The hottest Photoshop for selective color correcti

2022-08-17
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Photoshop selective color correction

in Photoshop, there are many methods to correct the scanned image, such as adjusting level, curve, hue/saturation, color balance, brightness/contrast, etc. The focus and object of various adjustments are different: lever adjusts the level of the whole image or a certain color channel; Curve focuses on hierarchical adjustment; Hue/saturation focuses on changing the hue and saturation of the colors in the image, which can not only adjust the hue and saturation of the whole image, but also adjust the hue and saturation of the basic color systems (C, m, y, R, G, b) in the image separately; Color balance can adjust the color balance relationship of the image, correct the color deviation in the image that is farther away from the fracture, the smaller the deformation is, or deliberately shift the image to a certain color, which acts on the color

in terms of the two attributes of image hierarchy and color, the focus of the above adjustment is different and has a certain pertinence. In photosho Cui Lixin said P, there is another color correction method - selective color correction (buy an experimental machine just like selective color ad), which is more targeted. It can increase or decrease the percentage of points according to the color of a specific color system in the image without making corresponding choices, and has little or no impact on other colors. This paper intends to conduct an in-depth discussion on relevant issues, hoping to provide some useful information to Photoshop users

first, the reason for selective color correction

it can be said that the fundamental reason is to correct the color error of the scanner. When the scanner recognizes colors, the error size is inconsistent for the colors of different color systems, especially for uniform color blocks, the recognition error caused by the color of lines gives more obvious stimulation to human vision

first, let's look at the flat-panel scanner. The core device of flat-panel scanner is photosensitive detector charge coupled device (CCD). Different CCD has different photosensitivity and noise, which directly affect the resolution of the scanner, the detected tone range, color information and so on. In fact, the quality of CCD used in different grades of scanners is different. Generally, CCD has the following defects:

① cheap CCD is sensitive to environmental noise, which will distort the detected color, especially the decrease of color saturation

② CCD has the phenomenon of "crosstalk". When "crosstalk" occurs, the colors of adjacent pixels will interfere with each other, resulting in the overall color distortion

③ the dynamic density range of CCD is small, which makes the flat-panel scanner reflect the hue distortion of high-density areas

④ the sensitivity of CCD devices to various colors is different. The recognition accuracy of flat-panel scanner for red and green systems is higher than that of blue, purple and yellow systems

look at the drum scanner. The key components of the drum scanner are the color filter and photomultiplier in the scanning head. Figure 1 shows the spectral transmittance characteristic curve of a group of scanner main filters. It can be seen that the color filter does not have 100% transmittance in the spectral region it should pass through, but has a certain error. At the same time, because the transmittance of the three color filters in their respective spectral regions are different, their recognition rates of different colors are also different. Figure 2 shows the corresponding characteristic curve of photomultiplier tube spectrum. It can be seen that its signal receiving sensitivity to different visible spectra is also different

in addition, the spectral energy distribution of the light source of the scanning head is also different in each splash length. The nickel tungsten lamp radiates the strongest red light, followed by green light, and the smallest blue violet light

the above three parts of the drum scanner will cause color errors during scanning because of different spectral reflectance and transmittance. Among them, the most obvious ones that give people a visual sense are uniform color patches, lines or some colors with high saturation

second, the theoretical basis and function of selective color correction

the theory of chromatics refers to the three primary colors of pigments (C, m, y) as primary colors, the pairwise mixing of the three primary colors as secondary colors (i.e., R, G, b), and more than three mixed colors as compound colors. For multicolor, the primary color is the color whose hue is closest to that of the primary or secondary color. For example, multicolor M90%, Y90%, C90%, K90%, whose hue is red, will mainly drive the rapid development of automotive electronics, smart home appliances, equipment industry and other fields, and the color is red. If the complex color is gray, the main color is neutral gray

the colors on an image can be divided into nine color systems: C, m, y, R, G, B, W, K, neutral (neutral gray). Selective color correction is to correct the colors in the image according to this classification

