Pixel Images and Vector Graphics in Prepress

Commonly used prepress information processing software mainly has three major categories, namely image processing, graphic processing and graphic layout software. Photoshop software is mainly used to process pixel images; Illustrator, CorelDraw, and FreeHand software are called vector graphics processing software. An image composed of pixels is often referred to as a bitmap. Image processing is generally understood as processing of a bitmap, and graphic processing is generally understood as processing of a vector. This article will briefly describe the characteristics, differences, connections, and mutual conversions of the two.


Pixel image


The image generally refers to an image that reflects the continuous density of scenes, characters, etc. The main feature of the image is that it reflects the true appearance. In general, image information is captured into a computer using a digital scanning device (scanner) or digital photography device (digital camera) to become a digital image.
When scanning a manuscript, the scanner collects color information line by line, that is, divides the continuous picture into discontinuous pixel lattices, as shown in FIG. 1 . In general, a color scanner can collect R, G, and B signals at a certain point on the manuscript. When the digital camera is exposed, the captured image is divided into a large number of pixels (eg, 3 million pixels, 5 million pixels, etc.), and each pixel is also It consists of three signals: R, G, and B. The R, G, B information of all points is also stored when the computer stores the image.
Since printing generally requires four colors of C, M, Y, and K inks to reproduce colors, it is necessary to convert image color separations of the RGB mode into images of the CMYK mode. After color separation, each pixel of the CMYK mode image contains four data. The so-called "separation" is to decompose the color of each pixel of the original image (usually 3 data of RGB or L*a*b* mode) into data suitable for printing and ink color separation (for CMYK mode It is 4 data). Commonly used Photoshop software has a color separation module that is used to complete the color separation of bitmaps.
If the computer expresses each data contained in each pixel by 1 byte, the total data amount is 3 times the total number of pixels for the RGB or L*a*b* mode images; for the CMYK mode image, The total amount of data is 4 times the total number of pixels. Obviously, the entire image takes up a lot of storage space. Of course, for a pure black-and-white "1-bit image", each pixel occupies only 1/8 of a byte, that is, 1 byte can contain 8 pixels.
For example, the original is an 8 inch by 10 inch photo, the number of screens printed and reproduced is 175 lines per inch, and the required print magnification is 2. The scanning resolution of the manuscript is required to be 2.0 x 175 x 200% = 700 ppi, where 2.0 is the quality factor. When scanning this photo, the number of rows of pixels obtained is 10×700=7000 rows, and the number of obtained pixel columns is 8×700=5600 columns. The total number of pixels of this image is 39.2 million (7000×5600=39.2 million). . If the data amount of the RGB or L*a*b* pattern image is 112MB, the color image after the color separation may be up to 150MB. The higher the printing accuracy, the larger the format of the print, the more points need to be sampled per unit length on the original, and the greater the amount of storage required by the computer. Similarly, a 5-megapixel digital camera acquires 15MB of uncompressed three-color image data after shooting.
The pixel image has its own inherent resolution. Excessive magnification can cause mosaicism or make the image unclear. When it is zoomed out, some sample points are lost.


Vector Graphics


Computer graphics are physical information based on a mathematical model, characterized by the shape of the structure, as shown in Figure 2a. In computer graphics, regular graphics (rectangular, circular, elliptical, etc.) are described by related parameters; free graphics are generally composed of nodes, lines, and curves. The curves can be Bezier functions or B-spline functions. As shown in Figure 2b), the function parameters, node coordinates, etc. can be described. In simple terms, vector graphics can be thought of as a set of mathematical formulas stored in a computer, or as parameters of related mathematical functions.
For example, if a circle is drawn on a computer using graphics processing software, the computer does not store the color of each coordinate point in the circle when storing the computer, but generally only the center coordinates, the radius of the circle, and the inside of the circle are remembered. Contour color information can be.
The feature of the graphics is that it is not related to the resolution, but it only performs the rasterization imaging according to the resolution of the device when it is output or displayed. Since graphics are stored in the form of mathematical formulas, the image does not appear halo or mosaic when zoomed in or out.
Color processing of graphics is often referred to as color, not separation. For example: printing a red circle, according to the four-color ink superposition principle, as long as Y version is 100%, M version is 100%. Therefore, when the color is specified, the four-color version Y of this circle can be specified as 100% and M as 100%. Both C and K versions are 0. Since the screen does not truly reflect the color after printing, the coloration is performed against the chromatogram.


The relationship between fonts and graphics


The glyphs of text are graphics, but such graphics are very special. When people see them, they produce semantic information in the mind.
In order to display or record text on a computer and output it to various media, it is necessary to store the font information and form a font. In fact, a font is a data body that stores character outline information.
The early fonts were dot fonts, and they were also called image fonts. The disadvantages are the same as the image, and the jagged edges appear after zooming in. Therefore, early dot matrix fonts are generally equipped with several sets of fonts according to the font size. Its wording program uses some image processing tools.
Today's fonts are generally vector fonts, which can also be called graphic fonts. Because their processing methods and graphics are basically the same, you can think of fonts as a set of mathematical formulas and their parameters stored in a computer. The advantage of this font is that neither amplification nor reduction is distorted.
The processing of fonts is also performed according to the standard colors. The word processing is often seen in computer font programs, and a number of graphic processing tools are also used during the writing process. Fonts are divided into fonts for on-screen display and fonts for output of laser imagesetters. The accuracy and price vary. Screen fonts are only used for typesetting, so the required accuracy is not very high.


Pixel image and vector graphics conversion


It is very common to convert graphics into visible images. A graphic is drawn in the drawing software, and the computer converts the drawing graphic to an image on the screen at any time. In RIP, according to the graphical description of the page description language, according to the recording resolution of the recording device, vector graphics are converted into information for recording exposure, and visible images (films or plates) are formed by the recording device.
Images can also be converted to graphics, called "vectorization of images." Included in the CorelDraw suite is a CorelTrace software that can be used to convert pixel images to vector graphics.
The pixel image shown in Figure 1 is placed in CorelTrace, and the corresponding parameter settings (complexity, noise filtering, color number, node reduction, node type, minimum object size, etc.) can be converted into vector graphics. As shown in Figure 3, Figure 4 shows the image part.
The correct understanding of the characteristics of pixel images and vector graphics, and the use of appropriate processing software and means to deal with them, in order to get a better copy of graphics and text.

Source: Ke Yin Media - "Printing Technology"

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