Photoshop color correction

As the most important application software in the field of image processing, photoshop is already one of the tools that the majority of computer designers and prepress personnel must master. It has powerful image processing and design functions. This article aims to make an introduction to the color correction function in photoghop and illustrate it with examples.

1. Image quality analysis

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

(1) Level. The gradation refers to the natural change range of an image from light to dark, or the gradient of the natural density change of the image from light to dark. The level of printed matter refers to the level of brightness that can be recognized within the range of copy density. The more levels, the richer the level. In an image, the level of the image is divided into bright tones, middle tones, and dark tones. We can see the image's hierarchical distribution from the histogram in Photoshop. If the pixels in the image are mostly concentrated on the right side of the histogram, the brighter part of the image is more. If they are concentrated on the left side, the darker parts are more concentrated in the middle and the middle tone is more. As shown in Figure 1, it is a picture and its histogram, from which we can see that the picture does not lack layers in dark and bright tones, but there is almost no transition in the middle tone of the whole picture, making it from dark to bright The change is very abrupt, which is the biggest shortcoming of the image storage level.

Figure 1 View the hierarchical distribution of the image through the "histogram"

(2) Color. The image color analysis includes two aspects: 1 to determine whether there is a color deviation, and 2 to determine which colors should be paid special attention to in the prepress processing.

(3) Clarity. In addition to the layers and colors of the image, we must also pay attention to the clarity of the image. See if the image is blurry, so that the details of many images can not be shown. Or the image has noise and noise, making the image unsmooth.

2. Photoshop color correction function

Photoshop has a powerful color correction function. Here we will introduce its color correction function in three aspects.

(1) Level correction function For the level problems in the image, there are two tools that can realize the level correction and the curve level 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 Tune. At the same time it can also realize the determination function of black and white field and neutral gray. When determining the black and white field, it should be noted that the black field of the image must have sufficient density, the dot percentage should be more than 95%, it should be the darkest place in the image. The whitest place in the world. Compared with the color scale tool, the curve tool has more powerful functions in adjusting levels. The curve tool can achieve the correction of the level by pulling up or down the curve and adding any number of control points in the middle, which can not be achieved by the color scale tool. If the tone scale is coarse adjustment, the curve tool is fine adjustment. Of course, in addition to these two tools can adjust the level, there is also an automatic color scale 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 realize the color correction function of the image, such as color balance, brightness / contrast, hue, saturation, replacement color, optional color, channel mixer, etc. The color balance tool is designed according to the principle of adding basic colors and decreasing opposite colors when calibrating colors. It can realize the adjustment of the colors of different gradations (bright, medium, and dark) without affecting other gradations, as well as the brightness protection function. However, this tool is difficult to control colors. When you want to adjust a certain color, it often affects other colors, so generally only use this tool for fine-tuning. The brightness / contrast tool is mainly used to adjust the brightness, gradation and contrast of the image. Hue / saturation is to adjust the image from the angle of hue, lightness, and saturation of the color. It can adjust all three colors of the image or the specific attributes of a specific color. When adjusting a certain color, it will not affect other colors. Strong selectivity. The Replace Color tool is a search and replace command. Its function is to create a mask in the image. The "tolerance" is used to adjust the size of the mask. First use the eyedropper tool in the "selection" to pick up the color to be replaced (this color will be displayed in the "sampling" in the "transformation" area, when the color is corrected, the adjusted color will be displayed in the "sampling"), Then you can correct its hue, saturation, and lightness in the Transform area. The optional color tool adjusts the color from the dot percentage of the four-color version, which is more suitable for printing. It can adjust the color for cyan, magenta, yellow, green, blue, white, black, and neutral colors in the image. , When one color is corrected, it basically does not affect other colors, has a very strong selectivity, and is the most commonly used tool in color correction. There are two methods for color correction, relative and absolute. The relative adjustment of color is small, and the absolute adjustment is large. For example, adjust the magenta color of 60% magenta by 30%. If the relative method is adopted, the final magenta color is 76% [60% + 60% x30%]. Using the absolute method, the final magenta color is The 90% (60% + 30% dry channel mixer) can add and subtract the color information of several channels, which can effectively adjust the image color cast.

