Photoshop color correction: restore the image of the real color

As the most important application software in the image processing field, photoshop is one of the tools that computer designers and prepressors must master. It has powerful image processing and design functions. This article aims to introduce the function of color correction in photoghop and illustrate it through examples.

Image quality analysis

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

(1) Level. Hierarchy refers to the natural change range of an image from bright to dark, or the natural density change gradient from bright to dark. The level of print refers to the level of recognizable lightness within the range of copy density. The more levels, the richer the level. In an image, there are bright, middark, and dark tone levels in the image. 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 concentrated on the left side, the darker parts are mostly concentrated in the middle, and the middle tones are more.

(2) Colors. The color analysis of the image includes two aspects: (1) judging whether there is a color cast, (2) determining which colors need special attention in prepress processing.

(3) Sharpness. In addition to the level and color of the image, we also need to pay attention to the clarity of the image. See if the image is blurry enough that the details of many images don't show up. Or the image is noisy and mottled so that the image is not smooth.

Photoshop color correction function

Photoshop has a powerful color correction function, here its color correction function is divided into three aspects to introduce one by one.

(1) Level correction function For the level problem in the image, there are two tools that can achieve the level of correction color scale and curve color scale 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. Tone. At the same time, it can also achieve black and white field and neutral gray determination function. In determining the black and white field, it should be noted that the black field of the image should have sufficient density, and the percentage of halftone dots should be above 95%. It should be the darkest part of the image. The white field should be close to the net, and the percentage of the dots is less than 5%. It is an image. The whitest place. Compared with the color grading tools, the curve tool has a more powerful function in adjusting levels. The curve tool can implement level corrections by pulling up or down the curve and adding any number of control points in the middle. This is not achievable by a color scale tool. If the color scale is coarse, the curve tool is fine-tuned. Of course, in addition to these two tools to adjust the level, there is also an automatic color 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 implement image color correction functions such as color balance, brightness/contrast, hue, saturation, replacement colors, selectable colors, channel mixers, and so on. The color balance tool is designed based on the principle of increasing the basic color and reducing the opposite color when coloring. It can adjust the colors of different tones (bright, medium, and dark) without affecting other tones, as well as the brightness protection function. However, this tool is difficult to control the color. To adjust a color, it often affects other colors. Therefore, the tool is generally used only for fine adjustment. The brightness/contrast tool is mainly used to adjust the brightness, darkness, and contrast of the image. Hue/Saturation adjusts the image from the hue, lightness, and saturation of the color. It can adjust all the colors of the image or the three properties of a specific color. When adjusting a color, 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. Tolerance adjusts the mask's size. First pick up the color to be replaced with the eyedropper tool in the “selection area” (this color will be displayed in “Sampling” in the “Transform” area. When the color is corrected, the adjusted color will be displayed in “Sampling”). Then the hue, saturation and lightness can be corrected in the "Transform" area. The optional color tool adjusts the color by adjusting the percentage of four-color dots, and is more suitable for printing. It can correct the color of cyan, magenta, yellow, green, blue, white, black and neutral colors in the image. Correcting one of the colors basically does not affect other colors and is extremely selective. It is the most common tool for correcting colors. There are two methods for its coloring, relative and absolute. The relative adjustment of the color is small and the absolute adjustment is large. For example, adjust its magenta color to 60% of magenta, and increase it by 30%. If you use the relative method, the final magenta color is 76% (60%+60% x 30%). In absolute terms, the final magenta color is 90% (60%+30% dry channel mixer can add or subtract color information from several channels, which can effectively adjust image color cast.

(3) Sharpness correction function. Adjusting the sharpness of the image in photoshop often uses sharpening and removal tools. Sharp Chemicals has USM sharpening, further sharpening, sharpening edges and sharpening. We often use USM sharpening, which has three parameters in its dialog box: Number Crab, Radius, and Threshold. Number refers to the degree of clarity of the emphasis, the greater the number, the greater the emphasis on the value of its value in the O% -500%. Radius refers to the radius of the role of sharpness emphasis, that is, with a certain pixel as the center of calculation, to achieve a clear range of adjustment, the greater the radius, the greater the role of clarity of emphasis. However, under normal circumstances, the radius should not be too large, otherwise there will be a very hard border. The threshold value refers to the minimum grayscale difference emphasized by the sharpness, that is, only if the gray value of the neighboring pixel is greater than a given threshold, there will be a USM sharpening effect. Obviously, the greater the threshold, the less pronounced the sharpening of the USM. Some noises appearing on the image such as noise, moiré, scratches, etc., need to use denoising tools in order to remove them and achieve smooth images. Denoising and sharpening are exactly the opposite of the two principles. When the sharpness emphasizes too much, noise will appear, and noise reduction will reduce the sharpness. The most commonly used noise removal tool is the dust and scratch tool, which has two parameter radii and thresholds, the same meaning as in the USM sharpening tool.

Source: HC Network Printing Industry Channel

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