The so-called trapping refers to the purposeful color overlapping adopted to compensate for the overprinting of the printing press, and to prevent the phenomenon of exposing colors between the overprinting colors caused by the printing overprinting. To achieve fully automatic trapping control is impossible. As a pre-press staff, you must understand the printing process to use trapping control software correctly. Due to the different requirements of the printed matter, the overprint deviation control can be divided into three types: thin line sleeve, loose sleeve and overlapping sleeve during production. Among them, the control can be fully achieved without strict application before printing. According to the situation of the object, the parts that need to be trapped can be concessioned, diffused, and contracted during design and production. The EPS file must be "overprinted" in the application software that generated it; the grayscale TIFF file is best specified in the "overprint" information palette; the image background and field must be processed as above, and the text on the mixed color can be omitted Shrink treatment.
Diffusion and shrinkage are commonly used means of trapping control. Sometimes, only slight changes to the elements can complete the correct printing. For example, a yellow circle is overprinted on a cyan background. If the diameter of the circle is 10mm, the diameter of the concession circle in the background color is also 10mm. If there is a slight deviation in printing, the paper color will appear around the circle. If the diameter of the concession circle remains unchanged, and the diameter of the yellow circle is increased to 11mm, the diffusion method is adopted, and the yellow is overprinted into the surrounding cyan area, and the problem of inaccurate overprinting is solved. Similarly, if the diameter of the yellow circle is kept constant, the diameter of the concession circle is reduced to 9mm, that is, the shrinkage method can also be used to solve the problem of overprint deviation. It is worth noting that when the background color is a continuous tone image, it is generally not suitable to adopt the shrinking method.
Diffusion and shrinkage should also follow certain principles. Generally, lighter colors (secondary colors) should diffuse into darker colors (primary colors). If the foreground color is lighter than the adjacent background color, the foreground color should be diffused; otherwise, the foreground color should be filled with the background color. Because trapping control produces black lines around page elements, spreading lighter colors into darker colors will minimize the visual effects of trapping control.
When the prepress system performs trapping processing, different application software has different operation techniques and methods. Trap processing can be done manually in some software, such as FreeHand and Illustrator. For example, when the magenta object is adjacent to the cyan object, the cyan needs to be trapped at the adjacent part of the magenta object to compensate for possible overprinting. You can use the drawing tool "stroke" or "keyline" and specify the "overprint" indicator for this tool. In some software, "overprinting" can be done automatically. For example, on some high-end systems, if the photo is deleted according to a certain color block, the software will consider the color information of the background photo, and then adjust the color information in the image according to the continuous Automatically generate trapping control colors, and then perform trapping control on the foreground color according to the color information of the area below the photo. Of course, high-end systems differ in the way they implement the main trapping control colors. Some require the operator to specify their own, while others are automatically generated under software control. Some systems can analyze the darkness of the contact color to produce a new trapping control color. However, all systems can control the color behind rich black (containing at least 10% cyan, magenta, yellow, and 100% black) to prevent over-coloring caused by overprinting. The continuous tone image is diffused under the line. Some systems can also extend half of a color block under the trapped black line, and the line width can be different.
The prepress staff should pay attention to summarize more practical methods and techniques, understand and study the printing process, and do trap control in prepress production to ensure the smooth progress of printing.
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