perform selective color u data fading and select a certain color system in 馛 m, y, R, G, B, W, K, neutral as the adjustment object, and increase or decrease the dot percentage (C, m, y, K) of each pixel in its color system, without affecting the colors of other color systems. The magnitude of the color change of the same color system is related to the degree to which the color is close to the main color. The closer it is to the main color, the greater the magnitude of the change. For example, to selectively adjust the R color system of the four color blocks in Table 1, to reduce the percentage of C in the R color system, when the four color blocks are in one image, the reduction of C is completed at one time, and the results are shown in Table 1

in addition, the change of the color in the same color system as the selected corrected color can be controlled by adjusting the amplitude. Table 2 shows the changes of the 4 color blocks in Table 1 when the percentage of C decreases less. It can be seen that there is no change in C in the colors far away from the main color

the convenience of selective color correction is that there is no need to make a selection area for the correction of specific colors in the image, and it has little or no impact on other colors

III. operation method of selective color correction

enter the image/adjust/selective color menu in Photoshop, and an operation dialog box as shown in Figure 3 will appear. In the color drop-down menu, you can select the main color to be corrected (including C, m, y, R, G, B, neutral, W, K and other color systems), and then adjust the small triangle at C, m, y, K to increase or decrease the percentage of C, M, y, K of the selected color. Note that there are two adjustment methods, namely, relative and absolute under method. When relative is selected, the closer it is to the main color, the greater its change. For example, two green blocks Y100%, C82%, Y100% and C50%, when G is selected as the main color and C is reduced by 48%, they become Y100%, C50%, Y100% and C38% respectively, and the change range of the first color C is larger. However, when making relative changes, if the original percentage of a color block is 0, there is no effect. If you want to add m to the above two green blocks, selecting relative cannot be achieved

when absolute is selected, its change range is larger than that of relative under the same adjustment range. For example, when the above two green blocks are reduced by 48% C, the two color changes are Y100%, C82% → Y100%, C43%, Y100%, C50% → Y100%, C26%. For the original color version without dots, you can add a certain number of dots. If green is selected as the color for selective correction, m should be increased by 31%, and the changes of the two green blocks are: Y100%, C82% → Y100%, M19%, C82%, Y100%, C50% → Y100%, M11%, C50%. Note that the increase is still different, which is related to the approximation between the color and the main color

when making selective color correction, there may sometimes be many colors to be adjusted, which can be completed at one time without having to carry out color by color, which can improve work efficiency

IV. examples

the following examples illustrate the operation and effect of selective color correction

the scanned manuscript is a packaging bag, the color is close to "bag 4", the vegetables on the packaging bag are dark green, the trademark colors are Y100%, M100%, the text color is Y100%, and the background color is gradually changed to cyan

after scanning, the image is "bag 1", the trademark color is y98%, M100%, C8%, K9%, the text color is y99%, C29%, and the bottom color gradually changes to C46% and M20% at the top. From the data, red is dark red, yellow is green yellow, and cyan turns blue purple

adjust the above problems as follows:

first, correct the trademark. Take red as the main color of selective color correction, reduce the percentage of C and K, and appropriately increase the percentage of Y. It needs to be done twice to achieve the goal. After adjustment, it is "bag 2"

the second step is to use yellow as the main color for selective color correction of text. Because some pixels in vegetables are yellow, use the square selection tool to box the yellow text. To subtract the C in the yellow text, it needs to be done 4 times to achieve the goal. After adjustment, the image is "bag 3"

step 3: adjust the background color, select blue purple as the main color for selective correction, and subtract m from it. The adjusted image is "bag 4"

v. problems that should be paid attention to when making selective color correction

1. Pay attention to the adjustment range, check the color data at any time, and don't affect the colors you don't want to change

2. The purpose is often not achieved at one time, and it can be completed only after multiple adjustments

3. In order not to affect the change of image hue, generally, the relative mode should be selected for correction

4. When making selective color correction for an object, it may be necessary to change and adjust the main color with the color change, which can improve the work efficiency. If a certain C color contains more m, B color can be used as the main color to correct the selective color and reduce M. when the content of M is small, C can be selected as the main color for selective color correction

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