(3) Sharpness correction function. Sharpening and de-noising tools are often used to adjust the sharpness of images in Photoshop. Sharp Chemical has USM sharpening, further sharpening, sharpening edges and sharpening. We often use USM sharpening. There are three parameters in the dialog box: counting crab, radius and threshold. Quantity refers to the degree of sharpness emphasis. The greater the number, the greater the emphasis. The value is between 0% and 500%. Radius refers to the action radius of sharpness enhancement, that is, the calculation of the resolution of a certain pixel as the center, the larger the radius, the greater the effect of sharpness enhancement. However, under normal circumstances, the radius should not be too large, otherwise a very hard boundary will appear. Threshold refers to the smallest grayscale difference in sharpness, that is, only the grayscale value of adjacent pixels is greater than the given threshold, will there be USM sharpening effect. Obviously, the larger the threshold, the less obvious the USM sharpening. Some noises such as noise, moire, and scratches appear on the image. In order to remove them and smooth the image, a denoising tool is needed. Denoising and sharpening are two processes that are exactly opposite in principle. Excessive sharpness will result in noise, and denoising will reduce the sharpness. The most commonly used denoising tool is the dust and scratch tool, which has two parameters radius and threshold, the meaning is the same as the USM sharpening tool.

3. Case 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. Below we take a look at some examples to see how to use the color correction tools in Photoshop to achieve image color correction.

Example 1

Figure 2 Original Figure 1

Figure 2 is a bonsai image. Since the tone of the entire image is too dark, some details on the wall are not reflected. In addition, the image is too dark and unclear, which makes people feel depressed. Our main job is to increase the brightness of the image and increase the contrast of the image, so that the details of the dark tone are revealed. Step 1 Increase the brightness of the entire image. There are four methods. First, use the curve tool to increase the brightness of the image. The volume operation is as follows: open the curve tool, select the CMYK mixed channel as the adjustment object, and pull down the tone curve of the image. The adjustment result is shown in Figure 3. Second, use tools. The method is to open the color scale tool as follows, select the CMYK mixing channel as the whole object, and move the brightness key of the input color scale to the left. At this time, the brightness of the entire image will increase. Third, use the brightness \ contrast tool to adjust the brightness of the entire image. In the image \ adjustment, find the brightness \ contrast tool, drag the brightness button to the right, the brightness of the entire image will increase accordingly, the adjustment results shown in Figure 5. Using this tool can adjust the brightness of the entire image, if the adjustment is small, there is no big problem. However, if the adjustment range is large, it is often easy for us to consider the brightness adjustment range in the picture. If a large-scale adjustment is required, we should 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 tools are available in the curve tool or the color scale tool. Here, open the color scale tool, select the white eyedropper tool, select a point C44% M24% Y18% K4% in the upper left part of the image A certain gray level (gray value is 34%), we use this point as the whitest point of the image, at this time all pixels in the image have a certain color shift and brightness change relative to this point, we can get what we think For the desired image, the adjusted result is shown in Figure 6. In general, the white point of the image is selected by selecting the whitest point of the image. Using this method to adjust the overall brightness of the image mainly depends on the grayscale of the pixels selected as the white field. This adjustment is a fool type, as long as the pixel Select the point and the brightness of the image will change accordingly. In the actual operation, it is difficult for us to select the appropriate pixel point as the white field to adjust to the brightness we need, so using this method to adjust, we should use it carefully. We make a comparison between the above four brightness adjustment methods. The third and fourth methods have certain limitations, and the adjustment effect is not satisfactory. The first and second methods can achieve better image adjustment.

