In Photoshop, there are many ways to correct the scanned image, such as adjusting Level, Curve, Hue/Sateration, Color Balance, Brightness/Contrast, and so on. The focus and object of the various adjustments are different: Lever adjusts the level of the entire image or a color channel; Curve is focused on level adjustment; Hue/Sateration focuses on changing the hue and saturation of colors in the image. , It can adjust the hue and saturation of the whole image. It can also adjust the hue and saturation of the basic color system (C, M, Y, R, G, B) separately. Color Balance can adjust the color of the image. Balance the relationship, correct the color cast in the image or deliberately shift the image to a certain color, which is the effect of the color.
From the aspect of the image's level and color, the above adjustments are not the same and have certain pertinence. There is also a color correction method in Photoshop - Selective Color Adjust, more targeted, it can be a percentage of the color of a particular color in the image to increase or decrease the number of dots, without the need to make the appropriate choice , and have little or no effect on other colors. This article intends to conduct in-depth discussion on relevant issues, hope to provide some useful information for Photoshop users.
First, the reason for the selective color correction can be said that the fundamental reason is to correct the color error of the scanner. When the scanner identifies a color, the error size is inconsistent with respect to the color of different colors, especially for uniform color blocks, the identification error caused by the color of the line gives the visual stimulation to the human eye more clearly.
First, let's look at the flatbed scanner. The core device of the flatbed scanner is a photosensitive detector-charge coupled device (CCD). Different CCDs have different photosensitivity and noise, and they directly affect the resolution of the scanner, the hue range of detection, and color information. In fact, the quality of the CCD used by scanners of different grades is different, and the general CCD has the following defects:
1 Cheap CCDs are sensitive to environmental squeaks, which can distort the detected colors. The obvious manifestation is the decrease of color saturation.
2CCD has a "crosstalk" phenomenon. When "crosstalk" occurs, colors interfere with each other between adjacent pixels, thereby distorting the overall color.
3CCD has a small dynamic density range, which makes flatbed scanners reflect tonal distortion in high-density areas;
The sensitivity of 4CCD devices to various colors is different. Flat panel scanners have higher recognition accuracy for red and green systems than blue-violet and yellow systems.
Looking at the roller scanner again, the key components of the roller scanner are the color filters and the photomultiplier in the scanning head. Figure 1 shows the spectral transmission characteristics of the main filter of a set of scanners. It can be seen that the color filter does not have 100% transmittance in the spectral region it should transmit, but there is a certain error. At the same time, because of the difference in the transmittance of the three color filters in their respective spectral regions, their recognition rates for different colors are also different. Figure 2 shows the corresponding characteristic curve of the photomultiplier tube. It can be seen that the signal receiving sensitivity to different visible spectra is also different.
In addition, the spectral energy distribution of the scanning head light source is also different in each of the long-sweeping segments. The nickel-tungsten lamp has the strongest radiation red light, followed by the green light, and the blue-violet light is the smallest.
The above three parts of the roller scanner will cause color errors during scanning because of the different transmittance of the spectral reflectance goods. The most noticeable visual sensation is uniform color blocks, lines or some highly saturated colors.
Second, the theoretical basis of selective color correction and functional color theory to color the three primary colors (C, M, Y) as a primary color, the two primary colors of the two colors called the secondary color (ie, R, G, B), The three or more mixed colors are called complex colors. For a complex color, which color is closest to the hue and primary or secondary color, the main color is the color. For example, multicolor M90%, Y90%, C90%, K90%, and the color reddish, the main color is red. If the complex color is gray, the dominant color is neutral gray.
The colors on an image can be divided into nine color systems such as C, M, Y, R, G, B, W, K, and Neutrals. Selective color correction is targeted to correct the colors in the image according to this classification.
Select the color of each color in the selected color (U, Fa, G, B, W, K, and Neutral) and the percentage of dots of each pixel in the color system (C, M, Y). K) Increase or decrease without affecting the colors of other colors. The magnitude of the color change in the same color system is related to the degree to which the color is close to the main color. The closer to the main color, the greater the change. For example, for the selective color adjustment of the R color system of the four color patches in Table 1, to reduce the C percentage in the R color system, when the operation is performed with four color patches in one image, the reduction operation of C is completed once. See Table 1.
In addition, the color of the same color as the selected corrected color can be controlled by adjusting the amplitude. Table 2 shows the changes of the four color blocks in Table 1 when the percentage of C drops less. It can be seen that there is no change in C in the color far from the main color.
The convenience of selective color correction is that the correction of a particular color in an image does not require a selection area and has little or no effect on other colors.
