Basic Theories of Printing - introduction of four-color and dot
Why discuss printing?----it is no doubt that post-process knowledge is very important to pre-process.
Four-color printing:
Black can be obtained by mixing R, G and B color together according to a certain proportion, then why black color is still needed? Yes, because black is widely used in actual design, so it is not economical to produce black by three color inks in actual usage. This is why four-color printing appears. Moreover, black produced by R, G and B looks false, while black produced by direct preparation of ink looks stark. 1. Four-color principle is well accepted by people. It equals to four films in output, and cyan, magenta, yellow, black (C,M,Y,K) channels in Photoshop. When we modify the channels in image procession, we actually change the film. ２.Mesh, dot, angle, flat screen and hanging screen. Mesh: dot number per square inch. Ordinary printed maters usually use 175 meshes; newspaper uses 60~100 meshes according to paper quality; special printed maters uses special meshes.
Dot: include round, oval, rhombi, square, diamond and frequency dot etc. The above figure is composed of round dots.
Screen angle: the included angle formed by the straight line linked by two closest dots and the horizon line. The angle in above figure is 45 °.
Flat screen: dots of the whole film are evenly and regularly distributed with the same size, which form a plane. The above figure is flat screen.
Hanging screen: dots are in different size. The screen has layers. The following screen angle is 60°.
What to do before outputting film?
Ⅰ File format
Most parts of input center adopt RIP(pspnt) output system and phototypesetting output systems like HQ510, Scitex of Israel, Heidelberg etc. All of them support postscript press description files, i.e. files with suffix ps. Other file formats which can be output directly by RIP are: s2, s72, ps2 (Founder Bookmaker) , EPS, TIF, PDF etc. Widely used design software currently are: photoshop、pagemaker、coreldraw、illustrator etc. All can produce a PS or PDF file through postscript virtue printer. Word, wps2000 and other word processing software can also produce ps files through postscript virtue printer. In fact, pre-press output shall go through some processions like making-up, adding register regulating line and cutting line etc. But word, wps2000 is incapable of such functions. For color script, it would bring great difficulty to do post-production processing after the script is done. Therefore, select pagemaker and a series of professional design and composing software as mush as possible.
II Format and precision of format
Offset printing (four-color overprint) is widely adopted in modern printing, i.e. divide color image into four colors: cyan (c), magenta (M), yellow (Y) and black (B) dot film, and print them down into PS plate. After 4 times of printing by offset printing machine, color printed product is completed. Images used for printing is different from ordinary computer-displayed images. The image shall be in CMYk mode, not RGB mode or else. Change image into dot during outputting, i.e. precision: dpi. The minimum precision of images for printing shall be 300dpi/pixel/inch in theory, but most fine images we see in computer are actually 72spi RGB mode. They are unprintable. Image can not be selected simply according to its display effect. Don’t judge an image’s printability by seeing its effect through acdsee or other software. Open the image by Photoshop, and confirm its precision by image size option. For example: resolution of an image is 600*600dpi/pixel/inch. It could be used after being amplified to twice of the original size or more. If the resolution is 300*300dpi, it can not be amplified, but can be minified or kept as the original size. If the resolution is 72*72dpi/pixel/inch, its size must be decreased (dpi precision would increase) till the resolution reaches 300*300dpi. (Define the “re-define pixel” as N/A in image size option in Photoshop). Usually seen image formats are: TIF, JPG, PCD, PSD, PCX, EPS, GIF, BMP etc. TIF multi-colored, black & white bitmap, EPS vectorgraph or JPG compressed format bitmap are the generally used image format by output software before printing. Choose the three formats as much as possible.
III Image Color
For overprint, lap-print, empty, spot color and other terminologies, please refer to relevant printing knowledge. Here we only talk about some common knowledge. 1. Empty
When there is a line of blue words on yellow base board, the position of blue words shall be emptied on the yellow plate of the film, vise versa. If print blue directly on yellow, blue words would change into green. 2. Lap-print
When there is a line of black words embossed on red base board, the position of black words shall not be emptied on the red plate of the film. Because black can cover any other colors, if empty the black content, white margin would be produced in case of any error, especially for some tiny words. The margin can be spotted easily, because white and black are contrasted to each other. 3. Four-color black
This is a common problem. Before output, you must check out whether black words in publication files, especially small words, only appear in black board. They shall not exist in plates with other colors; otherwise, the finished product would be affected in quality. Black words would definitely turn into four-color black when converting RGB figure into CMYK figure, which shall be solved except specially designated before outputting film. 4. RGB mode image
When RGB mode image is output, RIP system would turn it into CMYK mode automatically, while color quality would be spoiled greatly, and the finished product of printing would show in tint and dim colors. You’d better change the image into CMYK mode in Photoshop. If it is scanning script, you must calibrate colors to make the image usable.
