Trapping processing in QM DI46-4 (I)

In the printing process, the high-speed transfer of the paper will produce a slight shift and stretch. At the same time, the paper will have a certain deformation after absorbing the fountain solution and the ink. To compensate for this irresistible defect, the print quality belt In order to influence, there are trapping processes in the printing. In traditional printing, trapping settings are generally performed in prepress design and production software. This article takes Heidelberg QM DI46-4 as an example to discuss the trapping control in digital printing.

First, trapping basic theory

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 accuracy throughout the printing process. However, in the actual printing, due to the deformation of the paper, when the four-color overprinting is performed, the printing position of each color image on the paper will have a certain degree of offset, and the substrate color of the printing material will show a small amount. 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. The following discussion mainly discusses two trap types:

(1) Extend the object by expanding or contracting.
(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 two or more color blocks with large color differences are connected. When the "Easy Trap" threshold (ie, the degree of color difference) is defined for each trapping program, the trapping level is automatically activated. 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 exceptions are objects that cannot be used for pictures and gradients. For such objects, they can only be processed with trapping techniques that are expanded or shrunk. 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 they all contain 100% cyan color), the other color component values ​​do not differ greatly. At this time, 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 adjacent color patches are overprinted by the trapping technique of outer expansion and inner shrinking, the size of one of the color patches is unchanged, and the size of the other color patch is changed, and the color of the overprinted portion will be automatically formed and depends on the darker color patches. As for what kind of color blocks need to be expanded, what kind of color blocks need to shrink is described as follows:

(1) 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. If the foreground block size does not change, the hollow part of the background color is reduced. This method is that the background object shrinks the foreground object.

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 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, and according to this ratio, reduce the trap width.

10. Trapping of objects superimposed on images and gradients

Objects and texts that are superimposed on images and gradients will change their trapping direction due to the constantly changing brightness of the pixel colors. To avoid such situations, objects and texts superimposed on images and gradients will use trapping techniques that expand and shrink in "Easy Trap" mode. For small fonts, it is recommended to use centerline trapping 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 size of black and other 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 setting is "100% Overprint" (Overprint 100%). In RIP, the background is not hollowed out and foreground black is completely overprinted in the background color.

Second, "Easy trap" (Easy trap) mode

1. Using the RIP program, select the Configure RIP... option from the QM-DI-RIP pull-down menu. The Connigig RIP dialog box appears.

2. The Extras->Configure RIP Extras window opens. Select "EasyTrap" and click the "Add" button. The "Enable Feature" dialog appears. Enter the license number in the input box and confirm the exit. The state of the "Enable" column corresponding to "Easy Trap" can be switched between "Yes" and "No".

3. Turn off "Configure RIP Extras" and "Configure RIP" and the "Easy Trap" function will work.