Several Color Modes of Digital Images (I)

In course teaching, the “digital image” we usually refer to is a work drawn with graphics, image processing software or digital tools. Its types include two-dimensional "digital images" (mainly using software such as Photoshop, painter, and Coreldarw) and three-dimensional "digital images" (mainly using 3dsmax, Maya, and other software). In the image and graphics processing software, several color models of HSB, RGB, LAB, and CMYK are generally used, and there are a plurality of color modes to reflect different color ranges, and many of these modes can be converted to each other with corresponding commands. For ease of description, this article mainly uses Photoshop as an example.

A color model in color design and processing software

The color of the image can be formulated by different color matching methods. Various color matching methods are called color models.

(a) HSB model

The HSB color model is a set of color models based on the visual characteristics of human eyes in daily life, and it is closest to human thinking on color recognition. The HSB color model uses hue (H), saturation (S), and brightness (B) to describe the basic characteristics of the color:

1. Hue (H) refers to the color reflected from or transmitted through the object. On a standard color wheel from 0 to 360 degrees, hue is measured by position (see figure).

Green yellow

Cyan Red

Blue magenta

In normal use, hue is identified by a color name, such as red, orange, or green.

2. Saturation (S) refers to the intensity or purity of the color. Saturation represents the proportion of color components in the hue, measured as a percentage from 0% (gray) to 100% (fully saturated). On the standard color wheel, the saturation from the center to the edge is increasing.

3. Brightness (B) is the relative lightness or darkness of a color and is usually measured as a percentage from 0% (black) to 100% (white). Due to the limitation of the device, when it is displayed on a computer screen, it is converted to an RGB model and converted to a CMYK model as a printout. This limits the use of the HSB model to some extent.

(b) RGB model

The vast majority of the visible spectrum can be represented by a mixture of different ratios and intensities of red, green, and blue (RGB) trichromatic light. The RGB color model relies on mixing different red, green, and blue values ​​to create most of the colors in a chromatogram. The RGB model is primarily used for images displayed on computers. In the RGB mode, each image element (pixel) within the image is assigned three values ​​between 0 and 255, representing red, green, and blue, respectively. In the RGB model, the three color components can represent almost any color by adjusting, including black, white, and various shades of gray. For pure black, these 3 RGB values ​​are set to 0 (minimum); for white, all 3 values ​​are set to 255 (maximum); gray is set to the same value by setting all 3 values set up. All WEB images are initially developed in the RGB model because the format is suitable for all graphics on the computer. The color of the computer reproduced on the display is achieved by red, green, and blue light.

The RGB color model is one of the most widely used color systems. The RGB color gamut is very wide, and cannot be fully expressed with ordinary pigments, but ordinary color separation printing cannot be performed.

(III) Lab Model

The Lab color model was established on the basis of the International Standard for Color Measurement developed by the International Commission on Illumination (CIE) in 1931. In 1976, this model was revised and named CIE Lab. The Lab color design is device-independent and regardless of the device used to create or export the image (such as a monitor, printer, computer, or scanner), the colors produced by this color model are consistent. Lab color consists of the psychological brightness component (L) and two chrominance components. These two components are the a component (from green to red) and the b component (from blue to yellow).

(d) CMYK model

The CMYK model is based on the light-absorbing properties of ink printed on paper. When white light strikes a translucent ink, part of the spectrum is absorbed and part of it is reflected back to the eye.

Each pixel in the image is synthesized by different ratios of four colors: indigo (C), magenta (M), yellow (Y), and black (K). Each printing ink for each pixel is assigned a percentage value, the brightest (highlight) color distribution is assigned a lower printing ink color percentage value, and the darker (dark) color is assigned a higher percentage value. Theoretically, three pigments of indigo (C), magenta (M), and yellow (Y) can synthesize and absorb all colors and produce black color. For this reason, the CMYK model is called a subtractive model. Because all printing inks contain some impurities, these three inks actually produce an earthy gray that must be mixed with the black (K) ink to produce true black (use K or Bk instead of B to avoid confusion with blue) .

In color software, when the parameters of one model change, the parameters of other models also change.