Trapping in digital printing

During the printing process, the paper undergoes a slight shift and stretch during the high-speed transfer process. At the same time, the paper will undergo some deformation after absorbing the fountain solution and the ink. To compensate for this irresistible defect, With the impact of print quality, trapping processes have appeared in printing. In traditional printing, trapping settings are generally performed 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.

I. Trapping in digital printing
1. Trapping concept
In multi-color printing, in order to obtain the best printing effect, it must be ensured that there is a very accurate overprint during the entire printing process. However, in the actual printing process, due to the deformation of the paper, when the four-color overprinting, the printing position of each color image on the paper will have a certain degree of offset, then the background color of the printing material will be revealed (a small White line).

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

(1) Extend the object by expanding or shrinking;

(2) Calculate the color value and position of a color band from the edge pixel values ​​where the color patches intersect.

2. Trapping conditions
In general, trapping is required when at least two color patches with large color differences are connected. When the threshold of “Easy Trap” (ie, the degree of difference in color) is defined for each trapping program, trapping occurs. The degree of printing will automatically start. During color separation, QM-DI-RIP calculates and evaluates the entire process during the interpretation of the PS file. If the percentage value of the color difference is greater than or equal to the trap threshold, an extended ribbon will be automatically generated (that is, trapping ). The precondition for this process is that the separation process of the file must be performed in QM-DI-RIP.

This kind of trapping method is suitable for almost all objects. The only exception is that it cannot be used for objects located on pictures and gradients. For such objects, trapping techniques that use external scaling or shrinking can only be used.

If there are two adjacent objects, where one color component value of one object is the same as the color component value of another object (if both contain 100% cyan color), the difference of other color component values ​​is not great. There is no need to trap, and the common color components make it very unlikely that there will be blank edges between them.

3. External expansion and retraction
The trapping technology of outer expansion and inner shrinking is used to superimpose the adjacent color blocks, ie, the size of one color block is unchanged, and the other color is changed in size. The difference is that what kind of color blocks need to be expanded and what kind of color blocks need to be expanded. Color blocks need to shrink. The color of the overprinted portion will be automatically formed and depends on darker shades of color.

(1) External expansion
The size of the foreground object is enlarged so that it is overprinted on a hollow background, the background color patch size remains the same, and the foreground color block size expands to the hollow background.

(2) Contraction
4. Trap color
Trapping colors are calculated from the common color components of adjacent colors.

5. Trap color adjustment
Practice has shown that the calculated trapping color has a higher saturation and appears to be very inappropriate on the printed image, and its effect is far less than an extension of brightness between adjacent objects.

In "Easy Trap" mode, each color value of the trapping color will be reduced by a certain percentage. After the trap color value in Figure 6 is reduced by 40%, the trapping section is not so obvious.

6. Trapping direction
The trapping direction depends on the color brightness of the adjacent objects. The general rule is that it extends from light to dark.

7. Centerline Trapping
In order to obtain high-quality fonts, it is recommended to use centerline trapping. In this way, the font structure can be well preserved without being covered by traps. This trapping occurs evenly at the junction of color patches.

8. Small font trapping
Trapping will cause the edges of small fonts to have a certain degree of blurring, which will reduce the recognition rate of characters. In "East Trap" mode, if the size of the text is within the set font size, the trap width will be automatically reduced when trapping.

9. Thin line trapping
Thin lines will be covered in whole or in part if you use common trapping. In "Easy Trap" mode, the program will calculate the ratio between the trap width and the width of the thin line. Based on this ratio, the trap width will be reduced.

10. Trapping of objects superimposed on images and gradients
Objects and text overlayed on images and gradients will constantly change their trapping direction due to the constant change in the brightness of the pixel colors. In order to avoid such situations, objects and texts superimposed on images and gradients will use the trapping technique of expanding and retracting in the "Easy Trap" mode. For small fonts, it is recommended to trap the centerline to preserve the font structure.

11. Automatically shrinking black
In order to obtain maximum saturation in print, black is often used in ultra-black. Ultra-black contains 100% black and corresponding proportions of blue, yellow, and yellow. The general settings are 50%C, 50%M, 50%Y, 100%B.

If overprinting is not accurate, a colored border will appear around the black block.

By narrowing the overlapping range of cyan, magenta, and yellow, you can avoid such situations. This shrinking function, that is, reducing the range of other black color components, is provided by the "Autochoke Superblace" property.

12. 100% overprint
If a 100% black foreground is superimposed on a colored background, all backgrounds in black will be hollowed out (ie "knocked out") at RIP.

If overprinting is not correct, a white or light-colored border will appear between the colored background and the black foreground.

To avoid this, the black is set to "Overprint 100%", which will not be processed on the background at RIP, with the foreground black completely overprinted on the background color.

Second, automatic "auto trap" (Easy trap) mode
1. In the Merchant RIP program, select the Configure RIP... option from the QM-DI-RIP pull-down menu. The Connigig RIP dialog box appears.

2. Click on the "Extras" item in the window in Figure 22 to open the "Configure RIP Extras" window. Select "EasyTrap", click "Add" button, "Enable Feature" dialog box appears, enter the license number in the input box, confirm the exit. In FIG. 23, the state of the "Enable" column corresponding to "Easy Trap" can be switched between "Yes" and "No".

3. When you close the "Configure RIP Extras" and "Configure RIP" dialog boxes, the "Easy Trap" function mode is ready for use.

Third, trapping parameter settings
Trapping parameters can be set in the trapset. Based on different trap widths, there are three factory trapsets:

·Light narrow trap width
·Mid (moderate) trap width
·Heavy Wide trap width
The trapset can be copied, modified or preset (set according to the characteristics of the page setup or calibration)

Start/preset trapsets

Select "Trapping Manager" from the "OutpUt" drop-down menu. The "Trapping Manager" dialog appears. There are already three factory default settings.

Select the default trap settings name;

Click "Edit" to reset or edit the trapping parameters, or click "New" to create a new trapping setting name;

After selecting "Edit" or "New", the "Trap Specifications" dialog opens.

Name: Enter the trapping name in this box;

Centerline Iuminance threshold: If this box is activated, traps will be generated between two adjacent colors. Enter the difference in brightness of the adjacent objects in the input box.

Trap widty/stroke width limit: Enter the limit percentage between the desired trap width and the width of the retraction width.

Auto choke superblbCk: If this box is activated, the trap width of the color values ​​of C, M, and Y components will be reduced accordingly. This setting applies only to text and objects using super 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 trapping value of the object that intersects the picture and the gradient in this box;

Text<...pts trap width: Enter the font size in this box. All characters whose size is less than this limit will use the trap width set in the text. Units: Select the unit of measure;

Trap threshold: The input value in this box determines the threshold for trapping. For different component colors, each input its difference in the adjacent color.