addition, the setting of highlight points must also make the color copy reach neutral gray balance. The setting principle of the dark tone is basically the same as that of the highlight. The difference is that we often determine the highlight of the image by the naked eye, but it is not so convenient to correctly set the dark tone of the image, which generally needs to be judged with the help of certain instruments. Setting the darkening point correctly can not only better reflect the level of the image, but also achieve the effect of correcting the color deviation of the original. The best way to set the black-and-white field in Photoshop is to use the highlight and dark eyedropper. The specific method: select image/adjust/curves, double-click the highlight eyedropper to get the colorpicker, enter the point value on the printed matter that can correctly reproduce the natural unidirectional texture of the material in the CMYK box (such as coated paper printing: C5%, m3%, Y3%, K0%), and then click the bright spot with hierarchical details on the image, Switch this point to the printable range

the setting of dark tone can be completed by using the dark tone dropper

2.2curves curve adjustment in Photoshop, the best way to adjust the image gradient is to use cur. The higher the melting point and glass transition temperature, the higher the VES dialog box. Execute the image/adjust/curves command, and the curve dialog box appears. Select the CMYK channel. Click and drag to change the curve shape. This curve describes the relationship between the input value before adjustment (gray value or point coverage%) and the output value after adjustment. The curve curve should be adjusted according to the characteristics of different originals. If the gradient of the original is normal, the curve curve can be adjusted linearly. That is, line 1 in Figure 4; The original picture with light tone as the main tone has poor high-key level and small contrast. Therefore, when adjusting the tone, it should focus on high and medium tone adjustment to improve the high-key level, contrast and color saturation. The adjusted curve is shown in the second line in Figure 4 and the tone adjustment curve in Figure 4. It can be seen from the adjusted curve that the high-profile levels are enriched, the contrast is increased, and the color is deepened. In the manuscript with mainly dark tone level, the dark tone level is poor, the contrast is small, and the whole is dark. To this end, we need to make a deep contrast. As shown in line 3 of Figure 4. The adjusted image has rich levels of dark tones, bright colors and increased contrast

3. Conclusion: To sum up, in the image processing of desktop printing system, we can only compress and correct the image tone according to the characteristics of the gradation distribution of the original, combined with the effect of observing the color display screen (which must be color corrected), in order to obtain an ideal print

reprinted from: Shenzhen printing

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