1997 PROCEEDINGS
`
`Technical
`Association
`of the
`Graphic
`Arts
`
`TAGA
`
`"Disseminating
`graphic arts
`research
`internationally
`since 1948."
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`The 1997 T AGA Proceedings was edited and assembled by
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`P ublication Rights Agreement
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`

`STATUS OFCGATS.l2
`USING PDF FOR DIGITAL DATA EXCHANGE
`Stephen N. Zilles'
`
`Keywords: Data, Electronic, Files, Standards
`
`Abstract: The Portable Data Format (PDF) is a format for representing
`composited electronic documents for the purpose of exchanging the
`document between a sender and a receiver that may not have had any
`prior communication. PDF provides an object based representation of the
`content of the electronic document; that is, there are different
`representations for the objects, the text, geometric graphics and raster
`images, of which the document is comprised. These object based
`representations efficiently encode the various object types and also
`provide a reproduction-device-independent representation of the digital
`data. The Committee for Graphics Arts Technologies Standards (CGATS)
`is developing a standard, CGATS.12, for the exchange of digital data
`using PDF. Thjs work is motivated by the desire to allow the electronic
`transmission of the creative work of the graphic artist throughout the
`production workflow to final production either as a final image on media
`or a surrogate for that image, such as a printing plate. To avoid
`dependence on the set of applications used to create a graphic
`presentation, a standard format is necessary to allow transmission of the
`digital work through prepress shops to the publisher and on to the
`printer. The DDAP (Digital Distribution of Advertising for Publications)
`Association has been instrumental in encouraging and supporting this
`standardization. This paper provides an overview of PDF and a PDF
`workflow and describes how PDF can be used in conjunction with raster
`based workflows.
`
`Background
`There have been two approaches taken to the preparation of
`digital, electronic representations of graphics arts presentations. In one
`approach, raster-based, the entire presentation is represented in terms of
`raster images, both those scanned from traditional sources, such as
`
`* Manager of Standards, Adobe Systems Incorporated, 345 Park A venue,
`MS/Wl4, San Jose, CA 95110-2704, Email: szilles@adobe.com
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`photographs, and synthetically created images such as text composed on
`a computer system. In the other approach, object-based, the representation
`of the presentation is based on the type of object, such as text, geometric
`graphics and raster images, being represented. For example, text is
`represented as character strings plus a font to be used to render the
`character string; geometric graphics are represented using commands to
`draw the graphic and raster images are represented as rasters. Because
`the second approach uses distinct representation approaches for the
`object types within the document, the total representation is quite
`efficient and is independent of the resolution of the (often unknown)
`reproduction device on which the digital document is to be realized.
`A raster-based format in which all of the "objects" have been
`rasterized; that is converted into some portion of an array of color picture
`elements or pixels at a resolution suitable for the intended reproduction
`device. Because the raster representation is closer to the direct inputs of
`the reproduction device, the process of generating film, plates or final
`impressions is simpler and has a more predictable time scale.
`Both object-based and raster-based formats complement each
`other: a raster-based format provides the tightly bound representation for
`which most of the rasterizing decisions have been made and an object(cid:173)
`oriented format provides as representation that is more flexible, which
`can be adapted to more kinds of reproduction processes and resolutions.
`The flexibility of object-based formats also provides a greater capability
`for correction of errors in the content when that is necessary and it
`provides for personalization, such a local address lists, bingo card
`numbers, of the content of an advertisement.
`Publications and other presentations may combine material
`distributed in either format. For example, a publication may have its
`editorial content in an object based format and have two partial page
`advertisements, one in an object-based format and the other in a raster(cid:173)
`based format. This requires that both raster-based and object-based
`formats can be combined to produce a "final" presentation.
`The raster-based approach is the subject of the companion paper
`by David McDowell in this volume on the status of TIFF /IT. TIFF /IT is a
`standard raster-based format which is based on the output formats used
`by Color Electronic Pre-press Systems. This paper presents an overview
`of PDF which is an object-based format and the basis for CGA TS.12, a
`standard for PDF eXchange or digital pre-press data, also known as
`PDF-X.
`
`Object-based Exchange formats
`There are several widely used, object-based exchange formats
`based on what is now called the Adobe Imaging Model. This imaging
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`model is a powerful way to represent text, graphics, and images in a
`coherent and consistent manner. It was originally implemented in the
`PostScript Language, which is an exchange format for printing and is now
`at the core of a wide range of printing and publishing technologies,
`including the PostScript Language, the Portable Document Format (PDF),
`and a number of application products.
