Prepress Image Processing: Color Modeling in Photoshop (1)

In Photoshop, the color mode determines the color model used to display and print Photoshop documents. Photoshop's color mode is based on building models that describe and reproduce colors well. Common models include HSB (express hue, saturation, brightness); RGB (red, green, blue); CMYK (cyan, magenta, yellow, black); and CIE L*a*b*. Photoshop also includes patterns for special color output, such as indexed colors and two-tones.
In addition to determining the number of colors that can be displayed in an image, the color mode also affects the number of channels and file size of the image.

Based on human perception of color, the HSB model describes three basic characteristics of color:

Hue is the color that is reflected from or transmitted through the object. On a standard color wheel from 0 to 360 degrees, hue is measured by position. In normal use, hue is identified by a color name, such as red, orange, or green.

Saturation, sometimes called chroma, refers to the intensity or purity of a color. Saturation represents the proportion of gray 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.

• Brightness is the relative darkness of the color and is usually measured as a percentage from 0% (black) to 100% (white).

Although you can use the HSB model in Photoshop to define a color in the Palette or Color Picker dialog box, there is no HSB model for creating and editing images.

The vast majority of the visible spectrum can be represented by red, green, and blue (RG trichromatic light is a mixture of different ratios and intensities. Cyan, magenta, and yellow are produced where the colors overlap. RGB color synthesis produces white. They are also called Adding colors.Adding all the colors together to produce white—that is, all light is reflected back to the eye.Additions are used for lighting, video, and displays. For example, the display emits light through red, green, and blue phosphors to produce color.

Photoshop's RGB mode uses the RGB model to assign an intensity value ranging from 0 (black) to 255 (white) to the RGB component of each pixel in the color image. For example, a bright red may have an R value of 246, a G value of 20, and a B value of 50. When the values ​​of the three components are equal, the result is gray. When all components have a value of 255, the result is pure white; when all values ​​are 0, the result is pure black.

The RGB image uses only three colors and reproduces up to 16.7 million colors on the screen. The RGB image is a three-channel image, so each pixel contains 24 bits (8 × 3). The default mode for new Photoshop images is RGB, and computer monitors always use the RGB model to display colors. This means that when working in a non-RGB color mode (such as CMYK), Photoshop temporarily converts the data to RGB data and displays it on the screen.

Theoretically, pure cyan, magenta (M), and yellow (Y) pigments can synthesize and absorb all colors and produce black color. For this reason, these colors are called subtractive colors. Because all printing inks contain some impurities, these three inks actually produce a kind of earthy gray that must be mixed with the black (K) ink to produce true black. (K is used instead of B to avoid confusion with blue.) Mixing these inks to produce a color is called four-color printing.

Color reduction (CMY) and additive color (RG are complementary colors, each subtractive color produces an additive color, and vice versa.

In Photoshop's CMYK mode, each print ink for each pixel is assigned a percentage value. The brightest (highlight) color distribution has a lower percentage of printing ink color, and the darker (dark) color assigns a higher percentage value. For example, bright red may contain 2% cyan, 93% magenta, 90% yellow, and 0% black. In a CMYK image, pure white is produced when the values ​​of all four components are 0%.