Third, the selective color correction operation method In Photoshop enter the Image / Adjust / Selective Color menu, the operation dialog box shown in Figure 3 appears. In the Color drop-down menu, you can select the main color to be corrected (including C, M, Y, R, G, B, Neutrals, W, K, etc.), and then adjust the small triangles at C, M, Y, and K. It is possible to increase or decrease the percentage of C, M, Y, K for the selected color. Note that there are two methods of adjustment, Relative (Relative) and Absolute (absolute) under Method. When Relative is selected, the closer it is to the main color, the greater the change. For example, two green blocks Y100%, C82% and Y100%, C50%, when G is selected as the main color and C is reduced by 48%, they become Y100%, C50% and Y100%, C38% respectively, and the first color is C. Big changes. However, when making Relative changes, if the original percentage of a color block is 0, there is no effect. To add M to the above two green blocks, selecting Relative cannot be achieved.
When Absolute was selected, the change range was greater than Relative in the same adjustment range. If the above two green blocks are also reduced by 48%C, the two color changes are Y100%, C82%→Y100%, C43%, Y100%, C50%→Y100%, and C26%. And for the original color version without a dot, you can add a certain number of outlets. If you choose green as the color for selective correction, M will increase by 31%. The change of two green blocks is: Y100%, C82%→Y100%, M19%, C82%, Y100%, C50%→Y100%, M11%, C50%. Note that the magnitude of the increase is still not the same, or is it related to how close the color is to the dominant color.
When performing selective color correction, there may be many colors to be adjusted at one time, which can be completed in one operation. It is not necessary to perform one color and one color, which can improve work efficiency.
IV. Example The operation and effect of selective color correction are illustrated by examples below.
The scanning manuscript is a packaging bag, the color is close to the "bag 4", the vegetables on the packaging bag are dark green, the trademark color is Y100%, M100%, the text color is Y100%, and the background color gradient is a cyan gradient.
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 gradient is C46%, M20%. From the data point of view, red is dark red, yellow is green yellow, and cyan becomes blue-purple.
Adjust the above problem as follows:
First correct the mark. Using red as the dominant color for selective color correction reduces the percentage of C and K while increasing the percentage of Y appropriately. To do it twice to achieve the goal, after the adjustment is "bag 2".
The second step is to use yellow as the dominant color for selective color correction of text. Since some of the pixels in the vegetables are colored yellow, use a square selection tool to frame the yellow text. To subtract C from the yellow text, it takes 4 times to achieve the goal. After adjustment, the image is "Bag 3."
The third step is to adjust the background color, and choose the blue-violet color as the main color for selective correction, minus the M. The adjusted image is "pocket 4".
Fifth, for selective color correction should pay attention to the problem 1, pay attention to the extent of adjustment, at any time to check the color data, do not want to change the color has an impact.
2. When adjusting, the goal cannot be achieved once. It needs to be adjusted several times before it can be completed.
3, in order to not affect the image tone changes, under normal circumstances should choose Relative mode correction.
4. For selective color correction of an object, it may be necessary to change the main color with the color change, which can improve the work efficiency. If a C color contains more M, B can be the main color to start the selective color correction, reduce M, when the M content is small, you can choose C as the dominant color for selective color correction.
From the aspect of the image's level and color, the above adjustments are not the same and have certain pertinence. There is also a color correction method in Photoshop - Selective Color Adjust, more targeted, it can be a percentage of the color of a particular color in the image to increase or decrease the number of dots, without the need to make the appropriate choice , and have little or no effect on other colors. This article intends to conduct in-depth discussion on relevant issues, hope to provide some useful information for Photoshop users.
First, the reason for the selective color correction can be said that the fundamental reason is to correct the color error of the scanner. When the scanner identifies a color, the error size is inconsistent with respect to the color of different colors, especially for uniform color blocks, the identification error caused by the color of the line gives the visual stimulation to the human eye more clearly.
First, let's look at the flatbed scanner. The core device of the flatbed scanner is a photosensitive detector-charge coupled device (CCD). Different CCDs have different photosensitivity and noise, and they directly affect the resolution of the scanner, the hue range of detection, and color information. In fact, the quality of the CCD used by scanners of different grades is different, and the general CCD has the following defects:
1 Cheap CCDs are sensitive to environmental squeaks, which can distort the detected colors. The obvious manifestation is the decrease of color saturation.
2CCD has a "crosstalk" phenomenon. When "crosstalk" occurs, colors interfere with each other between adjacent pixels, thereby distorting the overall color.
3CCD has a small dynamic density range, which makes flatbed scanners reflect tonal distortion in high-density areas;
The sensitivity of 4CCD devices to various colors is different. Flat panel scanners have higher recognition accuracy for red and green systems than blue-violet and yellow systems.
Looking at the roller scanner again, the key components of the roller scanner are the color filters and the photomultiplier in the scanning head. Figure 1 shows the spectral transmission characteristics of the main filter of a set of scanners. It can be seen that the color filter does not have 100% transmittance in the spectral region it should transmit, but there is a certain error. At the same time, because of the difference in the transmittance of the three color filters in their respective spectral regions, their recognition rates for different colors are also different. Figure 2 shows the corresponding characteristic curve of the photomultiplier tube. It can be seen that the signal receiving sensitivity to different visible spectra is also different.