`The PostScript Language
`The PostScript Language has become the industry standard for
`interchanging printing files for producing high-quality output. Central to
`the success of the PostScript Language is the fact that it is a completely
`device-independent page description language. This means that the same
`file can be printed on desktop laser printers that cost a few hundred
`dollars or on high-end printing systems that cost hundreds of thousands
`of dollars, and the document will always print with the highest quality
`achievable by the particular output device.
`Another important aspect of the PostScript Language is its
`imaging model: the model by which the text, geometric graphics and raster
`data is imaged onto a medium. The imaging model has two components:
`a set of operations for painting colored regions onto the background and
`a graphics state which controls how the painting is done. The power of
`the model comes from the kinds of curves and areas that can be modeled
`and the level of control that the graphics state provides. Over its more
`than 10 years of use, this model has been shown to be both robust and
`powerful. Most electronic graphics arts presentations can be and are
`represented in the PostScript Language.
`Also important to the success of the PostScript Language is thnt is
`has been documented in a publicly available form since its creation in
`1984. The PostScript Language Specification Manual contains permission
`for anyone to implement to the specification. This has allowed multiple
`vendors to provide implementations and has encouraged competition to
`provide the best quality and highest performance.
`The PostScript Language is both a page description language that
`describes the format of a printed page, and a general-purpose
`programming language. The programming language part of the PostScript
`Language contains procedures, variables, and control constructs that
`must be interpreted to render a page description. The programmability of
`the PostScript Language has both positive and negative aspects. On the
`positive side, it provides a flexibility that has made it possible to adapt
`PostScript to almost every document production environment. On a more
`negative note, the procedural nature of PostScript files means that thE'y
`must be interpreted in the order in which they are streamed into an
`output device. One reason for this is that PostScript files are not
`inherently page independent; imaging operations performed on the second
`page of a document may depend on graphics state settings that were
`established on the first page.
`·
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`The Portable Document Format
`The Portable Document Format (PDF) was created to take the
`power of the PostScript Language beyond just printing. PDF and the
`software used to create, display and print PDF is also based on the
`Adobe Imaging Model. It was developed so that users could view and
`manage documents on-screen in a device- and application-independent
`manner. With PDF, documents can be created in virtually any
`application, on any platform, and easily converted to PDF, where they
`retain the full range of high-quality typography, graphics, images, and
`color. PDF files can be viewed, navigated, searched, printed, and
`archived in Macintosh, Windows,® and UNIX® environrnents1.
`PDF retains the Adobe Imaging Model, but removes the
`programming language portion of PostScript. This makes imaging PDF a
`simpler, more rapid process. PDF also explicitly represents the structure
`of the document: the content of the pages (and parts of pages) and the
`resources required to image these pages are all separately represented in
`the PDF file. With this structural information, each page can be
`independently imaged and the data required to do that imaging can be
`efficiently accessed. (There is even a form of PDF, linearized PDF, thnt is
`optimized for incremental access to the parts of the file necessary to
`image a particular page. Linearized PDF can be used to provide efficient
`display of randomly accessed individual pages of a document over i'l
`network such as the World Wide Web.) To reduce the size of PDF files,
`compression techniques nre built into the format to compress the contents
`of pages and the resources.
`PDF for Prepress Digital Data Exchange
`From early on in the process of standardizing the exchange of
`digital prepress data, it has been realized that there is a market need for
`both raster-based and object-based exchange formats. For example, the
`requirements prepared the association for the Digital Distribution of
`Advertising for Publications (DDAP) refer to both the work on ANSI IT
`8.8 using TIFF and the work using the PostScript Language. In 1993,
`CGATS SC 6 was created to develop standards to satisfy the DDAP
`requirements. The enrly object-based work undertaken by CGATS was
`based on the PostScript Language and Encapsulated PostScript (EPS)
`files. When PDF 1.0 was released in June of 1993, it became clear that
`PDF was a better solution than EPS for prepress exchange becnuse its
`simpler, more rigorous structure mnde rasterization easier and freer from
`anomalies that are sometime experienced with EPS files.
`PDF 1.0 did not, however, have the full functionality of the Level
`2 PostScript Language; it lacked some of the features needed for Graphics
`
`1 using, for example, the free Adobe Acrobat software (available on
`http:/ /www.adobe.com).
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`Arts uses. Work on an object-based standard using EPS began in 1993,
`as this work progressed, it became clear that it would be difficult to get
`agreement on how to constrain EPS suitably for reliable exchange. During
`this period, PDF was being extended to have the features required for
`graphics arts work. In 1995, it became clear that PDF would provide a
`more robust solution than EPS and the work of CGATS SC 6 shifted to
`use PDF as the reference for CGATS.12. With the publication of PDF 1.2
`in November, 1996, almost all of the features required for graphics arts
`usage were part of the PDF specification.
