Gradient correction of the hottest color image

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Color image gradient correction

Abstract: by analyzing and comparing the gradient range of color image originals and printed copies, this paper expounds the reasons why gradient correction should be adopted in desktop printing system image processing, and puts forward the general methods of gradient compression and gradient curve adjustment combined with the application of Photoshop software

1. Introduction

the three indicators for evaluating image reproduction quality are tone, color and sharpness. The reproduction of tone is the most important. Due to the restriction 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 best reproduction of the image tone within the limited brightness range is always a difficult point in color printing. At present, with the application of color desktop publishing system becoming more and more popular, people can use image processing software to adjust the image gradient. Tone reproduction of color images seems to be easy. However, this is not the case. Since the density range of the printed copy is generally smaller than that of the original, the gradient 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 step compression is small, which is not easy to be detected. However, when the gradient of the image is limited to a small printing density that reflects the brightness change of the original, the proportion of this compression will be amplified, resulting in a great difference in the visual perception produced by the color display screen and the printed image. 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 key points of the original tone reproduction and the content and theme of the original, and then correct the tone on this basis

2. Step correction step correction actually contains two meanings. The first is to perform artistic processing on the tone of the original to meet the customer's subjective requirements 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 the 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. The method of step compression and adjustment is usually adopted for step correction. 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 original, 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 reproduction process, so as to meet the requirements of reproduction. In Photoshop, an image processing software, curve compression and adjustment are mainly realized by setting highlight/darken points and adjusting curve curve. It can also be achieved in a small range by adjusting the brightness/contrast tool

2.1 for the setting of black-and-white field (setting of highlights/darkening points), since small points (1% ~ 5%) will be lost during printing, and points greater than 95% will become solid, it is necessary to first select highlights (the brightest point with hierarchical details) and darkening 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. The setting of highlight/darken points can accurately display the tonal level and detail table of the original. The limit of harmful materials in PVC coiled floor GB 18586-2001

setting the high/dark key correctly is the key factor for successful replication. Highlights directly affect the hue 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 make the medium brightness level on the original be well reproduced. In addition, the highlight points must be set so that the color copy reaches 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. Generally, we need to use certain instruments to judge. Setting the darkening point correctly can not only reflect the level of the image, but also correct the color deviation of the original. The best way to set the black-and-white field in Photoshop is to use the highlight and eyedropper. The specific method is as follows: select image/adjust/curves, double-click the highlight eyedropper to get the colorpicker, enter the point value that can be correctly reproduced on the printed matter in the CMYK box (for example, coated paper printing: c5%, m3%, y3%, k0%), and then click the light point with hierarchical details on the image to switch the point to the printable range. The dark tone setting can be completed by using the dark tone dropper

2.2 adjustment of curves in Photoshop, the best way to adjust the image gradient is to use the curves dialog box. Execute the image/adjust/curves command to open the curve dialog box. 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 original gradient is normal, the curve curve can be adjusted linearly. That is, line 1 in Figure 4; The original image with light tone as the main tone has poor high tone level and small contrast. Therefore, during the tone adjustment, the high and medium tone should be mainly adjusted to improve the high tone level, contrast and color saturation. The adjusted curve is shown in the 2 lines in Figure 4 and the 4 tone adjustment curve in Figure 4. It can be seen from the adjusted curve that the high-profile layers are enriched, the contrast is increased, and the color is deepened. In the manuscript with mainly dark tone levels, the dark tone levels are poor, the contrast is small, and the whole is dark. To this end, it is necessary to draw a large and medium-sized dark tone level 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 whether the supporting fuse of the original is loose in the world construction market, and in combination with the effect shown by the color display screen (which must be color corrected), so as to obtain an ideal print

color correction

images after color separation often need to be corrected to meet the requirements. There are many correction tools in Photoshop software. We usually use the following:

(1) use curves to correct the high and low key values of the image. Taking into account the gradient and gray balance of the whole image, the curves curve has powerful functions, and the gradient loss of the image is very small. I hope readers like it. For example, if the image is dimmed in the middle, we can use the curve to adjust

(2) use the selective color tool to adjust the local color block in the figure

for example, if we want to increase the saturation of the blue block in the image, we can adjust it as shown in Figure 7. For originals with rich color levels of continuous tone (more transitions and gradients), readers are expected to use this item with caution, and the adjustment amount should not be too large, otherwise there will be faults, which will damage the continuity of tone

(3) sharpen the image with sharpening tools. There are four sharpening methods under filter/sharpen. Unsharp mask is the most powerful. Amount indicates the amount (intensity) of sharpening, radius is the number of pixels participating in sharpening, and threshold indicates the starting point of sharpening. The reader can modify the three parameters according to the screen display. Ensure that the printed image is clear, natural and the granularity is appropriate. Of course, readers should first ensure that their monitors focus normally

after color correction, the bright and dark tones of the image should be within the normal range, and the middle tone should meet the visual needs of the human eye, faithfully restoring the gray balance. If you do this, you can say that the drawing is finished

Copyright © 2011 JIN SHI