Method for generating PDF trapping objects without knowing the description of the contours

Abstract

A method for creating PDF trapping objects from a PDF page description without a known contour description by using a computer for a printing process includes using the computer to generate a colored tiling pattern out of a part of the PDF page description without the top object to be choked, using the computer for clipping with a mask of the background object, using the computer for clipping with the top object or text to limit following frame additions to the top object, using the computer for adding a frame to the top object, using the generated colored tiling pattern as an inner frame between the background object and the top object, using the computer for adding a frame to the top object while activating overprinting of the background in the required object colors and using the computer for inserting the trapped top object into the PDF page description.

Claims

1. A method for creating PDF trapping objects from a PDF page description without knowing a contour description by using a computer for implementing a printing process, the method comprising the following steps: using the computer to generate a colored tiling pattern out of a part of the PDF page description without at least one object located on top to be choked; using the computer for clipping with a mask of a background object; using the computer for clipping with the at least one object located on top, or a text, to limit following frame additions to the at least one object located on top; using the computer for adding a frame to the at least one object located on top using the at least one generated colored tiling pattern in the form of an inner frame between the background object and the at least one object located on top; using the computer for adding a frame to the at least one object located on top while activating overprinting of a background in required object colors; and using the computer for inserting the at least one trapped object located on top into the PDF page description.

2. The method according to claim 1, which further comprises converting the PDF page description in the computer into an internal representation in the form of a number of objects to implement the trapping process.

3. The method according to claim 2, which further comprises trapping all objects located on a background object and to be choked and located on top and trapping all further background objects and their objects to be choked and located on top.

4. The method according to claim 1, wherein the object located on top is at least one text without a known contour description.

5. The method according to claim 1, wherein the object located on top is at least one text with a known contour description.

6. The method according to claim 1, wherein the colored tiling pattern that has been created contains all background objects that are located underneath all objects that are to be choked and are located on top.

7. The method according to claim 1, which further comprises creating at least one colored tiling pattern, each created color tiling pattern containing at least the background object that is located underneath the at least one object that is to be choked and is located on top.

8. The method according to claim 1, which further comprises accurately positioning the at least one colored tiling pattern that has been created relative to an original position in the PDF page, wherein a size is freely variable, and wherein the at least one colored tiling pattern that has been created contains at least the background object that is located underneath the at least one object to be choked located on top.

9. The method according to claim 1, which further comprises covering a rectangular area with the at least one background object to which the at least one colored tiling pattern that has been created refers.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

(1) FIG. 1 is a diagrammatic, top-plan view of a text on a background illustrating an example of a gap that has occurred in an image;

(2) FIG. 2 is a view similar to FIG. 1 illustrating an example of trapping to correct the gap in the image;

(3) FIG. 3 is a flow chart of the method of the invention;

(4) FIG. 4 is a top-plan view of a sample graphic of a document having background objects for the implementation of the method of the invention;

(5) FIG. 5 is a view similar to FIG. 4 of a diagram for the generation of a colored tiling pattern color space;

(6) FIG. 6 is a top-plan view illustrating a clipping result for text above the gradient.

(7) FIG. 7 is a view similar to FIG. 6 illustrating a stroking result for text above the gradient;

(8) FIG. 8 is a view similar to FIG. 7 illustrating a choking result; and

(9) FIG. 8A is an enlarged section of the result for better illustration.

DETAILED DESCRIPTION OF THE INVENTION

(10) Referring now in detail to the figures of the drawings in which corresponding elements have the same reference numerals and first, particularly, to FIG. 1 thereof, there is seen an undesired gap 3, which is clearly visible at a letter a where white paper shines through. For a better understanding it is to be understood that the background color is 100% cyan whereas the color of the letter a is 100% magenta. A lighter text 2 is located on a darker background 1. Since the print sheets of the two colors are not printed precisely on top of each other but slightly offset, a small region in between remains unprinted. Conversely, the colors cyan and magenta overlap on the other side of the letter, resulting in the creation of a blue tint.

(11) In order to eliminate this gap 3, trapping is carried out, in this case spreading 4. The result is shown in FIG. 2. To put it simply, a thin line is placed around the lighter object 2, in this case the letter a, creating a minimum amount of overlap between the border areas. In this case, the spreading causes the colors cyan and magenta to be printed on top of each other to compensate small alignment errors as long as the offset is not greater than the overlapping region. The amount of overlap depends on the printing process. In modern offset printing, very accurate alignment is possible, whereas screen printing requires more spread to effectively eliminate gaps.

(12) However, in particular for chokes 10, the contour descriptions of the objects to be trapped, mostly text 7, are unknown. In order to nevertheless be able to carry out automatic choking by using a computer 12, a so-called colored tiling pattern color space 5 is created for the light text background 6. Colored tiling pattern color spaces 5 may contain any desired PDF content including images, color gradients or graphics, and are treated as a special color space that may be used like the traditional known color spaces (CMYK, RGB, gray, spot colors . . . ). This pattern color space 5 contains some of the objects 6 or all objects 6 that are located underneath the text 7, but at least the object 6 that is to be choked.

