As the most important application software in the image processing field, photoshop is one of the tools that computer designers and prepressors must master. It has powerful image processing and design functions. This article aims to introduce the function of color correction in photoghop and illustrate it through examples.

1, image quality analysis

　　 Before we process an image, the first thing to do is to analyze the image quality.

(1) Level. Hierarchy refers to the natural change range of an image from bright to dark, or the natural density change gradient from bright to dark. The level of print refers to the level of recognizable lightness within the range of copy density. The more levels, the richer the level. In an image, there are bright, middark, and dark tone levels in the image. We can see the hierarchical distribution of the image from the histogram in photoshop. If the pixels in the image are mostly concentrated on the right side of the histogram, the brighter parts of the image are more, if concentrated on the left side, the darker parts are mostly concentrated in the middle, and the middle tones are more. Figure 1, is a picture and histogram, from which we can see that the picture does not lack levels in the dark tone and bright tone, but the whole picture has almost no intermediate tone transition, from dark to light tone The changes are very abrupt, which is the biggest drawback of the image in terms of hierarchy.

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Figure 1 Viewing the Hierarchical Distribution of Images via Histogram

(2) Colors. The color analysis of the image includes two aspects: (1) judging whether there is a color cast, (2) determining which colors need special attention in prepress processing.

(3) Sharpness. In addition to the level and color of the image, we also need to pay attention to the clarity of the image. See if the image is blurry enough that the details of many images don't show up. Or the image is noisy and mottled so that the image is not smooth.

2, photoshop color correction function

　　 Photoshop has a powerful color correction function, here its color correction function is divided into three aspects to introduce one by one.

(1) Level correction function For the level problem in the image, there are two tools that can achieve the level of correction color scale and curve color scale can adjust the level distribution of the main channel and each color separation channel, especially suitable for adjusting the brightness and darkness of the image. Tone. At the same time, it can also achieve black and white field and neutral gray determination function. In determining the black and white field, it should be noted that the black field of the image should have sufficient density, and the percentage of halftone dots should be above 95%. It should be the darkest part of the image. The white field should be close to the net, and the percentage of the dots is less than 5%. It is an image. The whitest place. Compared with the color grading tools, the curve tool has a more powerful function in adjusting levels. The curve tool can implement level corrections by pulling up or down the curve and adding any number of control points in the middle. This is not achievable by a color scale tool. If the color scale is coarse, the curve tool is fine-tuned. Of course, in addition to these two tools to adjust the level, there is also an automatic color level can also adjust the level, but the tool can not be artificially controlled, but automatically adjusted.

(2) Color correction function. There are many tools in photoshop that can implement image color correction functions such as color balance, brightness/contrast, hue, saturation, replacement colors, selectable colors, channel mixers, and so on. The color balance tool is designed based on the principle of increasing the basic color and reducing the opposite color when coloring. It can adjust the colors of different tones (bright, medium, and dark) without affecting other tones, as well as the brightness protection function. However, this tool is difficult to control the color. To adjust a color, it often affects other colors. Therefore, the tool is generally used only for fine adjustment. The brightness/contrast tool is mainly used to adjust the brightness, darkness, and contrast of the image. Hue/Saturation adjusts the image from the hue, lightness, and saturation of the color. It can adjust all the colors of the image or the three properties of a specific color. When adjusting a color, it does not affect other colors. Strong selectivity. The Replace Color tool is a find and replace class command that creates a mask in the image. Tolerance adjusts the mask's size. First pick up the color to be replaced with the eyedropper tool in the “selection area” (this color will be displayed in “Sampling” in the “Transform” area. When the color is corrected, the adjusted color will be displayed in “Sampling”). Then the hue, saturation and lightness can be corrected in the "Transform" area. The optional color tool adjusts the color by adjusting the percentage of four-color dots, and is more suitable for printing. It can correct the color of cyan, magenta, yellow, green, blue, white, black and neutral colors in the image. Correcting one of the colors basically does not affect other colors and is extremely selective. It is the most common tool for correcting colors. There are two methods for its coloring, relative and absolute. The relative adjustment of the color is small and the absolute adjustment is large. For example, adjust its magenta color to 60% of magenta, and increase it by 30%. If you use the relative method, the final magenta color is 76% (60%+60% x 30%). In absolute terms, the final magenta color is 90% (60%+30% dry channel mixer can add or subtract color information from several channels, which can effectively adjust image color cast.