`Some of the additional benefits of using PDF for prepress digital
`data exchange are editability and improved archivability. Because of the
`structure and object-based nature of PDF files, it is simpler to construct
`an editor that can make necessary, last-minute changes to the file. The
`textual data is stored in PDF as strings rather than images. This
`facilitates editing the content of the string to correct errors. The structure
`of PDF also allows parts of the file to be identified as replacement areas
`which would be varied to allow local vendor names and address or bingo
`card numbers to be added for a particular use of the file. Also because
`the text is still in string form, the PDF files can be indexed simplifying the
`search for a particular archived file based on the content of that file.
`
`PDF and Prepress Workflow
`One reason for choosing PDF as the basis for a digital prepress
`data exchange format is the recognition that PDF was also a better basis
`for the desktop publishing prepress workflow. Today, most documents
`are delivered to prepress or print shops in the authoring application
`format. Once received, the file enters a workflow process based on the
`PostScript Language or native application formats.
`Although the PostScript Language was initially developed as a
`language for describing pages and controlling printers, but its flexibility
`enabled it to become the data format for carrying prepress and
`production information as well. But, flexibility is a two edged sword; it
`can also lead to unpredictability, in part because so many different
`applications generate PostScript Language files in so many different ways
`and PostScript Language page descriptions can be arbitrarily complex. A
`typical imposition application may have to understand 200 or more
`different application versions of PostScript output. It is not uncommon
`for prepress application developers to spend half of their development
`time just keeping up with the latest application output streams.
`In contrast, PDF files are highly structured. A PDF file can be
`thought of as a database of objects with direct access to each object, and
`each page of a PDF document is independent of the others. If a prepress
`application uses PDF files instead of the PostScript language as its input
`and output, it is able to directly access the information needed and
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`incrementally update the file. The prepress application also has just one
`format to understand-PDF. The apparent arbitrariness of PostScript
`technology is eliminated, so PDF provides the foundation for a print
`production system that delivers consistent, predictable results. A PDF
`file delivers the single "digital master" for use in electronic, printed, and
`mixed workflows, ensuring the highest fidelity across all media.
`PDF 1.2 includes the features necessary for PDF files to work
`seamlessly in production printing for color and monochrome workflows.
`The full functionality of the Level 2 PostScript Language, including the
`features for high-end printing, such as control over screening, separations
`and image replacement is representable in PDF 1.2. This allows a
`PostScript Language file to be converted into PDF 1.2 and, where
`necessary to use existing PostScript Language devices, can be converted
`back again to the PostScript Language without loss of functionality,
`without loss of visual fidelity. A benefit of delivering documents as PDF
`files is that a PostScript Language file that has been created from PDF
`tends to print more reliably than the original PostScript file. In the
`conversion to PDF, the arbitrariness is removed from the file, so that
`when it is converted back to PostScript technology it is more tightly
`structured. Some of the features of PDF 1.2 that make these conversions
`possible are:
`
`The graphics state set in a PDF file has been extended to
`include relevant device-dependent parameters. This "extended
`graphics state" allows the specification of stroke adjustment,
`overprinting, black generation, undercolor removal, transfer
`functions, halftone screens, and halftone phase.
`The information in Open Prepress Interface (OPI) version
`1.3 comments can be represented in PDF files, enabling OPI image
`replacement to be preserved in the PDF file. Very large high(cid:173)
`resolution images can be stored separately from the PDF file itself,
`allowing small files to be maintained and routed with the large
`images replaced at print time.
`The full set of Level 2 PostScript Language color spaces is
`in PDF 1.2. This included adding separation and pattern color
`spaces. A separation color space can be specified for any
`separations (spot colors or process colors) to be produced by a
`given device. If the output device does not support the specified
`separation, it will use an alternate color space (specified in the
`PDF file) for predictable behavior. A second new color space, the
`pattern color space, allows the printing of PostScript language
`patterns.
