Definition of saturation, hue, and contrast in image processing
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In image processing, common color models include HSB (Hue, Saturation, Brightness), RGB (Red, Green, Blue), CMYK (Cyan, Magenta, Yellow, Black), and CIE, and so the corresponding colors The patterns are RGB, CMYK, Lab, and so on. In the HSB color model, hue, saturation, and contrast are basic descriptions of image attributes.
saturation
Saturation can be defined as the saturation divided by the brightness, and the same degree of saturation as the color represents the degree of color deviation from the same brightness gray. Note that it is not the same concept as chroma. However, since the color saturation determines the same effect that appears in the human eye, the same concept is used for the color saturation and saturation. 
Saturation refers to the vividness of color, also known as the purity of color. The degree of saturation depends on the ratio of the color component and the decolor component (gray color) in the color. The larger the color component, the greater the saturation; the greater the achromism component, the smaller the saturation. Pure colors are highly saturated, such as bright red, bright green. Mixed with white, gray or other shades of color, it is an unsaturated color, such as purple, pink, yellowish brown, etc. The fully unsaturated color has no shade at all, such as various grays between black and white.
Saturation
Used to evaluate the visual properties of components of pure color in the entire color vision (including achromatic colors). It is related to (or is closely related to) the physical psychology of darkness. It depends on the amount of white light mixed in the color light. The higher the amount of pure spectral light, the higher it is.
tone
Color tone refers to the overall tendency of the picture color in a picture, which is a large color effect. In nature, we often see such a phenomenon: objects of different colors are enveloped in a golden sunlight, or are enveloped in a hazy, light blue moonlight of muslin; It is enveloped in the charming golden yellow of autumn; or it is unified in the silver-white world of winter. This kind of color on different objects, enveloped with a certain kind of color, so that different colors of objects have the same color tendency, this color phenomenon is the color tone. 
Basic meaning
Hue refers to the wavelength of light reflected by an object, which is determined by which wavelength predominates. Different wavelengths produce different color sensations. Hue is an important feature of color tone, which determines the essential characteristics of color essence.
Hue does not refer to the nature of color, but a general assessment of the overall color of a painting. Hue refers to the basic tendency of the color appearance of a work. Among the three elements of lightness, purity (saturation), and hue, certain factors play a leading role. We call it a certain hue. Although a painting uses a variety of colors, there is a general tendency that it is bluish or reddish, warm or cold, and so on. This tendency in color is the tone of a painting. The hue of a work can usually be defined in terms of hue, lightness, warmth, and purity.
The color tone is divided into warm and cool colors in terms of heating and cooling: red, orange, and yellow - warm colors, symbolizing: the sun and the flame. Blue - a cool tone, symbolizing: forest, sea, blue sky. Black, purple, green, and white are the midtones; the warmer the brightness is, the warmer the overall impression is, the warmer it is, and the higher the coolness, the cooler the overall feel. Cold and warm colors are only relatively speaking. For example, in the red color system, when red and red are together, red is a warm color, while red is considered a cold color, and when rose red and violet are both present Rose is warm color.
Contrast
Contrast refers to the measurement of the different brightness levels between the lightest and darkest blacks in the light and dark areas of an image. The larger the difference is, the greater the contrast is. The smaller the difference is, the smaller the contrast is and the better the contrast is. : 1 can easily display vivid, rich colors, when the contrast ratio up to 300:1, it can support all levels of color. However, since the ratio suffers from the same predicament as the brightness, there is no effective and fair standard to measure the ratio. Therefore, the best way of identification is to rely on the user's eyes. 
In a dark room, the brightness at the white screen (at the brightest time) is divided by the brightness at the black screen (at the darkest time). More precisely, the contrast ratio is to subtract the white signal at 100% and 0% saturation, and divide by the white value of Lux (lightness, ie, lux, lumens per square meter) as the unit of measurement at 0% ( The 0% white signal is actually black) and the resulting value. The contrast is the division of the white and black brightness units. Therefore, the brighter the white and the darker the black, the higher the contrast. Strictly speaking, the contrast we refer to is the ratio of the brightest (white) and darkest (black) brightness of the same point on the screen, but usually the contrast of the product is for the entire screen. For example, a screen is completely white. The brightness of the screen is 500 cd/m2, and the brightness of the full black screen is 0.5 cd/m2. The contrast of the screen is 1000:1.