Chapter 7 ― Standard File Formats

Although a large number of files provided by customers are in native file formats (ex. Quark XPress QPS or QXD) or in PostScript, the variation, complexity, and resulting errors of native and PostScript file format exchange is leading an increasing number of CTP users to employ, in deed demand, standard file formats. The two significant file formats that have become standardized are the Portable Document Format (PDF as in the CGATS PDF/X1 standard) and TIFF/IT.

PDF. Adobe explains that PDF is an object-based file format that structures PostScript so that you can share PostScript files between computers as you share PostScript files with printing devices. PostScript was never intended to be an exchange format. Furthermore, PDF files tend to be smaller than PostScript and TIFF/IT files, and PDF’s object-based format allows users to incorporate workflow tools into their PDF applications. Adobe continues to develop workflow applications of PDF. PDF should simplify OPI (a so-called PostScript hack) and should replace EPS. Adobe works with a cataloger producing catalogs via CTP. The page size ranges from 500 KB to 4 MB. Acrobat Distiller acts as a PostScript preflight tool in that it RIPs the file; hence, it verifies the structure of the PostScript files that is distilled.

The PostScript level III specification is available as a PDF file, downloadable from www.adobe.com (click on developer tools and then go to technical documents.) Adobe is working on better ways to deal with full transparencies and better methods for handling three-dimensional blends. For instance, many artists use several blends for textures that flow into one another, and these files are very slow to RIP. Adobe has introduced a tool that meshes blends, with one command, into a single object that RIPs much quicker. Quark has licensed the Placed PDF library from Adobe, which allows them to incorporate PDF elements into Quark XPress pages.

Another advantage of PDF is that it encompasses elements, such as fonts, and they can stay compressed right up to the RIP. A cataloger tested Adobe’s compression at different source resolutions and asked clients to evaluate the output. The clients did not discern any differences in the output.

Adobe is working on handling ICC profiles within PDF and will be able to turn them off or on, and will be able to append them to one another. However, their biggest issue is that ICC profiles use three-color based color spaces and the printing world is CMYK. Adobe will allow users to pick between device specific color (names spaces such as “SWOP”) and using ICC profiles.

TIFF/IT. The Tagged Image File Format/Imaging Technology format is a CGATS and ISO Technical Committee 130 standard file format. TIFF/IT supports many high-end systems formats including continuos tone (CT), final page (FP), line work (LW) formats. The header file format is based upon the popular TIFF file format that was created by Aldus in the 1980s. The header file format can be read by any TIFF utility, however, the file types supported by TIFF/IT (CT, FP, HC, etc.) are unique to the print production workflow. TIFF/IT files can come in two flavors: the very flexible P2 flavor and the somewhat limited, but widely supported and portable P1. TIFF/IT P1 is recommended for CTP workflows.

One of the questions regarding the TIFF/IT file format is the whether the format can support conversion from the PostScript environment. PostScript input into the workflow does not preclude a prepress operation from using TIFF/IT file formats. For instance, Shira, which started out as a translation application provider for Scitex, Linotype, and other CEPS providers, offers conversion utilities for this purpose. Shira’s “jetstream” will take any CEPS or PostScript input and produces any output, including TIFF/IT, PostScript, and PDF.

One of the shortcomings of the raster-only workflow is that you must know the output resolution of the final output device and the file must be built accordingly. Problems are encountered when distributing files to several different output devices or when files are distributed and the output is not known.

CGATS Standards.  CGATS Subcommittee 6 (The Committee for Graphic Arts Standards is an ANSI organization and subcommittee six is working on file formats) is working to specify file formats for the exchange of final composite files. Some of the issues that the SC6 encountered include the inherent differences between application-based files and data-based files, as well as the differences between raster-based, vector, and object-based file files. The main guiding objective of the SC6 committee was to create “fool-proof” file formats for final-form printing. By final form printing, we mean that all high-resolution files have been added to the layout, all press and color adjustments have been made and the file is ready for output to plate.

The SC6 committee decided to use a variation of TIFF for raster-based data and PDF for object-based data. They divided the PDF task into two parts. The first part, PDF/X1, is for final composite images and the second part, PDF/X2, will cover partial image exchange. The SC6 committee has also added a PDF/X0 or Base-PDF file standard to its agenda. PDF/X0 will deal with graphic arts application of PDF without any modifications or restrictions to the format itself. In other words, PDF/X0 deals with PDF as provided by Adobe.

Complete (PDF/X1) files must include all fonts and font metrics, all images are incorporated as high-resolution files, and the file needs to be color calibrated for the output target (e.g., SNAP, GRACoL, & SWOP). Partial files (PDF/X2) are applied when all information necessary to render the document has been processed, but may not be part of the exchange. Furthermore, fonts and font metrics may already be at the receiver’s site. PDF/X2 is used to process OPI/APR images separately when color profiles are processed at the receiver’s site. PDF was selected, rather than PostScript, because it is a much richer language in that it allows for forms support such as job tickets and it verifies the document’s structure. The SC6 group is simply setting a standard for using PDF as published, rather than rewriting the format. Some of the SC6 standards for the use of PDF include:

·        Requires all components be part of the exchange

·        Specifies external file references

·        Defines the use of a preview image

·        Identifies compression methods

·        Defines trapping flags

·        Defines CMYK color space

·        Defines spot color usage

·        Recommends use of file identification

The standard for TIFF/IT is ISO 12639 Graphics Technology – Prepress digital data exchange – tag image file format for image technology (TIFF/IT). TIFF/IT may be updated in the future to include a preview image, allow compression, and with new P1 formats. One of the biggest things that the SC6 committee has pushed for is identifying the types of files that you can include in a PDF/X1 file: TIFF/IT P1 file types, TIFF 6.0 (limited to CMYK formats conforming to TIFF/IF P1 gray scale and bi-level files), EPS, and DCS version 1.0. Files must also us “Xform” and Ximage” PDF features for preview images.

CGATS.12/1, Part 1 “PDF/X1 for complete exchange” has been approved and is available in final published form from NPES (see http://www.npes.org). CGATS.12/1 Part 2, “PDF/X2 for partial exchange” is still in development. Anyone who wants to get directly involved with the SC6 subcommittee should call Mary Abbot of NPES at (703)264-7200 or via email at mabbott@npes.org. Direct input by CTP users, (printers, prepress services, and print customers alike), even if you can’t make it to all of the committee meetings, is greatly valued and critical to the success of standard file format development.