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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.
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 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.
How to Use Photoshop for Selective Color Correction
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.