(3) Sharpness correction function. Adjusting the sharpness of the image in photoshop often uses sharpening and removal tools. Sharp Chemicals has USM sharpening, further sharpening, sharpening edges and sharpening. We often use USM sharpening, which has three parameters in its dialog box: Number Crab, Radius, and Threshold. Number refers to the degree of clarity of the emphasis, the greater the number, the greater the emphasis on the value of its value in the O% -500%. Radius refers to the radius of the role of sharpness emphasis, that is, with a certain pixel as the center of calculation, to achieve a clear range of adjustment, the greater the radius, the greater the role of clarity of emphasis. However, under normal circumstances, the radius should not be too large, otherwise there will be a very hard border. The threshold value refers to the minimum grayscale difference emphasized by the sharpness, that is, only if the gray value of the neighboring pixel is greater than a given threshold, there will be a USM sharpening effect. Obviously, the greater the threshold, the less pronounced the sharpening of the USM. Some noises appearing on the image such as noise, moiré, scratches, etc., need to use denoising tools in order to remove them and achieve smooth images. Denoising and sharpening are exactly the opposite of the two principles. When the sharpness emphasizes too much, noise will appear, and noise reduction will reduce the sharpness. The most commonly used noise removal tool is the dust and scratch tool, which has two parameter radii and thresholds, the same meaning as in the USM sharpening tool.

3, example analysis

　　 Through the above analysis of image quality and the introduction of color correction tools in photoshop, we have a basic understanding of photoshop image correction. Here we look at some examples to see how to use the color correction tools in photoshop to achieve image color correction.

Example 1

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Figure 2 Original Figure 1

Figure 2 is a bonsai image. Because the tone of the entire image is too dark, some details on the wall are not reflected. In addition, this image is too dark and unclear, giving people a feeling of depression. Our main task is to increase the brightness of the image and increase the contrast of the image, so that the details of the dark tone reveals that step 1 increases the brightness of the entire image. There are four ways here. First, use the curve tool to increase the brightness of the image. The body operation is as follows: open the curve tool, select the CMYK mixing channel as the adjustment object, and pull down the image tone curve, and the adjustment result is shown in FIG. 3 . Second, use tools. The method is as follows: open the level tool, select the CMYK mixed channel as the whole object, and move the bright button of the input level to the left. At this time, the brightness of the entire image will increase, and the adjustment result is shown in FIG. 4 . The third uses the Brightness\Contrast tool to adjust the brightness of the entire image. In Image\Adjustments, find the Brightness\Contrast tool and drag the Brightness button to the right. The brightness of the entire image will increase. See Figure 5 for adjustment results. With this tool, the brightness of the entire image can be adjusted. If the adjustment is small, there is no problem. However, if the adjustment range is large, it is often easy to make the map when we consider the brightness adjustment, if you need a large adjustment we must consider using other adjustment tools. Fourth, use the image black and white field positioning tool to adjust the brightness. The black and white field calibration tool is available in the curve tool or the color scale tool. Open the color scale tool and select the white straw tool. Select a point C44% M24%Y18% K4% in the upper left part of the image. This point is blue and has a With a certain grayscale (greyscale value of 34%), we use this point as the whitest point of the image. At this time, all the pixels in the image produce a certain color shift and brightness change with respect to this point, and we can get what we want. For images, the adjusted results are shown in Figure 6. In general, the white point of the image is selected to select the white point of the image. Using this method to adjust the overall brightness of the image mainly depends on the gray level of the selected pixel as a white field. This adjustment has a fool type, as long as the pixel When selected, the brightness of the image will automatically change accordingly. In actual operation, it is difficult for us to select the right pixel as the white field to adjust to the brightness we need. Therefore, we must use this method carefully. The above four brightness adjustment methods, we do a comparison, the third and fourth methods have some limitations, the adjustment effect is not satisfactory, the first and second methods can better achieve image adjustment.

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Figure 3 Using the Curve tool to adjust the image's effect

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Figure 4 Effect of Adjustment Using Levels Tool

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Figure 5 Adjusting the Image Using Brightness/Contrast

Step 2 Deepen the red of the pot. We refer to the results of the adjustment in Figure 4, we can see that the red color of this tile becomes light at the same time as adjusting the overall brightness of the image. It is not very eye-catching. Select a bit of C21% M41% Y45% K53% on the tile basin. To come, red contains more of the opposite color C and black K, but the basic color M and Y content is less, for which we need to deepen. Here we just deepen the red, so choose a tool that is more specific to the specific color. In the selection tool, red is used as an adjustment object to reduce the content of the opposite color green, and to increase the basic color magenta and yellow. The adjustment parameters and results are shown in FIG. 7. Compared with Figure 4, the adjusted results are more eye-catching.

Now, when we look back, comparing Fig. 7 with Fig. 2, we can see that the whole image is very clear. There is no original darkness, and the details of the image can be well represented, giving people a much better feeling than the original.