A paper image manuscript is scanned and input to the computer. If the color seen on the screen is different from the color on the manuscript, we call it color cast. This method is based on the color difference that can be felt by human eyes. But for the digital manuscript image, it is inaccurate to observe with the human eye without the comparable physical objects. At this time, it is necessary for the computer to accurately tell us whether the image is color shifted, so as to make targeted corrections. This article mainly analyzes how to analyze the color cast of images in PhotoShop software without considering the color space of hardware devices.
1 Basics of color theory
In the color system, the three primary colors of red (R), green (G) and blue (B) are mixed according to the brightness value of the color from 0 (black) to 255 (white). Kinds of colors. Among the three colors, when the brightness values ​​of the three colors are equal, gray is generated; when the three brightness values ​​are 255, pure white is generated; and when all the brightness values ​​are 0, pure black is generated.
On the color wheel, red and green are mixed to get the middle color yellow; red and blue are mixed to get the middle color magenta; green and blue are mixed to get the middle color cyan. .
The three primary colors of the colorant, cyan (C), magenta (M), yellow (Y), and black (K), are mixed according to the percentages of different ink concentrations to form a print or printing color. Among the three colors, when the ink concentrations of the three colors are equal, a gray color is produced; when the concentrations of the three inks are all 100%, a gray-black color is produced (due to impurities in the ink itself, only gray-black color can be obtained, and Not pure black); when all three inks are 0, white (ie no ink) is produced.
On the color wheel, cyan and magenta are mixed to get the middle color blue; cyan and yellow are to get the middle color green; magenta and yellow are to get the middle color red.
The above six colors of red (R), green (G), blue (B), cyan (C), magenta (M), and yellow (Y) are reflected on a color wheel, and the relationship between them is:
1. The diagonal colors on the color wheel are complementary to each other. For example, the complementary color of blue is yellow.
2. The two colors separated on the color wheel are mixed to get the intermediate color of the two colors, such as red and green to get yellow.
3. In R, G and B, one of the two colors is mixed to obtain the complementary color of another color, such as red and green to get the complementary color of blue is yellow.
4. In C, M, and Y, one of the two colors is mixed to obtain the complementary color of another color, such as cyan and magenta to get the complementary color of yellow is blue.
For an RGB color mode image, when there is a lot of color information at a certain point on the image, the image is biased toward this color. However, when the color information is little or 0, the image is biased to the complementary color of this color.
Also for the CMYK color mode image, when there is a lot of color information at a certain point on the image, the image is more focused on this color. However, when the color information is few or 0, the image is more focused on the complementary color of this color. [next]
2 According to the histogram of the image to determine the color cast of the image
In the PhotoShop software, open the image histogram palette. In the expanded view of the histogram, the "average" of the composite channel of the image is the average brightness value of the entire image. If the "average" of a single color channel is close to it, it means No color cast; if the "average" of a single color channel differs greatly from it, it generally indicates that the image is color cast.
For images in RGB color mode, if the "average" of a single color channel is greater than the "average" of a composite channel, this image is biased toward this color; if the "average" of a single color channel is greater than the "average" of a composite channel "Small" indicates that this image is biased towards the complementary color of this color.
The CMYK color mode image is opposite to the RGB color mode image. If the "average" of a single color channel is greater than the "average" of a composite channel, it indicates that the image is biased to the complementary color of this color; if the "average" of a single color channel "Is smaller than the" average "of the composite channel, indicating that the image is biased towards this color.
3 Identify the color cast of the image based on the neutral gray theory
From the perspective of color theory, equal amounts of red (R), green (G), and blue (B) and equal amounts of cyan (C), magenta (M), and yellow (Y) inks will produce gray.
R255 + B255 + G255 = White R254 + B254 + G254 = Gray ...
R128 + B128 + G128 = Gray ...
R1 + B1 + G1 = Gray R0 + B0 + G0 = Black C0 + M0 + Y0 = White C1 + M1 + Y1 = Gray ...
C50 + M50 + Y50 = Gray ...
C99 + M99 + Y99 = grey C100 + M100 + Y100 = grey black
Under normal light irradiation, if the pixels at a position that should be gray on an image do not display gray, then the image must be color cast.
The above two methods of analyzing the color cast of the image through the histogram of the image and the neutral gray theory can effectively help us analyze the color cast of the image in the prepress job. Color theory is the basis for analyzing image color cast. In a computer, the color information of an image is expressed in the form of numbers. If the color theory is inaccurate, it is difficult to analyze the color cast of the image. In addition, the mastery of color theory is also the basis for our later adjustment of image color cast, so readers should strengthen their study of color theory.
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