Pixel image and vector graphics in the pre-process

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Pixel images and vector graphics in prepress processing

prepress information processing software used to accelerate the transformation and upgrading of key materials is mainly divided into three categories, namely, image processing, graphics processing and graphic typesetting software. Photoshop software is mainly used to process pixel images; Illustrator, CorelDRAW and freehand software are called vector graphics processing software. The image composed of pixels is often referred to as bitmap, image processing is generally understood as the processing of bitmap, while graphics processing is generally understood as the processing of vector graph. This paper will briefly explain the characteristics, differences, connections and mutual transformation of the two

pixel image

image generally refers to the image that reflects the continuous density of scenery, people, etc., and its main feature is to reflect the real face. Usually, image information is collected into the computer by digital scanning equipment (scanner) or digital photography equipment (digital camera) and becomes a digital image

when scanning the original, the scanner should collect color information point by point and line by line, that is, divide the continuous picture into discontinuous pixel lattice, as shown in Figure 1. Generally, the color scanner can collect the R, G and B signals of a certain point on the original. When the digital camera is exposed, the photographed image is divided into a large number of pixels (such as 3million pixels, 5million pixels, etc.), and each pixel is also composed of R, G and B signals. When storing images, the computer should also store the R, G and B information of all points

since printing usually requires four-color inks of C, m, y and K to copy colors, it is necessary to convert RGB mode image color separation into CMYK mode image. After color separation, each pixel of CMYK mode image contains four data. The so-called "color separation" is to decompose and convert the color of each pixel of the original image (generally three data in RGB or l*a*b* mode) into data suitable for printing and distinguished by ink color (four data in CMYK mode). Commonly used Photoshop software has a color separation module, which is used to complete the printing color separation of dot matrix

if the computer represents each data contained in each pixel with 1 byte, the total amount of data for RGB or l*a*b* mode images is 3 times the total number of pixels; For images in CMYK mode, the total amount of data is 4 times the total number of pixels. Obviously, the whole image takes up a lot of storage space. Of course, for pure black-and-white "1-bit image", each pixel only occupies 1/8 of 1 byte, that is, 1 byte can contain 8 pixels

for example, the original is 8 inches × 1 is the maximum 0 inch photo that can be accepted by the host. The number of lines added for printing and copying is 175 lines/inch, and the magnification of the printed matter is required to be 2. The scanning resolution of the original is required to be 2.0 × one hundred and seventy-five × 200% = 700ppi, where 2.0 is the quality factor. When scanning this photo, the number of pixel rows obtained is 10 × 700 = 7000 rows, and the number of pixel columns obtained is 8 × 700 = 5600 columns, and the total number of pixels in this image is 39.2 million (7000 × 5600 = 39.2 million). If the data volume of RGB or l*a*b* mode image is 112mb, the four-color image after color separation can reach 150MB. The higher the printing accuracy is, the larger the print size is, the more points need to be sampled per unit length on the original, and the larger the storage capacity of the computer is. Similarly, the uncompressed trichromatic image data obtained by a 5-megapixel digital camera is 15MB

pixel images have their own inherent resolution. Over amplification will lead to mosaic phenomenon or make the image not clear enough, and some information of sampling points will be lost when shrinking

vector graphics

computer graphics are shape information based on a mathematical model, which is characterized by the structure of shapes, as shown in Figure 2A. In computer graphics, regular graphics (rectangle, circle, ellipse, etc.) are described by relevant parameters; Free graphs are generally composed of nodes, lines and curves. The curves can be described by Bezier function or B-spline function (as shown in Fig. 2b), which can be described by function parameters, node coordinates, etc. In short, the vector graph can be regarded as a set of mathematical formulas stored by the computer, or the parameters of the mathematical function related to the large suction resistance of the oil pump

for example, if a circle is drawn with graphics processing software on the computer, the computer does not store the color of each coordinate point on the circle when storing, but generally only remembers the coordinates of the center of the circle, the radius of the circle, the filling in the circle and the color information of the outline

The characteristic of

graphics is that it has nothing to do with resolution, but only when it is output or displayed, the dot matrix imaging is carried out according to the resolution of the equipment. Because the graphics are stored in the form of mathematical formulas, there will be no virtual halo or mosaic when the graphics are enlarged or reduced

the color processing of graphics is usually called color marking, not color separation. For example, if you print a red circle, according to the superposition principle of four-color ink, as long as the Y version is 100%, the M version is 100%. Therefore, when marking the color, you can specify that the four-color version of this circle is 100% y, 100% m, and 0 for both versions C and K. Since the screen cannot truly reflect the color after printing, the color shall be marked with reference to the chromatography

the relationship between font and graphics

the font of text belongs to graphics, but this kind of graphics is very special. When people see them, they will produce semantic information in their minds

in order to display or record the text on the computer and output it to various media, its font information needs to be stored, which forms a font. In fact, a font is a data body that stores character shape information

the early font is lattice font, which can also be called image font. Its disadvantage is the same as that of the image, and jagged edges will appear after magnification. Therefore, early dot matrix font libraries were generally equipped with several sets of font libraries according to different font sizes. Its word making program uses some image processing tools

the current font is generally a vector font, which can also be called a graphic font. Because its processing method is basically the same as that of graphics, the font can be regarded as a set of mathematical formulas and parameters stored in the computer. The advantage of this font is that it is undistorted when zoomed in or out

the font is also processed according to the standard color. Word formation processing is often seen in computer word library programs, and some graphic processing tools are also used in word formation. The font is divided into the font for screen display and the font for laser Imagesetter output. Its accuracy and price are different. The screen display font is only used for typesetting, so the required accuracy is not very high

conversion between pixel image and vector graphics

it is very common to convert graphics into visible images. Draw a graph in the drawing software, and the computer will convert the drawn graph into the image on the screen at any time. In rip, according to the graphic description of page description language and the recording resolution of the recording device, the vector graphics are converted into the information of recording exposure, and the visible image (film or printing plate) is formed through the recording device

images can also be converted into graphics, which is called "vectorization of images". The CorelDRAW suite includes a CorelTrace software that can be used to convert pixel images into vector graphics

put the pixel image shown in Figure 1 into CorelTrace and set the corresponding parameters (complexity, speckles filtering, number of colors, node reduction, node type, minimum object size, etc.). During the rainy season, you should pay more attention to wiping, which can be converted into vector graphics, as shown in Figure 3, and Figure 4 is the image part

only by correctly understanding the characteristics of pixel images and vector graphics, and using appropriate processing software and means to process them, can we obtain better image and text reproduction effect

source: keyin media -- "printing technology"

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