IV Precision of film hanging screen
Precision of hanging screen is called hanging mesh. Higher precision means finer product, while paper and ink also play important role. If images with higher hanging mesh are printed onto newspaper paper, images would become blurred very badly. Therefore, you shall know the printing paper before output, and then decide precision of hanging screen. Usual paper and hanging mesh precision: imported copper-plate paper or self-adhesive paper etc---175~200 lines; imported offset paper etc--- 150~!75 lines; ordinary offset paper etc—133~150 lines; newspaper paper—100~120 lines. Worse paper quality means lower hanging mesh, vise versa.
Conclusion
You shall pay attention to the following before outputting printed maters:
1. Select commonly used design software, like pagemaker. Don’t use Word.
2. Image mode must be CMYK; minimum precision shall be 300*300dpi/pixel/inch.
3. Check if words in publication materials are four-color words, if they are, change them into single-color 100% black.
4. When use pagemaker and illustrator, you must copy the linked image files and original files together.
5. Photoshop files are better to be psd (without layer combination) files in order to make modification easier.
6. Change words in coreldraw or illustrator into Outline to avoid output failure due to inexistence of such typeface in output center. Copy inlay files together.
7. Inform the output center of product size, printing volume, used paper etc, to make make-up and screen hanging easier.
8. If content of two pages is the same, only black words are different (which is usually seen), you only have to output CMYK four-color of the first page and K color of the second page. This would cut cost. If there are only two colors (like red and black), and requirement on printing is not high, you may output them in a color plate at a time. There is no need to output them after separating them into two colors. Some treatment in printing plate-making is necessary. It can reduce cost too.
Dot angle
Cyan: 15°, magenta 75°, yellow: 90°, black: 45°
Screen dots are usually aligned regularly, so there are angle differences in application. In single-color printing, screen line angle usually adopts 45° , because dots printed in this angle is the most comfortable. It is not easy to spot the existence of dots, which would yield continuous grayish effect. In printing with two colors or more, you have to notice angle combination of two screens. Otherwise, unnecessary patterns would occur, called as “Moier”. Moier could be avoided by making angles of the two screens differ in 30°. Therefore, dominant color or deep color use 45°, tint color uses 75°, yellow uses 90°, cyan uses 15° and black uses 45°. There are no restrictions on these angles which are subject to change upon requirement.
What is PostScript?
Postscript is a program language designed specially for figure and text printing. It is irrelevant to media. PostScript is suitable for printing on paper, film or for screen display. It is a page description language, similar to HTML language. PostScript is promoted by Adobe in 1985, being first applied in LaserWriter press of Apple. The major target of PostScript is to provide a language which can describe image and is independent from equipment. Independent from equipment means not to rely on any specific equipment performances (e.g. press resolution) to describe an image. Therefore, the description can be printed directly in other PostScript presses without any modification. As a language, PostScript has a whole set of grammar and format regulations. Please go to Adobe website .http://www.adobe.com, if you are interest. You can find complete information there. PostScript files are stored in text, which is similar to HTML file. You only have to open a PostScript file by wordpad to see it. PostScript file uses Postscript language to describe characteristics and parameters of image to be displayed or printed. Detail printing or displaying are interpreted and conducted by PostScript interpreter to obtain needed image. Figure 2 is the display of PostScript file of figure 1 in PostScript file reader. You can see that the PostScript file has gain multi-color pages after being interpreted and displayed. PostScript files have many advantages compared with other format files. Aside from being independent from equipment, which is an important point, they are independent from operating system platform. Many figure environment of Unix regards support to PostScript as a part of the core functions. Both Windows and Unix operating systems can read and print PostScript files, which enables easy communication. PostScript file is stored in text format, thus it is small and suitable for transmission on internet. What’s more, printing and displaying by PostScript equipments (press, monitor) have unique advantages and can achieve the best quality.
Methods for generating PostScript files:
(1) You may refer to language format of PostScript to write a PostScript file directly by text editor according to your need. You have to undertake big workload if the file is complex, and shall know PostScript language well.
(2) You can use special functions provided by PostScript file editing software or some other text and figure editing software to generate PostScript file.
(3) The simplest method is to add a PostScript press (virtue) in your operating system. In text editor or figure editor, the file you obtained by printing the image you need through PostScript press is the PostScript you need.
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