The important role of the hottest trapping process

The important role of trapping technology in the process of color printing

in the process of color printing reproduction, the superposition of text and graphics often occurs. When these objects (text, graphics and images) overlap each other, the objects in front of them will cover the ones behind them. In printing, the objects in front of them are usually called foreground, and the objects behind them are called background. If the colors of the upper and lower "layers" are different, It requires us to achieve accurate alignment in the future assembly and printing process, otherwise there will be white exposure or color overlap in the overprint part, which will eventually affect the effect of the output image

however, due to the reasons of equipment and technology, people can not achieve absolute registration in the real sense, even in the era of digital printing. Because when the paper is transferred at a high speed on the printing machine, it will generally produce slight displacement and stretching. In addition, the paper will also have certain deformation after absorbing the fountain solution and ink, which will bring registration problems to color products

in order to avoid the influence of the deformation of the substrate material or the relative displacement of the material on the printing quality, people apply trapping control at the lap joint of various objects (which also reports the strength and toughness of the metal, which is called trapping) to solve this problem, that is, when making prepress, use computer software to do appropriate processing, adjust the overprint range of each color block, so as to avoid the phenomenon of white edges, In printing, this process is called trapping process

1. Trapping principle

the simplest understanding of trapping principle is to make the foreground and background colors overlap each other through the methods of internal contraction and external expansion, so as to ensure that the two will further improve our position as a leading country in global environmental improvement and climate change, and there will be no white edge phenomenon between different colors. Outward expansion corresponds to the enlargement of the size of the foreground object, and the size of the background color block remains unchanged, so that the outer edge of the foreground color block is overprinted on the hollowed out background. Shrinking corresponds to keeping the size of the foreground color block unchanged, so that the hollowed out part of the background color is reduced, and the foreground color block is overprinted on the reduced edge of the hollowed out part of the background color block. Generally speaking, in trapping processing, the use of inward shrinkage or outward expansion mainly depends on the contrast between the foreground color and the background color. Generally speaking, the following principles are followed in outward expansion: expanding the background color without expanding the foreground color, expanding the light color without expanding the dark color, and expanding the flat without expanding the field. The reason is that the background color, light color and flat have less visual impact on people than the foreground color and dark color. Otherwise, people will easily detect the change of the object shape

what is the value of the expanded pixel? This mainly depends on the performance of the printing materials, the registration accuracy of the printing equipment and the printing method. Due to the improvement of machine processing technology, the precision of the general printing machine is very high, so it is not very difficult to ensure the registration of the printing book, but the factors of the printing materials are not controlled by the printing personnel. Generally, the trapping value can be appropriately increased for the easily deformed printing materials, such as paper and plastic printing materials. For coated paper with small expansion ratio, the trapping value can be appropriately reduced. For the situation that the registration accuracy of the old equipment decreases due to machine wear and other reasons, the trapping value can also be appropriately increased. In addition, the trapping value can also be appropriately increased for the rotating equipment

in fact, in the process of prepress production, it is not necessary to set the trapping value. In many cases, it is not necessary to set the trapping value at all. The trapping function has a good effect only when there is an obvious connection between two colors in the file and the contrast is very strong, but trapping in places with low contrast is easy to cause the edge of the image to blur. Therefore, images with continuous transition tones or gradual graphics generally do not need trapping

2. Trapping processing method

generally, trapping processing can be achieved in two ways. One is related prepress software in the field of desktop publishing, such as image processing software Adobe Photoshop, graphics software Adobe Illustrator, Macromedia freehand and edition software Adobe pagem. Please do not open the box door. Trapping functions are set in Aker, industry and QuarkXPress. With the help of the "trap" function, people can apply the trapping effect at the overlap of various objects. Second, professional trapping software is used. At present, a considerable number of printing process providers and some output device providers provide professional trapping software. In the past, this kind of professional trapping software generally processed postscript files, such as Adobe trapwise launched by Adobe company. It processed postscript files on the application workstation (just like rip in imagesetter), and converted some image information into pixels and points in the files, Then make trapping settings on all adjacent objects. At present, digital processes generally support PDF file format, so there are a large number of PDF trapping processing tools on the market, such as Agfa's in RIP trapping and Heidelberg's

supertrap and supertrap plus trapping processing tools

no matter which software is used to deal with trapping, it is mainly realized in two ways in principle, that is, expanding the object by expanding or shrinking, or calculating the color value and position of a color band from the edge pixel value of the intersection of color blocks. Specifically, the first method includes layering method and shift method. Layering method is to trap elements by shrinking or expanding them on different layers; The shift method is to shrink in or expand out by moving the inflection point position in the color block, which is used in trapping related to gradient