At present, pictures have become one of the important means of information transmission, but many pictures are not very good quality, such as layer compression, color cast, lack of contrast, etc., mainly because these pictures come from poor picture discs and digital cameras with lower resolution And scanner or internet, so it is necessary to correct these pictures. At present, the most used image correction software is Adobe's image processing software Photoshop, and its leadership in the field of image processing has been accepted by people. Curve is an important color gradation correction tool in Photoshop, many people are relatively unfamiliar with it, here we give a detailed introduction to its role in Photoshop.
The Curve tool has a powerful image color gradation correction function. It can provide any level adjustment, and the level adjustment curve is the core. It can adjust not only the main channel, but also each monochrome channel. The Curve tool is more convenient than the Level tool when adjusting color shift. The Level tool changes the level of the image by moving the light and dark tone buttons, which is more suitable for high-key and low-key adjustment of the image: The Curve tool can add some nodes on the adjustment curve and arbitrarily change the shape of the curve to produce the corresponding level change. The high, medium, and low key have good adjustment effects, which is much more powerful than the Level tool. If Level is coarse adjustment, Curve is fine adjustment. The curve curve should be adjusted according to the characteristics of different manuscripts. It can be adjusted linearly or nonlinearly.
Open the image \ adjust \ curve (shortcut key Ctrl + M) in the Photoshop menu bar, the curve dialog box pops up, as shown in Figure 1. You can see that the tone scale adjustment curve is located in the middle of the dialog box. Its horizontal axis represents the tone scale of the original image (input), and the vertical axis represents the corrected tone scale (output). In the default state, the two coordinate axes range from 0 (black) to 255 (white), and the curve in the coordinate axis is called a tone curve. The initial value (ie, the default state) of the input and output values ​​are consistent, so the tone curve is a straight line of 45 °. The gradient bar below the tone curve is used to describe the gray value of the horizontal axis. At the bottom right of Figure 1, there are three straw tools, namely black field, gray field and white field calibration straws, they can realize the black and white field calibration of the image.
Curve can achieve the following three functions
1. Adjustment of image level
Curve curve can effectively adjust the color depth of the image, especially for the adjustment of some boring images. Let's take an example to see how to use this tool to adjust the image level. Figure 2 is a dull image. The whole image is gray and seems to be raining, so the details in the dark tones of the image are lost, and the contrast of the dark tones is too small and the level is insufficient. To this end, we need to increase the contrast of the dark tone, and at the same time improve the overall brightness of the image, so that the color of each layer becomes lighter. Open the curve dialog box, take the CMYK mixed channel as the adjustment object, and pull down the curve. The adjusted parameters and results are shown in Figure 3. It can be seen that the overall effect of Figure 3 is much better than that of Figure 2. The whole image is brighter, the details are more abundant, and the contrast of the dark tone is also opened.
2. Correct color cast
The Curve tool can correct the overall color shift of the image or the color shift of a certain tone, but the ability to correct a certain color shift is poor. Because it is based on the channel, not the color. Let us look at an example. Figure 4 is a picture of a woman. We take a little C2% M35% Y26% K0% from its face to know that its M is larger than Y. There is such a rule for people's skin color: Oriental and Western people's skin color is mainly M and Y or the main color is red, containing a small amount of C and K. The difference is that Orientals have a yellowish complexion, and their skin tone is slightly larger than M; Westerners have a slightly larger complexion than Y; but their Y and M generally do not exceed 40%, so that they can reflect the complexion Fair-skinned, especially for women, except for special circumstances. It can be seen that Figure 4 shows an Oriental woman, so its Y should be slightly larger than M, and Figure 4 is just the opposite, so we need to lower the M in Figure 4, here we use Curve tool. Open the curve dialog box, select the magenta channel as the adjustment object, and pull down the curve to get the desired result. The adjusted parameters and results are shown in Figure 5. Compared with Figure 4, the skin color is slightly yellowish and more white and tender, which is in line with the skin color of Orientals. The adjustment of the M version here will affect the gray balance of the entire image, so the adjustment range should not be too large.
3. Black and white field calibration
When determining the black and white field, it should be noted that the black field of the image must have sufficient density, the dot percentage should be above 95%, and it should be the darkest place in the image; the white field should be close to the dead net, and the dot percentage should be below 5%, which is The whitest place in the image. Once a certain point on the image is selected as a black and white field, then the brightness of other pixels on the image will be changed relative to that point. If point a on the image is selected as the white field, the dot percentage of the point with higher brightness than a is 0%, the gradation disappears, and the point with lower brightness than point a will increase the brightness relative to point a. If point b on the image is selected as the black point, the dot percentage of the point with lower brightness than point b is 100%, and the point with higher brightness than point b will decrease the brightness relative to point b. It can be seen that if the black and white fields of the image are not selected properly, the brightness and color of the entire image will change. Taking a woman wearing a white gauze skirt as an example, if a point is selected as a white point on the scarf she wears, the percentage of dots at that point is C27% M18% Y4% K0%. As can be seen from its dot percentage, this dot is not the brightest spot on the image and contains more gray components. Therefore, the dot percentage of the skirt with higher brightness than this dot is 0%, it becomes a white, the layer is lost, and the character's face The level above is also lost. If you choose a point on the white gauze skirt on her chest as a white field, compare the dot percentage Cll% M1% Y2% K0% of the point with other pixels on the image. This point is basically The top is the whitest point on the image, so the overall brightness of the image does not change much from this point, and the brightness is only slightly improved. There is no loss of the level of the skirt and face, the transition is very natural, and because the brightness of the face is increased, the character's skin color is more white and delicate.
The Curve curve can also be drawn as needed to make some special effect words, we will not elaborate here. When using this tool, be sure to clarify your purpose, and then consciously adjust, do not arbitrarily adjust. Only by using this tool in practice, can you have a deeper understanding of its functions.
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