Desktop publishing software such as image processing software Photoshop, graphics processing software Illustrator, Freehand, and group edition software Pagemaker, QuarkXpress, and Indesign all support trapping processing functions, but there are differences in the trapping processing methods.
1 Trap processing of image processing software
Image processing software uses the dot matrix method to describe the "content information" to be expressed, and its trapping processing function is also implemented using the dot matrix method. There are two different ways of trapping using the dot matrix method: First, the foreground and background colors are color blocks, the computer samples and analyzes all the border pixels of the image, and then selects the average of all pixels to complete the trap. The advantage of this trapping method is that the trapping processing speed is fast, but the printing overprint error is obvious; Second, the image is continuously adjusted for the foreground and background colors. In this case, the trapping process is usually set in advance at both ends of the image boundary. The trap width of each pair of neighboring pixels on the border selects the intermediate color as the trap color, so that the border is almost invisible on the printed product.
The steps to set the trap in Photoshop are as follows: open a color image and convert it to CYMK color mode; type the magenta Chinese character "printer" on the cyan background, and then merge the layers; select the menu Image / Trap, The dialog box shown in Figure 3 appears. Width in the dialog box is the width of the compensation boundary. This value is directly related to the offset of the color plate during printing. The value is equal to the sum of the four-color printing overprint errors. There are three units of Width: Pixels, ranging from 1 to 10; Points, ranging from 0.01 to 9.99; mm (millimeters), ranging from 0.01 to 3.53mm. Set the trap width value to 3 Pixels and click OK to get the result shown in Figure 4. It can be seen from the figure that the M and C colors penetrate each other to form a blue edge.
In fact, in addition to trapping between color patches in Photoshop, you can also perform trapping processing on continuous tone images. The result of this processing is generally to expand the relatively pure colors with low lightness around to form a certain trap. width. However, it is not recommended for people to use in normal production.
2 Graphics processing software and group version processing
Trap processing in software
Graphic processing software and group processing software are based on the vector method to deal with trapping. The vector processing method of trapping is for a layout composed of vector graphics and text elements. The page description of this layout adopts a mathematically strict geometric description method, that is, using straight lines, circles, and cubic Bezier curves and their coloring to complete the Description of all outlined objects. Trap processing is on the boundary of two different colors, the lighter side expands to the darker side, thus forming a new boundary line, this trapping method can better handle fine graphics and text trapping Printing, but cannot process bitmap color images.
(1) Set the trap in Illustrator
Select two or more objects and choose Window> Show Path Manager. In the Path Manager palette, click the "trap" button in the lower right corner of the dialog box (if the "trap" icon is not displayed, select "Display Options" from the pop-up menu). The dialog box shown in Figure 6 will appear.
In the dialog box, you can set the values ​​of trapping thickness, height / width, and tone reduction. The specific setting of the thickness range in printing production depends on the printing paper and printing equipment. The "height / width" ratio refers to the ratio of horizontal trapping and vertical trapping. The inconsistent setting of horizontal and vertical trapping values ​​can compensate for unevenness during printing, such as paper stretching. The "tone reduction" value is used to change the color tone of the trap, usually the default value is 40%. The "Hue Decrease" value will reduce the value of the trapped brighter colors; the darker colors remain at 100%.
In addition, there are two options for trapping in the dialog box, whether to use "trap with process color" and whether to use "reverse trap", select "trap with process color" item, will create a brighter color Trap into darker colors, the object "reverse trap" is used to trap darker colors into lighter colors.
(2) Trap processing in PageMaker
PageMaker's trapping information is controlled by the Trapping Preference dialog box in the File menu. The trap value that can be defined in the trap parameter dialog box is also a range. You can set the trap width, trap threshold, and black attributes.
There are two settings in the trap width setting box: â‘ The default trap width setting; â‘¡ The width of black for trapping, the width is generally set to 2 times the default trap width. There are three settings in the trap's threshold setting box: â‘ The "step limit" value determines the necessary point of PageMaker trapping. Increasing or decreasing this value determines the amount of each color ink component before PageMaker trapping. This The lower the percentage, the more PageMaker traps between the colors. The value range of the "centerline trap threshold" is 0% to 100%, which means that we can set it anywhere from 0% to 100% threshold. At 0, all traps are set on the centerline. At 100%, the centerline traps are avoided, resulting in complete expansion or contraction. â‘¢Trap Text Above determines the minimum font size to use trapping in PageMaker.
In the Black Properties dialog box, one can set the black limit value and the automatic overprint black setting. The setting of the black limit value determines solid black or super black. Automatically overprinting black allows us to decide that black lines and fill colors will trap or overprint. At the bottom of the dialog box, there is an option to trap images entered by PageMaker.
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