Causes of Digital Image Quality Analysis II

Second, the pixel depth

The cable depth refers to the number of bits used to store each pixel (ie, bits), which is also used to measure the resolution of the image. The pixel depth determines the number of possible colors for each pixel of the color image, or determines the number of possible gray levels for each pixel of the grayscale image. The more bits used to represent a pixel, the more colors a pixel can express and the deeper it is. Although the color image can be deep, the deeper the pixel, the greater the storage space required. The pixel depth is too shallow and affects the quality of the image. The image looks very rough and unnatural.

Bits are the basic elements of digital data. Each Bit is either on or off, usually represented by 1 or 0, ie there are only two changes. Each pixel of the scanned image has a pixel depth, such as 1 to 32-bits. The 1-bit image is a black-and-white image (such as the black-and-white line drawing mentioned above). A 4-bit pixel has 4 variations (00 01 10 11), which represents a range of colors from white-light gray-dark gray-black.

An 8-bit pixel can express all the grays in 256-level grayscale, which can be printed by Postscript(R) Level 2 and Level 3 printers. Each pixel of an image is represented by three components: R, G, B. If each pixel is 8-bits deep, then each pixel shares a 24-bit representation and each pixel can be one of 16,777,216 colors.

When a pixel is represented by a value of 32-bits, if R, G, and B are represented by 8-bits, the remaining 8-bits are often called alpha channel bits. There is an alpha channel in Adobe Photoshop software. More commonly, there are four 8-bit channels in a CMYK mode, which are cyan, magenta, yellow, and black.

Third, the image color model

Different color models have different color representations and have an effect on color digital images. The following are the main common color description models.

RGB color model

Red, green, and blue are the three primary colors of color light. The three wavelengths red, green, and blue are the basis of all colors in the natural world. Most of the visible spectrum can be mixed with different ratios and intensities of red, green, and blue (RGB) three-color lights. To indicate that cyan, magenta, and yellow are produced where the colors overlap. Because RGB color light synthesis produces white, the RGB color model is additive. The RGB color model is commonly used for lighting, video, and displays. For example, the system of colors produced on a display has the same basic characteristics as the light produced in nature: colors can be generated from the red, green and blue colors. This is the basis of the RGB color model. Most scanners can also use RGB color models to record digital image data. The color display can emit three kinds of light beams with different intensities, so that the interior of the screen can cover the phosphor materials of red, green and blue colors to generate light. For example, when you see red in Photoshop, the monitor turns on its red beam, and the red beam stimulates red phosphor to display a red pixel on the screen.

In Photoshop, using the RGB color picker, various combinations of red, green, and blue color values ​​can be used to change the color of a pixel. The three primary color values ​​range from 0 to 255. R: 255, G: 255, B: 255 is overlaid to produce white, but R: 0, G: 0, B: O is superimposed to produce black (no color light). R: 185, G: 132, B: 234 The superposition produces the color as shown in the figure.

In connection with the previous knowledge about the pixel depth of the image, 16777216 colors are sufficient for reproducing a crystal clear digital image on a display connected to a computer equipped with 24-bit color, although this is only the color visible in nature. a part of.

CMYK color model

Blue, yellow, and yellow are secondary colors and are complementary colors of red, green, and blue. The CMYK color model is based on the light absorption properties of the ink printed on the paper. When white light strikes the translucent ink, part of the spectrum is absorbed and part of it is reflected back to the eye. Theoretically, pure cyan (C), magenta (M), and yellow (Y) pigments can synthesize and absorb all colors and produce black color. For this reason, the CMYK model is called a subtractive model. But actually the printing ink will contain some impurities. The three inks actually produce an earthy gray that must be mixed with the black (K) ink to produce a true black color (using K or Bk instead of B is to avoid confusion with blue. ). A print made up of 39% cyan, 47% magenta, 0% yellow and 1% black (black absorbs all light), this print will reflect 60% red, 52% green, and 99% blue .

Lab color mode

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 revisited and named CIELab. The Lab color design is device-independent; no matter what device is used to create or output an image (such as a monitor, printer, computer, or scanner), the colors produced by this color mode are preserved. Consistent. The 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). The Lab image is a three-channel image containing 24 (8 x 3) bits/pixel.

You can use Lab mode to process Photo CD images, individually edit height and color values ​​in images, transfer images between systems, and print to Postscript® Level 2 and Level 3 printers. To print Lab images to other color Postscript devices, first convert them to CMYK. In general, Lab colors are internal color modes that Photoshop uses when converting between different color modes.

HSB color mode

HSB is based on human perception of color, not computer values ​​of RGB, nor is it the CMYK percentage of the printer. The human eye considers color to be composed of chroma, saturation, and brightness. The HSB model describes three basic characteristics of color:

1. Hue H, on a standard color wheel from 0 to 360 degrees, hue is measured by location. In normal use, hue is identified by a color name, such as red, orange, or green. The chromaticity is based on the wavelength of the light wave reflected by the object or the wavelength of the light wave transmitted through the object.

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. Saturation is often called the work color. The higher the saturation, the lower the gray component and the higher the intensity of the color.

3. Height B, which is the relative intensity of the color, is usually measured as a percentage from 0% (black) to 100% (white).

The above four color models are several models that are often used in image processing. The color model of the image is different, and of course there is a difference in the color of the image on the table. (to be continued)