Photoshop color correction
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As the most important application software in image processing, photoshop has become one of the tools that computer designers and prepressers must master. It has powerful image processing and design functions. This article aims to introduce the color correction function in photoghop and explain it by example.
1, image quality analysis
Before we can process an image, the first thing we need to do is to analyze the image quality.
(1) Level. Hierarchy refers to the range of natural variation of an image from light to dark, or the natural density of images from light to dark. The level of printed matter refers to the level of lightness that can be recognized within the range of copy density. The more the level of the print, the richer the level. In an image, the layers of the image are highlighted, midtone, and dark, and 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 the focus is on the left side, the darker parts are more concentrated in the middle, and the intermediate tones are more. As shown in Figure 1, is a picture and its histogram, from which we can see that the picture is not lacking in the dark and bright, but the whole picture has almost no intermediate adjustment, so from the dark to the bright The change is very abrupt, which is the biggest shortcoming of the image storage hierarchy.
(2) Color. The color analysis of the image includes two aspects: 1 to determine whether there is color cast, and 2 to determine which colors are of particular interest in prepress processing.
(3) clarity. In addition to the level and color of the image, we also pay attention to the clarity of the image. Look at the image so that the details of many images are not shown. Or the image is noisy and mottled, making the image not smooth.
2, photoshop color correction function
Photoshop has a powerful color correction function, and its color correction function is introduced in three aspects.
(1) Hierarchical correction function For the hierarchical problems existing in the image, there are two tools to achieve the level of correction of the gradation and curve gradation to adjust the hierarchical 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 should have sufficient density. The percentage of the dot is above 95%. It should be the darkest place in the image. The white field should be close to the net. The percentage of the dot is below 5%. The whitest place in the middle. Compared to the Levels tool, the Curve tool has more powerful features in adjusting the hierarchy. The curve tool can achieve level correction by pulling up or down the curve and adding as many control points in the middle, which is not possible with the level tool. If the color scale is a coarse adjustment, the curve tool is a fine adjustment. Of course, in addition to these two tools can adjust the level, there is also an automatic 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 perform image color correction functions such as color balance, brightness/contrast, hue, saturation, replacement color, optional color, channel mixer, and more. The color balance tool is designed to increase the basic color and reduce the opposite color according to the color correction. It can adjust the color of different tone (light, medium and dark) without affecting other tones, as well as brightness protection. However, it is difficult to control the color of the tool. When you want to adjust a certain color, it often affects other colors. Therefore, it is generally only used to fine-tune the tool. The Brightness/Contrast tool is mainly used to adjust the brightness, level and contrast of the image. Hue/Saturation adjusts the image from the hue, brightness, and saturation of the color. It adjusts all the colors of the image or the three properties of a specific color. When a certain color is adjusted, 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 and the Tolerance is used to adjust the size of the mask. First use the pipette tool in the "Selection" to pick up the color to be replaced (this color will be displayed in the "Sampling" section of the "Transform" area. When this color is corrected, the adjusted color will be displayed in "Sampling"). The hue, saturation and lightness can then be corrected in the Transform area. The optional color tool adjusts the color by adjusting the percentage of dots in the four-color version. It is more suitable for printing. It can color the cyan, magenta, yellow, green, blue, white, black and neutral colors in the image. It corrects one of the colors and does not affect other colors. It has strong selectivity and is the most commonly used tool for correcting colors. There are two ways to color it, relative and absolute. The relative adjustment of the color is small, and the absolute adjustment is large. For example, adjust its magenta by 60% of magenta by 30%. If the relative method is used, the final magenta is 76% [60%+60%x30%). In absolute terms, the final magenta is 90% (60% + 30% dry channel mixer can add and subtract the color information of several channels, can effectively adjust the image color cast.
(3) Sharpness correction function. Sharpening and removing tools are often used to adjust the sharpness of images in photoshop. Sharp Chemicals has USM sharpening, further sharpening, sharpening edges and sharpening. We often use USM sharpening, which has three parameters in the dialog: number of crabs, radius and threshold. The quantity refers to the degree of emphasis emphasis. The larger the quantity, the greater the emphasis, and the value is between O% and 500%. The radius refers to the radius of action emphasized by the sharpness, that is, the calculation of the definition of the resolution centered on a certain pixel. The larger the radius, the greater the effect of the sharpness emphasis. However, in general, the radius should not be too large, otherwise a very hard boundary will appear. The threshold value refers to the minimum gray level difference emphasized by the sharpness, that is, only the gray value of the adjacent pixel is larger than a given threshold, and the USM sharpening effect is obtained. Obviously, the larger the threshold, the less obvious the USM sharpening. Some noises appearing on the image, such as mottled, moiré, scratches, etc., are used to remove the image to achieve smoothness of the image, and a denoising tool is required. Denoising and sharpening are two opposite processes in principle. When the sharpness emphasizes excessive, noise will appear, and denoising will reduce the definition. The most commonly used denoising tool is the Dust and Scratch tool, which has two parameter radii and thresholds, meaning the same as in 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 image correction in photoshop. Let's take a look at some examples to see how to use the color correction tool in photoshop to achieve image color correction.
