Applying traps to a printed page specified in a page description language format

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FIELD OF THE INVENTION

This invention relates generally to computer-assisted publishing and particularly to a technique for trapping pages specified in a page description language format.

BACKGROUND OF THE INVENTION

For many years, the preferred process for color printing on paper has been to process each page in multiple exposures, or separations. Each separation exposes selected areas of the paper to an ink of a different color-the usual process ink colors are cyan, magenta, yellow and key (CMYK). Additional color inks are often used as spot colors in additional separation.

High-quality printing, such as that required by the publishing industry, poses many difficult problems in controlling the separations. For example, there is often a problem with positional registration of the various separations. As a result, the several inks are never perfectly aligned with one another, and a small gap between regions of different colors occurs, which is often visible to the eye as a white line.

One common solution to the registration problem is to perform a technique known as trapping. As the separations are built, regions of a particular color are expanded or "spread" beyond their nominal boundaries, or contracted or "choked" within such boundaries. Various photographic trapping techniques are well known. Although these are sometimes tedious to apply, they are justified in expensive publications, such as magazines and newspapers, where time and labor is available to create individual traps for each primed page.

With the current widespread use of computers in publishing, various systems now perform choking and spreading electronically. The usual approach is to first render the page in pixels at the desired output resolution and then store the rendition in a memory referred to as a "frame buffer." Typically, a frame buffer is assigned to each of the four process separations. If spot colors are used, an additional frame buffer is necessary for each spot color. Each frame buffer is choked and spread on a pixel-by-pixel basis, and the result used to control the printing of the respective color. Because this approach requires a frame buffer for each of the output inks, it requires a large hardware expenditure, the cost of which is justified in expensive publications, such as newspapers and magazines.

In recent years, a new class of low-cost applications in the publishing industry has emerged, namely "desktop" publishing. A desktop publishing system user typically uses a standard personal computer to specify his output page as a computer in a page description language (PDL) such as Postscript.TM.. (Postscript is a trademark of Adobe Systems, Incorporated, of Mountain View, Calif.) The Postscript PDL file is typically sent to a PDL-compatible automatic page setter, which then interprets the file and renders the several color separations.

The typical PDL user usually does not know or care about registration of separations--all the user understands is that the color page displayed on the computer screen often does not look the same as the page printed by the priming press due to color alignment errors. In order to avoid errors, a desktop publishing user can manually examine the pages as they are displayed on a computer screen, predict where the registration errors are likely to occur, and then create traps by specifying additional vectors to cover the errors. Unfortunately, this procedure is tedious and undesirable to perform manually. Further, it is difficult to automate, especially for pages having multiple intersecting colored objects.

Thus, what is needed is a way for a desktop publishing user to automatically produce a properly trapped page from a PDL file. This would provide predictable results from a desktop publishing system without the need to provide special instructions to the page setter or incur the cost of using expensive trapping hardware.

SUMMARY OF THE INVENTION

The invention is a method of electronically trapping a printed color page including a plurality of color regions. The method includes the steps of: (a) creating, according to a set of trapping rules, a proposed trap area for a color region of interest; (b) modifying the proposed trap area if one or more color regions are in such close proximity to the color region of interest that an undesirable trap would result without the modification; and (c) creating a trap for the color region of interest which is equivalent to the proposed trap area less any modifications made to the proposed trap area.

In accordance with other aspects of the invention, the method further includes the step of comparing the proposed trap area to color regions to determine if there are any intersections between the two areas and, if one or more intersections are present, modifying the proposed trap area to exclude the color region or regions from the proposed trap area. In another aspect of the invention, the proposed trap area is further modified based upon an exclusionary zone that is defined relative to each color region that intersects the proposed trap area. As an example, for a given color region that intersects the proposed trap area, the exclusionary zone is equal to one-half the distance between the color region of interest and the intersecting color region.

In accordance with still further aspects of the invention, some of the color regions include existing trap areas that were generated prior to determining a trap for a color region of interest. In this case, the method further includes the step of comparing the proposed trap area to existing trap areas to determine if there are any intersections between the two areas and, if one or more intersections are present, modifying one of the trap areas according to a set of trapping rules. In one aspect of the invention, the trap area with the lower neutral density is the area that is modified.

In accordance with still further aspects of the invention, the proposed trap area is modified based upon an exclusionary zone that is defined relative to any color region that necessitates modification of the proposed trap. Such a region is termed a proximate color region. Also, in one embodiment, the exclusionary zone for any proximate region is equal to one-half the distance between the color region of interest and the proximate color region.

