The first line of printing plate: DTP designers pay attention to the problem

Since the DTP puts all prepress work from the original to the four-color film on the computer; since the DTP runs through everything from design to printing; due to the electronic pages described in pure mathematics on the computer, there is no alternative. This physical process of high-speed printing of ink on paper, so DTP will push computer designers to participate in the first line of printing plate.

This puts higher requirements on computer designers and requires them to require other knowledge in addition to the professional design capabilities of the arts, especially in the printing of color separation, hanging nets, overprints, imposition knowledge, etc., in order to be able to accurately The original is faithfully reproduced.

The first thing to pay attention to is separation, hanging nets and other issues

Computer designers should be aware that in the printing process, various colors are represented by superimposing different amounts of cyan, magenta, yellow, and black inks in the form of dots. As we look closely at a colorful newspaper, we can see that it consists of many points. In the darker areas, the points are larger; in the middle area, the points are moderate in size; in the lighter places, the points are smaller. However, the distance between the adjacent points is equal, and the distance between the adjacent points is determined by the screen hanging on the picture. This distance is called the number of screen lines. The number of screen lines is generally expressed as the number of one-way parallel lines per inch. The typical value of the number of screen lines used in newspapers is 80-100 lines per inch, the color prints such as magazines are 120-150 lines per inch, and the high-quality art albums are 175 lines and 200 lines per inch. In general, he stated that the resolution (dpi) of a color picture input with a scanner is based on the principle of twice the number of output screen lines. If you want to print the cover of a magazine on line 150, the dpi value of the scanned image should be 150 x 2 = 300 (dpi) during production. Therefore, the designer should first understand what the designed work is to be printed. It is a newspaper or an album to determine the dpi value of the image scan. Scan dpi value is set high, printed prints will be "paste", dpi value is too low, the work is not beautiful. The commonly used values ​​are:

Artwork carrier screen line scan dpi
Newspaper 80 -- 100 150 -- 200
Magazine 120 -- 150 250 -- 300
Photo album 175 350 350

In the four-color film, the dots of different colors are arranged at different angles so as not to overprint different colors of ink during printing. The angle between the vertical line and the horizontal line at the center of the screen is called the screen angle. These screen angles have been set in the phototypes, and they are based on a carefully selected and clever compromise: cyan is 15, magenta is 45, yellow is 90, and black is 75. Some daring designers will give up on these default perspectives and make their own choices. This should be carefully considered and trial-and-error so as to avoid the occurrence of “moiré” due to inappropriate screen angles. In order to accurately and faithfully reproduce the original, the increment of printed dots is also a problem that should be paid attention to. The so-called dot increment is a slight percentage change in the size of the dot of the printed matter compared with the dots in the original design. It is visually the depth and shade of the printed matter and the color has changed. Especially in the bright part.

The general network will generate increments in the following stages.

1) Image-setter generates dot gain on the output film

2) Exposure increments during exposure

3) Dot gain from printing ink

Of course, the type of printing machine, the type of paper is also an important factor in the creation of dot increments.

In order to obtain prints that are as faithful as possible to originals, dot gains have to be considered and overcome in designing. The increment of dots generated by the imagesetter can be overcome by regularly correcting the percentage of dots on the output film. The following set of grayscale meshes can be output regularly, measured by density, and then corrected for this set of values ​​for the imagesetter. This correction is very important.

The increment of printing outlets is about 10%. In the design, the dots to be designed should be 10% smaller than the actual outlets to be obtained in order to ensure that the printed materials can be faithful to the originals. The best method is for different printing plants. Different inks, after repeated experiments, determine the percentage increment of printing dots. And in the production of more accurate to overcome.

Percentage of mesh sites (%) 0 10 20 30 40 50 60 70 80 90 100

Mesh Density Value 0.00 0.05 0.10 0.16 0.22 0.30 0.40 0.52 0.70 1.00 3.50

The second problem is overprinting

When there are two colors adjoining on the page, for example: a blue circle under a yellow background, a white round hole must be dug on a yellow background under the blue circle, otherwise the blue circle presses on the yellow background. It will turn into a green circle, which is not what we want to achieve, but in the printing process, the paper on the printing press is stretched, and there will be a slight shift, and it is also difficult for the printing machine to always maintain a good position. This makes it impossible to print the blue circle on the white hole, resulting in a white gap between the circle and the background. Overprinting is the solution to this problem.

Overprinting can be used to create a transition between two colors, filling the seams that are not allowed to appear. When there are two colors on the paper, the deeper color determines the light-colored border. So there are two common overprinting methods. Dilation is used to superimpose light-colored objects on dark backgrounds; contraction is used to superimpose dark objects on light-colored backgrounds. It seems that dark colors always determine the boundaries of objects. So both methods use superficial color for superimposition while the dark part remains unchanged.

In DTP design and production, computer software such as FreeHand and Illustrator draw a graphic, its internal color and border color can be defined separately. Using this feature, you can complete the superimposition of graphics. The specific method is to set the border of the graphic as overprint, and the color is the darkest color in the graphic and background colors. For example, when superimposing, the principle of light color and dark color should be observed so that the shape of the figure does not change significantly. White--yellow--cyan--green--magenta--red--blue--black Some black celebrity inscriptions or logos can be directly defined as imprints. Comrade Deng Xiaoping's name for my school title: higher education press, we use directly defined as imprinting, or to make it into a four-color ink with a background component.

The third problem is imposition

Previously, the imposition was the last process before platemaking. It is time-consuming and labor-intensive to finish it by hand. It is easy to cause quality problems due to slippage. Now it can be used as the last step in the DTP design, with the corresponding imposition software and a large photo-setter.

Imposition is to arrange multiple pages of electronic pages on a printed text sheet. The commonly used printing paper size is 787 x l02 mm and 850 x ll68 mm. Through computer imposition, a piece of film that meets the requirements of printed paper is produced.

In order to ensure that the printed text is cut and folded, the pages can be arranged in the correct order, and the order of the impositions should also be noted:

When binding a book, 3mm of raw edges should be cut from the sky, corner, and book mouth of the book. So when making a make-up, there is a 6mm incision between the pages. This part of the work plays a very important role in four-color printing. It is extremely difficult to try to combine 16-page 16-color four-color film on one-page printing paper, and it is much easier to use computer to complete it.

In short, color desktop publishing eliminates pure computer creation. Post-processing of electronic pages is inseparable from the separation of color and printing expertise. Designers should be aware that a considerable part of their work is professional work of plate making and printing. Only when these aspects reach a professional level can we meet the requirements of color desktop publishing design to capture the market and advance the development of color desktop publishing.