Some details on the wall were not reflected because the tone of the entire image was too dark. In addition, this image is too dark and unclear, giving people a feeling of depression. 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 reveal step 1 to increase the brightness of the entire image. There are four ways to do this. 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 gradation curve. Second, use tools. The method is as follows. Open the level tool, select the CMYK mixing channel as the whole object, and move the light button of the input level to the left. At this time, the brightness of the whole image will increase, and the adjustment result is shown in Fig. 4. The third brightness/contrast tool adjusts 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, the adjustment results are shown in Figure 5. This tool can be used to adjust the brightness of the entire image. If the adjustment is small, there is not much problem. However, if the adjustment is large, it is often easy to make the brightness of the adjustment when considering the map. If you need a large adjustment, you should consider using other adjustment tools. Fourth, the image black and white field positioning tool is used to adjust the brightness. The black and white field calibration tool is available in the curve tool or the level tool. Open the level tool here, select the white pipette tool, and select a point C44% M24%Y18% K4% in the upper left part of the image. The point is blue and has A certain gray level (the gray value is 34%), we use this point as the white point of the image. At this time, all the pixels in the image produce a certain color shift and brightness change relative to this point, we can get what we think. The desired image, the adjusted result is 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 depends mainly on the gray level of the selected pixel as the white field. This adjustment has a fool type, as long as the pixel When the point is selected, the brightness of the image will automatically change accordingly. In actual operation, it is difficult to select the appropriate pixel as the white field to adjust to the brightness we need. Therefore, we should use it carefully. The above four brightness adjustment methods, we make a comparison, the third and fourth methods have certain limitations, the adjustment effect is not satisfactory, the first and second methods can better achieve image adjustment.
Use the Curve tool to adjust the effect of the image
Adjust the effect using the Levels tool
Adjust the image using Brightness/Contrast
Step 2 Deepen the red color of the tile. Referring to the results of the adjustment, we can see that the red color of the tile becomes lighter while adjusting the overall brightness of the image. It is not very conspicuous. Select a C21% M41% Y45% K53% on the tile. You can see that The red contains more of the opposite color C and black K, while the basic color M and Y are less, so we need to deepen it. Here we only deepen the red color, so choose a selection tool that is more specific to the specific color. In the selection tool, red is used as the adjustment object, the content of the opposite color cyan is lowered, and the basic color magenta and yellow are improved. The adjustment parameters and results are shown in Fig. 7. The adjusted result is more eye-catching than Figure 4.
Now we make a comparison, the whole image is very clear, no original gray, the image details can also be very good, giving people a much better feeling than the original.
Mainly adjust the image tonality, and Example 2 mainly adjusts the color cast of the image.
The image of a bird, the whole picture gives people a good feeling, but the bird's feathers are yellowish, not so white, and should be lowered. At this time, the color of the feather is white with more Y. We can choose a point from the feather to choose C38% M26% Y50% K11%.
Use the black and white field positioning tool to adjust the effect of the image
Deepen the effect of 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 ways to do this: first, reduce the yellow color on white with white as the adjustment object: second, adjust the yellow color as the adjustment object, and directly reduce the yellow color. The first method uses the relative adjustment method. The yellow color is reduced by 100%, but the yellow color reduction on the painting is not obvious. There is still a large amount of yellow color deviation, and due to the decrease of yellow color, the feathers have a faint blue color, which makes the feathers more white. The second method is used to lower the yellow color, and the yellow color is obviously disappeared. The effect is very good, and the bird's feathers are also very white. Repeated 3 times, the yellow color shift can basically disappear completely. It can be seen that when making a selection tool adjustment, it is important to choose which color to correct. However, when the second method is used to reduce the yellow color, the yellow color of the bird's head also changes. On the original bird's head, a little C2% M33% Y97% K0% is adjusted to become C2% M33% Y34% KO%, visible, The yellow border is lowered, but it doesn't matter. The adjustment of the future will fix the yellow of the bird's head. Second, use the curve tool to correct the yellow color. The content of yellow is too much on the whole image, so as long as the curve is adjusted, the amplitude is not too large, otherwise the feather will appear blue, which is difficult to see. Of course, when the curve tool is used to reduce the content of the yellow plate, the yellow of the head It will also decrease, but the reduction will be small and will not affect the appearance of the image.
Step 2 Improve the whiteness of the feathers
After correcting the color shift, the feather on the map is still not white enough to improve its whiteness. Still use the selection tool, choose white as the adjustment object, and reduce the amount of CMY three inks, the feathers appear whiter and at the same time pay attention to the adjustment, the amount of CMY three color plates should not be reduced too much, otherwise the white level of feathers will loss. Once it is lost, it is more difficult to recover.
Step 3 Adjust the yellow of the head
We are here to adjust the color of the bird's head. Select a point C0% M30%Y30% K0% in the head, you can see that the head is quite red, in order to restore the color of a gray, you need to open the distance between magenta and yellow, use the selection tool, in red In order to adjust the object, keep the magenta unchanged, increase the yellow color and repeat the operation a few times, the yellow color gradually emerges, as shown in Figure 8.
Finally, we can see that the image has been adjusted, the bird's feathers are very white, without the original yellow, to achieve our ultimate goal. The adjustment of the definition is relatively simple, and the problem is in the space. Here we will not give examples.