In still further aspects of the invention, the color page is expressed in a page description language format. The method further comprising the steps of: (a) reading an input data file which represents the color page expressed in the page description language format; (b) translating the page description language instructions comprising the input data file into a format suitable for detection and analysis of color edges between color regions; and (c) analyzing the translated input data file to identify color edges between the color regions, and using the translated input data file to create the proposed trap. Preferably, the a trapped output file is produced which includes the existing trap areas, with the traps being expressed in the page description language format.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and further advantages of the invention may be better understood by referring to the following description together with the accompanying drawings where in:

FIG. 1 is a hardware block diagram of a desktop publishing system that traps images according to the invention;

FIG. 2 is a flow chart of the steps performed by the system;

FIG. 3 is an example page, consisting of overlapping objects, which is to be trapped

FIG. 4 is the desired trapped output page;

FIG. 5 is a Postscript code segment describing the page;

FIG. 6 depicts one tile and the analytic description of the edges within that tile produced by the edge tile converter;

FIG. 7 depicts a list of edge vectors and associated color data for the tile;

FIG. 8 depicts the resulting list of trap vectors for the tile;

FIG. 9 depicts vectors in Postscript form;

FIGS. 10A-10C illustrate a disadvantage of traditional electronic trapping methods wherein the resultant trap can extend beyond the extreme edge of a nearby page object;

FIG. 11 illustrates an exemplary solution to the trapping problem set forth in FIGS. 10A-10C in accordance with the invention;

FIG. 12A is a page having a number of illustrative objects that are used in describing a trapping solution in accordance with the invention;

FIG. 12B illustrates a candidate trap area and proximity trap area for Object A;

FIG. 12C illustrates a prior art solution to the trapping problem set forth in FIG. 12A;

FIG. 12D illustrates an exemplary trapping solution for the trapping problem set forth in FIG. 12A in accordance with the invention;

FIG. 13 is a flow diagram of a second exemplary routine that may be used by the desktop publishing system for creating traps in accordance with the invention;

FIG. 14 is a flow diagram of a routine for determining those boundaries to be trapped and the size of the trap for use in the routine of FIG. 13;

FIG. 15 is a flow diagram of a routine for determining when the proximity of nearby page objects or existing traps is such that a modification should be made to the trap area;

FIG. 16 is a flow diagram of a routine for determining if two traps are in such close proximity that the trapping rules will be compromised if the traps remain as originally created;

FIG. 17 is a flow diagram of a third exemplary routine that may be used by the desktop publishing system for creating traps in accordance with the invention;

FIG. 18 is a flow diagram of a routine for determining those boundaries to be trapped and the size of the trap for use in the routine of FIG. 17.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The following is a description of the architecture of a general purpose computer system used to perform traps in page description language (PDL) form according to the invention. It should be understood that the Postscript PDL, the illustrated exemplary page, and trapping rules discussed herein are for illustration only, and are not meant as limitations on the scope of the invention. As will be better understood shortly, the invention lies in the way in which the input file and trapping rules are used to generate a list of trap vectors which are added to the input file.

FIG. 1 shows a block diagram of a desktop publishing system 100 including a central processing unit (CPU) 102, a memory 104, and a mass storage device such as a disk drive 106. The system 100 may, for example, be an International Business Machines (IBM.TM.) or compatible personal computer. Peripheral equipment associated with the desktop publishing system 100 includes standard input devices such as a keyboard 110 and mouse 112, and output devices such as a display 114.

Data describing the format of a desired primed page is typically input to the system 100 by a user who interactively specifies the page layout by using the keyboard 110, mouse 112, display 114 and suitable software. The input page data 300 is typically stored in the memory 104 or disk drive 106 in the form of a page description language (PDL) such as the well-known Postscript format. Other devices and methods not shown may also be used to assemble the input page data 300. The input page data 300 may typically include text, graphics, line art and/or scanned images.

After the input page data 300 is entered into the system 100, the CPU 102 then uses a set of trapping rules 160 to produce an output trapped page 400. The trapping rules 160 may be a set of rules specified by the user via the keyboard 110, or may be a preconfigured set of rules stored on the disk drive 106.

The output trapped page 400 is produced in the same PDL format as the input page data 300. The output trapped page 400, being a PDL file, may then be sent to any standard PDL-compatible page-setter 120 or other output device.

More particularly now, refer to the system software block diagram shown in FIG. 2. In the preferred embodiment, an initial set of instructions, step 206, selects a first subportion, or tile, of the output page.

In the next three steps 208, 210, and 211, the CPU 102 interprets each of the PDL instructions in the input page data 300 to build a representation of the tile which indicates the possible boundaries, or edges, between regions of different color as it will appear on the output page. This is typically done by first selecting the next Postscript instruction in step 208, and then interpreting the instruction by evaluating it within a clipping window which corresponds to the currently selected tile.

This process continues by looping back through step 211 to convert all of the input PDL instructions. The tile representation is then processed by an edge converter 212, which analyzes the tile representation and outputs information about the edges in the tile in the form of edge data 614 (FIG. 6).

The next set of instructions, referred to as the trapping analyzer 214, uses the edge data 614 and the set of trapping rules 160 to produce a description of the trap vectors 810 in any convenient form.

Step 216 then converts the trap vectors 810 to PDL form, and the process continues by looping back through step 218 until all tiles have been trapped.

The trap vectors for various tiles may be combined or merged to produce a condensed list of trap vectors, if desired, to combine trap vectors from adjacent tiles or from tiles which are overwritten by later PDL instructions. This merging step may typically be part of the generator 216.

A final step 220 appends the trap vectors in PDL form to the input page data 300, to produce the output trapped page data 400.

As a further example of how the invention operates, consider an exemplary input page 300 shown in FIG. 3. This input page 300 consists of a key (black) diamond 302 overlapped by a cyan triangle 304 on a yellow background 306. FIG. 4 depicts the desired trapped output page, including the key diamond 302, cyan triangle 304, and yellow backgro