The method of pre-press graphic and text processing software to fill in leaks
In pre-press platemaking, there are often different color patches in the image, one of which is covered by another color patch. When printing, each time a color is printed, there is often between color patches and color patches. It cannot be perfectly overlapped. This kind of mechanical overprinting inaccuracy problem must be dealt with before printing. We call it pre-press trapping.
Trapping is a very cumbersome thing, it is only used to correct the misalignment between pure colors in CMYK images. Generally, for continuous-tone images such as photographs, traps should not be created. If traps are used, the image quality will be compromised. Excessive trapping of color patches is also unnecessary, it will produce key lines (even cross lines) in the CMYK printing plate. These problems are not visible in the composite channel and will be revealed when the film is output. It can be seen that trapping will not make the image more exquisite, nor can it be used to solve the mechanical problems of printing registration. It can only properly cover the defects caused by the inaccurate registration.
The main method of trapping is to slightly expand the color block area, so that there is some overlap between different color block areas. This overlap should be very narrow, so that it is not easy to detect its existence on the printed matter. General vector software (such as Adobe lllustrator, Freehand, etc.) use this method to achieve trapping.
Adobe lllustrator trap method
The last one in the "Mix / Trap" under the Pathfinder panel of Adobe lllustrator is the trap button (Trap).
1. Three parameters need to be set (1) Thickness. Thickness refers to the input value of the trapping width, which is between 0.01 and 500Points, and the system default value is 0.25Points. Normally, the input value is 0.5 ～ 1Points, the specific use value should be determined according to the error measurement value of the printing press.
(2) Height / Width. Refers to the ratio of horizontal and vertical trapping.
(3) Tint Reduction. Reduce the value of the brighter color among the trapped colors, keep the darker color value at 100%, and the default value is 40%. In order to reduce the obvious darker borders of trapping areas, the amount of trapping ink can be controlled by reducing the hue percentage.
2. Option parameter (1) Select Traps with process color to trap in dark areas; select Reverse Traps to trap in bright areas;
(2) The order of trapping white color selection in Adobe lllustrator is M, C, Y from dark to light;
(3) When creating multiple spot color plates, determine the trapping in the dark or light areas according to the actual ink color.
The plug-in function \ Create \ trap under the Freehand software menu is basically equivalent to the Adobe lllustrator trap dialog box.
Adobe Photoshop trap method
Adobe Photoshop trapping is done by channel color diffusion, and there is only one trap width option in Menu \ Image \ Trap. Its usage rules are: all colors expand to black; light areas expand to dark areas; yellow expands to cyan, magenta, and black; pure cyan and pure magenta extend to each other on an equal basis.
Such trapping will inevitably produce obvious dark borders between colors. How to use percentages to control the shade of the trapping tone, like the vector software Adobe lllustrator to reduce the tone option, to control the amount of trapping ink? We believe that through practice, the following steps can achieve this effect.
(1) Open an image named "TP".
(2) Copy into "TP copy" and "TP copy 2" under menu \ Image \ Copy; first execute the "TP copy 2" file \ image \ trap, the width is determined to be 1Points; select all, cut and copy To the "TP copy" file.
(3) Get a new layer 1 in the "TP copy" file, press F7 to activate the layer panel, and change the layer 1 mode to the difference mode. Since the difference mode is the absolute value of the brightness difference of the corresponding pixels in each channel, the layer 1 and the background will get a black window after the operation.
(4) After merging the layers, inverse to get the white background and trapped white areas. Then select all, cut and copy to "TP" file.
(5) Change the mode of the new layer 1 obtained from the "TP" file to Multiply, changing its non-trapping is necessary for pre-press processing, but the width of the trapping white and the percentage of trapping white tones are reduced The setting should be determined according to the error measurement value of the actual printing machine, and through appropriate treatment, to avoid the white leakage of the printed matter.
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