Figure 3 Use the "curve" tool to adjust the effect of the image

Figure 4 The effect of using the color scale tool to adjust

Figure 5 Use "Brightness / Contrast" to adjust the image

Step 2 Deepen the red color of the tile basin. We refer to the adjustment results in Figure 4, we can see that the red color of this tile basin becomes lighter while adjusting the overall brightness of the image, which is not very eye-catching. Choose a little C21% M41% Y45% K53% on the tile basin By the way, red contains more of the opposite colors C and black K, while the basic colors M and Y content is less, so we need to deepen it. Here we only deepen the red, so choose a selection tool that is more specific to the specific color. In the selection tool, use red as the adjustment object, reduce the content of the opposite color cyan, and increase the basic colors magenta and yellow. The adjusted parameters and results are shown in Figure 7. Compared with the adjusted result in Figure 4, the red color is more eye-catching.

Now let's go back and make a comparison between Figure 7 and Figure 2. The whole image is very clear, without the original grayness, and the image details can be well expressed, giving people a much better feeling than the original image.

Example 2

Example 1 mainly adjusts the tone of the image, and example 2 mainly adjusts the color shift of the image.

Figure 8 is an image of a bird. The whole picture feels good, but the feathers of the bird are yellowish and not so white. The yellow should be lowered. At this time, the color of the feather is white with more Y, and we can choose C38% M26% Y50% K11% from the feather to verify it.

Figure 6 Use the black and white field positioning tool to adjust the effect of the image

Figure 7 The effect of deepening the tile basin

Step 1: Correct the yellow color cast. Here you can use the selection tool or the curve tool to correct the yellow color cast. First, use the selection tool to adjust. There are two methods: first, use white as the adjustment object to reduce the yellow on the white: second, use yellow as the adjustment object, directly reduce the yellow. Use the first method to use the relative adjustment method. The yellow has been reduced by 100%, but the yellow on the painting is not obvious. There are still a lot of yellow casts, and due to the reduction of yellow, the feathers appear a light blue, making the feathers more white. The second method is used to reduce the yellow color, the yellow color cast disappears obviously, the effect is very good, and the feathers of the birds are also very white. Repeat 3 times repeatedly, the yellow color cast can basically disappear completely. It can be seen that when adjusting the selection tool, it is very important to choose what color to correct. However, when the second method is used to reduce the yellow, the yellow of the bird's head also changes. After the original bird's head is selected, C2% M33% Y97% K0% is adjusted to C2% M33% Y34% KO% The yellow horizon has been lowered, but it does not matter, we will repair the yellow of the bird's head in future adjustments. Next, use the curve tool to correct the yellow color. There is too much yellow content in the whole image, so as long as the curve is adjusted, the amplitude should not be too large, otherwise the unsightly blue will appear on the feathers. Of course, when using the curve tool to reduce the content of the yellow version, the yellow of the head It will also be reduced, but the reduction is very small and will not affect the appearance of the image.

Step 2 Improve the whiteness of feathers

After correcting the color cast, the feather on the picture is still not white enough. You can increase its whiteness. Still use the selection tool, select white as the adjustment object, and at the same time reduce the amount of CMY three inks, feathers appear more white. At the same time, pay attention to the adjustment, the amount of CMY three color versions should not be reduced too much, otherwise the white level of feathers will be loss. Once lost, it is more difficult to recover.

Step 3 Adjust the head yellow

Now let's adjust the color of the bird's head. Select a bit of C0% M30% Y30% K0% on the head, it can be seen that the head is quite red, in order to restore the gray color to yellow, you need to open the distance between magenta and yellow, use the selection tool In order to adjust the object, keep the magenta unchanged, and increase the yellow to repeat the operation several times, the yellow will gradually reveal, as shown in Figure 8.

Figure 8 (1) Original picture (2) Final adjustment effect

Finally, we can see that after the image is adjusted, the feathers of the bird are very white, and the original yellow is lost, which achieves our ultimate goal. The adjustment of clarity is relatively simple, because of space issues, we will not give examples here.

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