In computer image processing, the image must be represented by data, and color space is the method of representing color by data. For a color representation, there can be multiple data representation modes, namely multiple color spaces. Some color spaces represent the colors independently of the performance of the color rendering equipment used, and are called device-independent color spaces (such as Lab and XYZ, etc.); some color space representations of colors vary with the color rendering equipment , Called the color space associated with the device (such as RGB, CMYK, HSB, etc.). The following is an in-depth interpretation of the color space commonly used in color science.
The color space is based on the visible spectrum
We can only see a small part of the electromagnetic spectrum, which is usually called the visible spectrum. We can see all light, because light is defined as the part of the electromagnetic spectrum that we can see. The color model attempts to describe the colors we see and process. Each color model represents a different method for describing and classifying colors. All color models use numerical values ​​to represent the visible spectrum. The color range that can be generated using a specific color model (such as RGB or CMYK) is the color space. Other color models are HSL, HSB, Lab, and XYZ.
â— RGB color space
The basic law of color mixing shows that any color in nature can be produced by mixing and matching the three primary colors of red, green, and blue. This can be geometrically formed by R, G, and B, the three perpendicular axes The coordinate system is called RGB tristimulus space, or RGB color space. For any color, it can be matched with a certain proportion of red light, green light and blue light.
RGB color space
The RGB color space uses the additive color mixing method because it describes the ratio of various "lights" to produce colors. The light continues to add color from dark to dark. Common color spaces based on RGB mode are sRGB, Adobe RGB and Adobe Wide Gamut RGB. Since different devices have different color display results for the same image, the RGB color space is the color space related to the device.
â— CMYK color space
The CMYK color space describes the values ​​of the four inks cyan, magenta, yellow, and black. Based on the light absorption characteristics of ink printed on paper. When white light strikes the translucent ink, some visible light wavelengths are absorbed (subtracted), while other wavelengths are reflected back to the eye. These colors are therefore called subtractive colors.
CMYK color space
Theoretically, cyan (C), magenta (M) and yellow (Y) pigments can be synthesized to absorb all light and produce black. Since all printing inks have some impurities, these three inks will actually produce earthy brown. Therefore, in addition to cyan, magenta, and yellow inks, black ink (K) is used in four-color printing. And the CMYK color space is the same as the RGB color space, and is a color space related to the device. [next]
â— HSB color space
The HSB (or HSV) color space is somewhat similar to Monsell's hue, value, and chromaticity system. It also uses three similar axes to define colors. HSB is derived from the RGB color space and is the color space associated with the device. The three basic color features in HSB are as follows:
H: Hue is the color reflected from or transmitted through the object. On the standard color wheel from 0 to 360 degrees, the hue is measured by position. In normal use, the hue is identified by the color name, such as red, orange, or green.
S: Saturation (sometimes called chroma) refers to the intensity or purity of the color. Saturation represents the proportion of gray components in the hue, which is measured as a percentage from 0% (gray) to 100% (fully saturated). On the standard color wheel, the saturation increases from the center to the edge.
B: Brightness is the relative lightness and darkness of the color, usually measured from a percentage of 0% (black) to 100% (white).
HSB color space
Because the HSB model can directly reflect the relationship between colors, it is very suitable for color design. Most design software provides this color model, including the Windows system palette. This color model is also used.
â— Lab color space
Lab color space (also known as CIE Lab) is one of the most common color spaces for measuring the color of objects, and can be widely used in all fields. It is one of the uniform color spaces and was formulated by CIE in 1976. In this color space, L is the brightness and a and b are the chromaticity coordinates.
Lab color space
The picture above shows the a and b chromaticity diagrams. In this figure, a and b indicate the color direction: + a is the red direction, -a is the green direction, + b is the yellow direction, and -b is the blue direction. The center is the achromatic zone; when the a and b values ​​increase, the color point moves away from the center and the color saturation increases.
The advantage of the CIELAB uniform color space is that when the color difference of the color is greater than the visual recognition threshold (just perceivable) but less than the color difference of the adjacent poles in the Munsell system, it can better reflect the psychological color of the object Feel the effect. Moreover, CIELAB's color table is intuitive and can evaluate colors intuitively. And because CIELAB is a color space that has nothing to do with the device, in color management, this feature can be used to communicate and calculate the correspondence between the original color, screen color and printing color in the color space, to achieve the visual consistency of colors and achieve different Color conversion between devices. [next]
â— XYZ color space
Because the "standard chromaticity observer" used to calibrate the spectral colors, negative stimulus values ​​appeared, which was inconvenient to calculate and difficult to understand. Therefore, in 1931, CIE established three imaginary primary colors X, Y, and Z based on the RGB system. A new chroma system. It is matched to the tristimulus value of the isoenergy spectrum, and is named as "CIE1931 standard chromaticity observer spectral tristimulus value", referred to as "CIE1931 standard chromaticity observer". This system is called "CIE1931 standard chromaticity system" or "2 ° field of view XYZ chromaticity system".
XYZ color space
The CIEXYZ color space is slightly transformed to obtain the Yxy color space, where Y takes the value of Y among the three stimulus values, which means brightness, and x, y reflect the color chromaticity characteristics. The XYZ tristimulus value and the associated Yxy color space form the basis of the current CIE color space. The concept of XYZ tristimulus value is based on the ternary theory of color vision, which shows that the human eye has a receiver that receives three primary colors (red, green, and blue), and all colors are regarded as a mixture of the three primary colors. The XYZ tristimulus value is calculated using these standard observer color matching functions and is a device-independent color space.
Summary: We all know that there are many ways to measure a color, and each method is measured in a coordinate system called a color space. The above color spaces are the most common color conversion spaces in color management, and color management is to solve the problem of color conversion and matching between various devices, so it is very important to understand them and their relationship.
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