`A typical scenario using PDF for document delivery would be: o
`graphic designer creates a design for an brochure. He obtains the images
`needed in photographic form and sends them the printer for scanning and
`storage of the high resolution images. The printer returns EPS files with
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`FPO (for placement only) images and OPl comments describing the high
`resolution files. The graphic designer creates a document in a page layout
`program, includes the FPO images received from the printer, and then
`outputs the document to a PDF file. Any printing control features
`specified in the authoring application are maintained. OPI comments
`specified in the EPS files are included in the PDF file so high-resolution
`images can be added back into the file before going to press. The (smC~ll)
`PDF file is then transmitted to the printer, reducing the time Clnd effort it
`takes to transfer. When the PDF file is received at the printer, initial
`preflight is streamlined because all of the components are in one neat
`package and viewable on-screen. The document is generally output to Cl
`PostScript language file at this stage, maintClining the print controls
`originally specified in the authoring application. It is routed through Cl
`prepress workflow, high-resolution images that remained at the printer
`are replaced in the file, C~nd the document is output to final film, piClte or
`paper.
`
`Commercial experience with PDF Ad Delivery
`The Associated Press (AP) operates a digital advertising delivery
`service called AP AdSEND. This is one of several services providing for
`the delivery of advertisements in PDF format. The AP delivers
`Cldvertisements via satellite to newspapers throughout the United States.
`Using PDF provides tremendous advantages for retailers and advertising
`agencies because of the cost savings, ability to make important last(cid:173)
`minute changes, faster time to market, and higher reproduction quality.
`At present, AP AdSEND supports more thCin 1,300 newspClpers
`receiving advertisements in PDF format from more than 400 major
`advertisers. The volume is now more than 70,000 full-page
`Cldvertisements per month; and up to 4,600 advertisements per day at
`peClk times. Up to this year, almost all of these PDF format
`advertisements have been gray scale advertisements. In 1997, AP
`AdSEND has begun to distribute color advertisements as well as gray
`scale advertisements, using the ability of the PDF format to transmit
`color, including device independent color.
`In addition, newspapers that receive PDF advertisements can use
`the "Export to EPS" feature of Acrobat Exchange to incorporate an EPS
`version of the PDF ad into a page layout. This capability eliminCltes the
`need for mC~ny of the manual production processes where errors are likely
`to occur. Combining PDF advertisements directly in a page layout can
`result in a substantial cost savings to newspapers, becCluse doing so cCln
`eliminate errors and the need to offer rebates or make good on misrun
`advertisements.
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`Summary
`CGATS.12 is really two standards efforts that are closely related.
`CGATS.12-1 is a standard for the exchange of complete PDF files. A
`complete file is one in which all the visual elements and all the resources
`needed to present those visual elements are included in the file. This
`means embedding all fonts used, the high resolution data for all
`replacement images and adjusting the color data for the intended printing
`conditions. CGA TS.12-2 is a standard for "incomplete" exchange in which
`some specific elements and resources may be omitted from the exchange
`by prior agreement between the participants in the exchange. For
`example, the high resolution images used in image replacement may
`already be at the printer when a PDF-X file is sent to him. In this case,
`there is no need to embed the high resolution data in the file as would be
`required for a complete exchange.
`A draft of CGATS.12 -1 is being prepared for ballot by the
`CGATS SC 6 subcommittee during the summer of 1996. This proposed
`standard specifies how to use PDF for the exchange of complete digital
`prepress data files among graphics arts establishments. This standard
`references the PDF 1.2 specification with a small number of extensions.
`These extensions include indications of whether the PDF file dntn has
`been trapped or not, what CMYK color space the data was prepared for
`and a mechanism that allows the high resolution replacement images to be
`transmitted with the PDF file. The standard specifies the use of PDF with
`only a few restrictions that are intended to insure successful exchange.
`These are primarily the use of CMYK color spaces and the limitntion of
`compression to loss less compress techniques, such as Flate and
`Run Length.
`Assuming normal progression of the standard and no new
`problems, CGATS.l2-1 should be an approved ANSI standard around
`May, 1998. This document has also been submitted as a new work item
`to the ISO TC 130 Committee which is the international graphics arts
`standards organization. It seems likely that TC 130 will accept this work
`item and progress PDF-X in the international arena on a similar, but
`slightly delayed time scale.
`
`Selected Bibliography
`Adobe Systems Incorporated
`1 990 Post Script Language Reference Manual, Second Edition,
`Addison-Wesley, ISBN 0-201-10174-2, 1990.
`Bienz, T., Cohn, R. and Meehan, J. R.
`1997 Portable Document Format Reference Manual, Version 1.2,
`Adobe Systems Incorporated, San Jose, CA, (November
`1996)
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`ISO
`
`1997
`
`ISO 12639 Graphic technology - Prepress digttal data
`exchange - Tag image file format for image technology
`(TIFF/IT), International Standards Organization, Geneva,
`1997
`McDowell, David
`1997 "Status of TIFF /IT", TAGA Proceedings, Technicnl
`Associntion of the Grophic Arts, Rochester, NY.
`
`316
`
`