(13) Such a pattern 5 is created as follows: During the automated trapping process, the PDF page description is analyzed and converted into an internal representation. This is formed of a stack of objects, i.e. of instances of different classes in a suitable programming language, which are saved in the original order of the page description and each contain a drawing element of the page. Such drawing objects are images, texts, color gradients or graphics, for instance. The objects always know their positions relative to the page and, depending on the type, among other features, their color, text, font, or graphics and all required parameters of the effective graphic state.

(14) The stack of objects that has been created in this way allows the content of a PDF page to be reproduced in its entirety or in parts in such a way that the content completely or partly corresponds to the original content. In the PDF, a page description is formed of a so-called content stream (a series of commands, coordinates or other instructions) and the associated resource elements (images, fonts, color spaces, or color gradients among others).

(15) During the trapping process, objects that are in contact with each other are examined in terms of whether traps are required, depending on their color and the selected trapping rules. Traps are inserted in the stack of objects as new objects, which are accordingly marked as trap objects that also know the relevant original objects, for instance. If in this process it is found that traps need to be made by using colored tiling patterns 5, the region in question is marked as a rectangle 11 and initially a preliminary trap description is created internally. When the trapping process has been completed, the stack of objects is asked to create one or more page descriptions for all regions concerned. In this page description, all objects that are not required for this specific purpose are left out, i.e. all trap objects that have been created so far, all objects that are completely outside the required rectangular area 11, and all objects for which these special pattern traps are to be created. Just like a regular PDF page, a colored tiling pattern 5 is formed of a content stream and the resources required therein and may thus easily be created from the data that have just been generated. The bounding box of the pattern required for this purpose corresponds to the requested rectangular area 11 of the page. The tiling pattern color spaces 5 that have been created in this way are inserted into the pattern resource of the original page and the preliminary trap descriptions may now be completed by adding the corresponding pattern name.

(16) The colored tiling patterns 5 that have been created in this way may now be used to carry out the choking 10. A flow chart of this process is diagrammatically shown in FIG. 3.

(17) The preferred embodiment of the method described therein is further illustrated in FIGS. 4 to 8.

(18) FIG. 4 illustrates an exemplary PDF document having two background objects 6 (background gradient 8 and background image 9) as well as a text that extends over both backgrounds 6. A text 7 (100% K, 0% CMY) is to be choked into gradient 8 (smooth shade) and image 9 using the CMY colors of the objects 6 located underneath. The font 7 is not embedded and no contour description is available. A contour graphic is available for the image 9 and the gradient 8.

(19) As is shown in the flow chart of FIG. 3, in a first step, the colored tiling pattern 5 is created. The manner in which this is technically done has been described above. For the current exemplary embodiment, the colored tiling pattern is created from a part of the page description without the texts to be choked. This preferably includes the region underneath all texts 7 to be trapped (see FIG. 5). The resultant pattern 5 may be composed of all elements available in the PDF document. What is important is that the pattern 5 is created so as to be correctly and properly positioned with respect to the original position on the page. A colored tiling pattern 5 has a freely definable size and a description of the associated elements. This tile 11 is repeated in all directions.

(20) Two clip commands are placed in order to choke the text 7 into the gradient 8:

(21) Clip No. 1: gradient mask minus image mask,

(22) Clip No. 2: text clip.

(23) The result is shown in FIG. 6. The following text stroke commands only become effective in the overlap of both clips, i.e. in the text region located above the gradient 8. The two clip commands and the subsequent stroke commands are only carried out across the gradient background 8 and across the text 7 located above the gradient. The text 7 above the image background 9 is choked in a second run of the process.

(24) In the next step, a text stroke command is placed in the desired double trapping width. In this process, the previously created pattern color space 5 is set as the stroke color. As is shown in FIG. 7, in the border region of the text 7 located above the gradient 8, the text has been covered in an inward direction with the smooth shade located underneath. Thus optically, the text 7 is narrowed by the trapping width, which is classical choking behavior 10.

(25) In a last step, another text stroke command is placed. The separations of the text 7 that are to be effective in the choke 10, which are the text color black in the illustrated example, are taken as the overprinting stroke color. The result is shown in FIG. 8: the black separation of the text 7 now covers the pattern 5 lying underneath. FIG. 8A illustrates a section of the desired result, including the choked text 10. In the border region of the text 7 within the gradient 8, CMY come from the gradient and K comes from the text.

(26) The choking 10 for the image background 9 is implemented in an analogous way using the same colored tiling pattern color space 5.

(27) The completely trapped image available in digital form in the computer 12 may now be used for the printing process.