In addition, the spectral energy distribution of the scanning head light source is also different in each of the long-sweeping segments. The nickel-tungsten lamp has the strongest radiation red light, followed by the green light, and the blue-violet light is the smallest.
The above three parts of the roller scanner will cause color errors during scanning because of the different transmittance of the spectral reflectance goods. The most noticeable visual sensation is uniform color blocks, lines or some highly saturated colors.
Second, the theoretical basis of selective color correction and functional color theory to color the three primary colors (C, M, Y) as a primary color, the two primary colors of the two colors called the secondary color (ie, R, G, B), The three or more mixed colors are called complex colors. For a complex color, which color is closest to the hue and primary or secondary color, the main color is the color. For example, multicolor M90%, Y90%, C90%, K90%, and the color reddish, the main color is red. If the complex color is gray, the dominant color is neutral gray.
The colors on an image can be divided into nine color systems such as C, M, Y, R, G, B, W, K, and Neutrals. Selective color correction is targeted to correct the colors in the image according to this classification.
Select the color of each color in the selected color (U, Fa, G, B, W, K, and Neutral) and the percentage of dots of each pixel in the color system (C, M, Y). K) Increase or decrease without affecting the colors of other colors. The magnitude of the color change in the same color system is related to the degree to which the color is close to the main color. The closer to the main color, the greater the change. For example, for the selective color adjustment of the R color system of the four color patches in Table 1, to reduce the C percentage in the R color system, when the operation is performed with four color patches in one image, the reduction operation of C is completed once. See Table 1.
In addition, the color of the same color as the selected corrected color can be controlled by adjusting the amplitude. Table 2 shows the changes of the four color blocks in Table 1 when the percentage of C drops less. It can be seen that there is no change in C in the color far from the main color.
The convenience of selective color correction is that the correction of a particular color in an image does not require a selection area and has little or no effect on other colors.
Third, the selective color correction operation method In Photoshop enter the Image / Adjust / Selective Color menu, the operation dialog box shown in Figure 3 appears. In the Color drop-down menu, you can select the main color to be corrected (including C, M, Y, R, G, B, Neutrals, W, K, etc.), and then adjust the small triangles at C, M, Y, and K. It is possible to increase or decrease the percentage of C, M, Y, K for the selected color. Note that there are two methods of adjustment, Relative (Relative) and Absolute (absolute) under Method. When Relative is selected, the closer it is to the main color, the greater the change. For example, two green blocks Y100%, C82% and Y100%, C50%, when G is selected as the main color and C is reduced by 48%, they become Y100%, C50% and Y100%, C38% respectively, and the first color is C. Big changes. However, when making Relative changes, if the original percentage of a color block is 0, there is no effect. To add M to the above two green blocks, selecting Relative cannot be achieved.
When Absolute was selected, the change range was greater than Relative in the same adjustment range. If the above two green blocks are also reduced by 48%C, the two color changes are Y100%, C82%→Y100%, C43%, Y100%, C50%→Y100%, and C26%. And for the original color version without a dot, you can add a certain number of outlets. If you choose green as the color for selective correction, M will increase by 31%. The change of two green blocks is: Y100%, C82%→Y100%, M19%, C82%, Y100%, C50%→Y100%, M11%, C50%. Note that the magnitude of the increase is still not the same, or is it related to how close the color is to the dominant color.
When performing selective color correction, there may be many colors to be adjusted at one time, which can be completed in one operation. It is not necessary to perform one color and one color, which can improve work efficiency.
IV. Example The operation and effect of selective color correction are illustrated by examples below.
The scanning manuscript is a packaging bag, the color is close to the "bag 4", the vegetables on the packaging bag are dark green, the trademark color is Y100%, M100%, the text color is Y100%, and the background color gradient is a cyan gradient.
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 gradient is C46%, M20%. From the data point of view, red is dark red, yellow is green yellow, and cyan becomes blue-purple.
Adjust the above problem as follows:
First correct the mark. Using red as the dominant color for selective color correction reduces the percentage of C and K while increasing the percentage of Y appropriately. To do it twice to achieve the goal, after the adjustment is "bag 2".
The second step is to use yellow as the dominant color for selective color correction of text. Since some of the pixels in the vegetables are colored yellow, use a square selection tool to frame the yellow text. To subtract C from the yellow text, it takes 4 times to achieve the goal. After adjustment, the image is "Bag 3."
The third step is to adjust the background color, and choose the blue-violet color as the main color for selective correction, minus the M. The adjusted image is "pocket 4".
Fifth, for selective color correction should pay attention to the problem 1, pay attention to the extent of adjustment, at any time to check the color data, do not want to change the color has an impact.
2. When adjusting, the goal cannot be achieved once. It needs to be adjusted several times before it can be completed.
3, in order to not affect the image tone changes, under normal circumstances should choose Relative mode correction.
4. For selective color correction of an object, it may be necessary to change the main color with the color change, which can improve the work efficiency. If a C color contains more M, B can be the main color to start the selective color correction, reduce M, when the M content is small, you can choose C as the dominant color for selective color correction.
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