Trapping treatment in the hottest digital printing

2022-08-14
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Trapping treatment in digital printing

in the printing process, the paper will produce slight displacement and stretching in the high-speed transmission process. At the same time, the paper will also have certain deformation after absorbing the fountain solution and ink. In order to make up for the impact of this irresistible defect on the printing quality, trapping technology has appeared in the printing. In traditional printing, trapping is usually set in prepress design and production software. This article takes Heidelberg QM di 46-4 as an example to discuss the trapping control in digital printing

first, trapping in digital printing

1. Trapping concept

in multicolor printing, in order to obtain the best printing effect, we must ensure that there is a very accurate overprint in the whole printing process. However, in the actual printing process, due to the deformation of the last sheet of paper, during the four-color overprint, the printing position of various colors of graphics and texts on the paper will be offset to a certain extent, and then the background color of the substrate material will be exposed (a small white line)

in order to avoid similar displacement, the overprint range of each color block must be adjusted. The overlap or expansion of this color block is called trapping. Here, we discuss the following two types of trapping:

(1) expand objects by expanding or shrinking

(2) calculate the color value and position of a color band from the edge pixel value of the intersection of color blocks

2. Trapping conditions

generally speaking, trapping is required when at least two or more color blocks with large color differences are connected. When the threshold of easy trap (that is, the degree of color difference) is defined for each trapping program, the trapping degree will start automatically. During color separation, qm-di-rip calculates and evaluates the whole process during the interpretation of PS files. If the percentage value of color value difference is greater than or equal to the trapping threshold, an extended color band (i.e. trapping) will be automatically generated. The prerequisite of this process is that the color separation process of the document must be carried out in QM - di-rip

Compared with the previous system,

this trapping method is applicable to almost all objects, the only exception is that it cannot be used for objects located on pictures and gradients. For such objects, we can only use the trapping technology of external expansion or internal contraction

if there are two adjacent objects, the value of one color component of one object is the same as that of the other object (e.g. both contain 100% cyan), and the value of other color components is not very different. At this time, there is no need to trap, and the common color component has little possibility of blank edges between them

3. Expansion and contraction

through the trapping technology of expansion and contraction, the adjacent color blocks are overprinted, that is, the size of one color block remains unchanged, and the size of the other color block changes. The difference is that what color block needs to be expanded and what color block needs to be retracted. The color of the overprint part will be formed automatically and depends on the darker color block

(1) expansion

the size of the foreground object is expanded, so that its outer edge is overprinted on the hollowed out background, and the size of the background color block remains unchanged, while the size of the foreground color block is extended to the hollowed out background

(2) shrinkage

4. Trapping color

trapping color is calculated from the common color components of adjacent colors

5. The practice of trapping color adjustment

shows that the calculated trapping color has a high saturation, which is very inappropriate to appear on the printed image, and its effect is far inferior to the extension of brightness between adjacent objects

in easy trap mode, each color value of trapping color will decrease by a certain percentage. After the trapping color value in Figure 6 is reduced by 40%, the trapping part is not so obvious

6. trapping direction

trapping direction depends on the color brightness of adjacent objects. The general rule is: extend from bright color to dark color

7. Center line trapping

in order to obtain high-quality fonts, it is recommended to use center line trapping. This method can well preserve the font structure and will not be covered by trapping. This kind of trapping is evenly located at the junction of color blocks

8. Trapping of small fonts

due to trapping, the edges of small fonts will be blurred to a certain extent, which will reduce the character recognition rate. In the East trap mode, if the text size is within the set font size range, the trapping width will be automatically reduced during trapping processing

9. Trapping of fine lines

fine lines will be completely or partially covered if they are used and there is no accurate standard for parameters such as experimental temperature and electromechanical torque. In the easy trap mode, the program will calculate the ratio between the trapping width and the thin line width, and reduce the trapping width according to this ratio

10. When using and operating this machine, please 1 be sure to use the trapping of objects on images and gradients according to the process specifications of the manual

for objects and text superimposed on images and gradients, the trapping direction will be constantly changed due to the continuous change of the brightness of pixel colors. In order to avoid this kind of situation, the object and text superimposed on the image and gradient will adopt the trapping technology of expanding and contracting in the easy trap mode. For small fonts, it is recommended to trap the centerline to preserve the font structure

11. Auto shrink ultra black

in order to obtain the maximum saturation of black on printed matter, people often use ultra black, which contains 100% black and the corresponding proportion of cyan, magenta and yellow. Generally, it is set to 50% C, 50% m, 50% Y and 100% B

if the overprint is not on time, a colored edge will appear around the black block

this kind of situation can be avoided by reducing the overlapping range of green, magenta and yellow. This retraction function, that is, to narrow the range of other color components of ultra black, is provided by the auto retract superblack attribute

12. 100% overprint

if a 100% black foreground is superimposed on a color background, all black backgrounds will be hollowed out (i.e. hollowed out) during rip

if the overprint is not on time, a white or light edge space will appear between the color background and the black foreground

to avoid this kind of situation, black is set to 100% overprint (overprint 100%), and blank processing will not be carried out on the background during rip, and the foreground black is completely overprinted on the background color

II. Easy trap mode

1. Run the rip program. Select the configure rip option in the QM - di - rip drop-down menu, and the configure rip dialog box appears

2. Click the extras item in the window in Figure 22 to open the configure rip extras window. Select easytrap, click the Add button, the enable feature dialog box appears, enter the license number in the input box, and confirm to exit. In Figure 23, the status of the enable bar corresponding to easy trap can be switched between yes and No

3. Close the configure rip extras and configure rip dialog boxes, and the easy trap function mode can be used

III. trapping parameter setting

trapping parameter setting can be carried out in trapset. Based on different trapping widths, three kinds of traps are set at the factory:

light (low) narrow trapping width

mid (medium) moderate trapping width

heavy (large) wide trapping width

traps can be copied, modified or preset (set according to the characteristics of page setup or calibration)

start/preset traps

select trapping manager in the output drop-down menu, Then the trapping Manager dialog box appears, in which there are three factory default settings

select the default trapping setting name

click Edit to reset the trapping parameters, or click new to create a new trapping setting name

after selecting Edit or new, the trap specifications dialog box opens

name: enter the trapping setting name in this box

centerline threshold: if this box is activated, trapping will occur between two adjacent colors. Enter the brightness difference percentage value of adjacent objects in the input box

trap width/stroke width limit: enter the limit percentage value between the desired trap width and the shrink width here

auto choose superblbck: if this box is activated, the trapping width of the color values of C, m and y components will be reduced accordingly. This setting is only applicable to text and objects using ultra black

normal trap width): enter the trap width in this box

blacsk trap width: enter the black trap width in this box

image/vignette trap width: enter the trap value of the object intersecting the picture and the gradient in this box

text

trap threshold: the input value in this box determines the threshold for generating traps. For different component colors, enter the difference of their adjacent colors

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