Prepress image processing method
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In prepress plate making, it is often encountered that there are obviously different color blocks in the image, one of which is covered by another color block. When printing, each color is printed, and between the color block and the color block often occurs. Can not be perfectly superimposed, this mechanically caused overprinting problem is not allowed, it must be handled before printing, we call it pre-press trapping.
Trapping is a very cumbersome thing, it is only used to correct the misalignment between solid colors in CMYK images. Generally, for continuous-tuning images such as photos, trapping should not be created. If trapping is used, the image quality will be impaired. It is also unnecessary to over-trap the color patches, which will produce key lines (even crosshairs) in CMYK plates. These problems are not visible in the composite channel and are revealed when the film is output. It can be seen that trapping does not make the image more beautiful, nor can it be used to solve the mechanical problem of printing registration. It can only properly cover the flaws caused by overprinting.
The main method of trapping is to slightly enlarge the color patch area so that there is some overlap between different color patch 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 Adobelllustrator, Freehand) Etc.) are using this method to achieve trapping.
Adobelllustrator trap method
The last one in "mixing/trapping" under the Pathfinder panel of Adobelllustrator is the trap button.
1. Need to set 3 parameters
(1) Thickness. Thickness is the input value of the trapping width. Between 0.01 and 500 points, the system default is 0.25Points. Usually, the input value is 0.5~1Points, and the specific use value is determined according to the error measurement value of the printing machine.
(2) Height/Width (Height/Width). Refers to the ratio of horizontal and vertical trapping.
(3) Reduce the color tone (TintReduction). Reduces the value of the brighter color in the trapped color. The darker color value remains at 100% and the default value is 40%. In order to reduce the apparent darker boundary of the trapping area, the amount of ink to be trapped can be controlled by reducing the percentage of tones.
2. Option parameters
(1) Select the print color trapping (Trapswithprocesscolor), then fill in the dark area; select ReverseTraps, then fill in the bright area;
(2) Adobelllustrator trapping color selection order from deep to shallow order is M, C, Y;
(3) When making multiple spot color plates, it is decided to fill in the dark or bright areas according to the actual ink color.
The plugin function \Create\trapping under the Freehand software menu is basically equivalent to the Adobelllustrator trap dialog.
AdobePhotoshop trapping method
Adobe Photoshop trapping is done by channel color interdiffusion. There is only one trap width option in Menu\Image\Trap. Its rules of use are: all colors expand to black; light areas expand toward dark areas; yellow expands toward cyan, magenta, and black; pure blue and pure magenta expand equally to each other.
This trapping inevitably creates a distinct dark border between the colors. How do you control the shade of the trap with a percentage, like the color reduction option in the vector software Adobelllustrator, to control the amount of trapping ink? We pass In practice, the following steps can be used to achieve this effect.
(1) Open an image with the file name "TP".
(2) Copy "TP copy" and "TP copy 2" under menu \image\copy; first execute "image copy" in "TP copy 2" file, the width is determined as 1Points; select all, cut, 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 difference of the brightness of the corresponding pixels in each channel, the layer 1 and the background will be blacked out after the operation.
(4) After merging the layers, invert (Inverse) to get the white background and the trap area. Then select all, cut, and copy to the "TP" file.
(5) Change the mode of the new layer 1 obtained by the "TP" file to Multiply. Changing the non-trapping is necessary for prepress processing, but the width of the trap is reduced and the percentage of trapping white is reduced. The setting should be determined according to the error measurement value of the actual printing machine, and the whitening phenomenon of the printed matter should be avoided by proper processing.