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Download the white paper - Ghent Workgroup

Ghent Workgroup

PDF Specification

in Practice:

GWG2012_CMYK

Author Jason Lisi

Education Member, Ghent Workgroup

jlisi@ryerson.ca

Date 5 February 2013

Status Final

info@gwg.org

www.gwg.org


Table of Contents

1 Introduction ................................................................................................... 3

1.1 About the Ghent Workgroup .......................................................................... 3

1.2 About the Specifications ................................................................................ 4

1.3 About This Document .................................................................................... 4

1.4 More Information ......................................................................................... 4

2 The GWG2012_CMYK Specifications .................................................................. 5

2.1 What are the GWG Specifications? Why a new standard? ................................... 5

2.2 Why a New Standard? ................................................................................... 5

2.3 GWG2012_CMYK and PDF/X .......................................................................... 5

2.4 Comparison with GWG Version 4 Specifications ................................................ 6

2.5 How can you use the GWG2012_CMYK Specification? ........................................ 6

2.6 Which 2012 Variant Should You Use? .............................................................. 7

3 Significant Changes from v4 ............................................................................. 9

3.1 Transparency ............................................................................................... 9

3.2 Optional Content .......................................................................................... 9

3.3 Color Management ....................................................................................... 9

4 Conclusion and Further Information ................................................................. 10

4.1 Conclusion ................................................................................................. 10

4.2 Further Information ..................................................................................... 10


Ghent Workgroup PDF Specification in Practice: GWG2012_CMYK

1 Introduction

The purpose of this white paper is to highlight important information regarding the latest

release of the Ghent Workgroup (GWG) specifications entitled GWG2012_CMYK.

GWG2012_CMYK replaces the GWG Version 4 Specifications.

1.1 About the Ghent Workgroup

The Ghent Workgroup (GWG) is a worldwide assembly of graphic arts stakeholders (user

associations, vendors, consultants, educational institutions, service providers, and end

users) founded in 2001. It was formed in response to increased needs for standardization

of the different processes in graphic arts workflows, especially in an increasingly

globalized service provider landscape. The rules of the group have been carefully

conceived to ensure that the group remains practically oriented, and the priority is

focused on the needs of the end users.

The GWG focuses on developing best practice guidelines and specifications for graphic

arts workflows. While initially focusing mainly on quality control and preflight for PDF

workflows in commercial print, that focus has broadened to also include metadata

specifications, workflow test suites and increased support for market segments such as

packaging.

All material created by the GWG is disseminated free of charge through the website of

the GWG (www.gwg.org) and through the vendors and user associations partaking in the

work of the group. Development happens through quarterly face to face meetings and

intermediary online discussions.

The mission statement of the Ghent Workgroup states that the group will “establish and

disseminate process specifications for best practices in graphic arts workflows”. In

practice this means that the group:

• Develops and maintains process specifications and associated documentation for best

practices in graphic arts workflows.

• Develops and maintains reference implementations to ensure the specifications it

develops are usable in the real world.

• Actively promotes adoption of its work in both the graphic arts user and vendor

communities.

• Streamlines and coordinates the decision process between its members.

While the group started its work developing guidelines for PDF quality control, it has

expanded its scope. The group is now involved in magazine, office, and packaging specific

specifications, the development of metadata specifications for delivering PDF files (job

tickets) or proofing them, and in developing test suites to ensure workflows and

applications are configured and used correctly.

Most of the work of the group is done through discussions on email based forums and

using virtual meetings. Four times a year, the members come together for a two-day

face-to-face discussion. To streamline the work and decision process, subcommittees

have been organized around specific topics do the actual work. To learn more about the

different subcommittees, or to find out how you can contribute to this effort, visit the

Ghent Workgroup website (www.gwg.org).

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Ghent Workgroup PDF Specification in Practice: GWG2012_CMYK

1.2 About the Specifications

At its core, each specification is a technical (and sometimes rather theoretical)

description of what PDF files should contain so they can successfully be processed in a

particular market segment or using a particular output technology. These technical

descriptions are translated by the software and hardware vendors who support the

specifications in their tools. They form the basis for the derived material like application

settings, whitepapers, and test suites that are usable by graphic arts users.

The specifications are not developed from scratch; each specification is built upon PDF/X

ISO standards. Because the GWG specifications add additional restrictions on top of the

restrictions imposed by the PDF/X standards, they are often referred to as PDF/X-Plus

specifications.

The technical descriptions developed by the GWG for each specification are not of direct

use to most printers, designers, or publishers. The application settings derived from

those specifications are the item they typically want to use. Application settings are

tested configuration files which can be downloaded from the GWG website and imported

in the most commonly used graphic arts applications, like Adobe Acrobat, Adobe

InDesign, QuarkXPress, Enfocus PitStop Pro, Callas pdfToolbox to make it easier to

create and deliver good (and GWG compliant) PDF files. The following application settings

are available from the Applications Settings section of the GWG website

(http://www.gwg.org):

• PDF export settings

• Preflight Profiles

• Color management settings

It should be noted that some vendors are already shipping 2012 specification setting with

their current releases of software. Each download contains a documentation file

explaining how to install and use the settings files in that specific application or workflow

system.

1.3 About This Document

The purpose of this whitepaper is to provide information on the GWG2012_CMYK

Specification, the variants within it, and how they can be used in common workflows

employing popular DTP, PDF generation, and PDF verification applications.

1.4 More Information

The GWG website contains tons of information regarding the GWG and its specifications

and deliverables and caters to vendors, associations and end-users alike. It also features

a user forum where you can interact with your peers and ask questions.

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Ghent Workgroup PDF Specification in Practice: GWG2012_CMYK

2 The GWG2012_CMYK Specifications

2.1 What are the GWG Specifications? Why a new standard?

In its search for specifications that could be recommended to its members, the Ghent

Workgroup concluded that existing ISO standards were too wide to cater to the specific

requirements of their vertical markets and for the specific problems with (older)

equipment and software used by their members.

The GWG2012_CMYK Specification currently contains seven different specifications to be

used in various markets:

1. GWG_SheetCmyk_2012

2. GWG_SheetSpot_2012

3. GWG_WebCmyk_2012

4. GWG_WebSpot_2012

5. GWG_WebCmykNews_2012

6. GWG_MagazineAds_2012

7. GWG_NewspaperAds_2012

In the future, additional variants may be added to address other market segments, such

as packaging for example.

2.2 Why a New Standard?

The GWG2012_CMYK Specification was created because of the new technology being

entered in the graphic arts workflows. Specifically, many current workflows are using

native PDF RIPs, and there has been increasing requests from the market (mainly

designers) to send and/or receive PDF files that were created using PDF 1.4 (or higher).

By basing this specification of the PDF/X-4 standard, the functionality of native PDF RIPs

can be utilized. As with previous PDF/X standards, PDF/X-4 is quite broad in its scope.

This new GWG specification is based on PDF/X-4, but applies sufficient restrictions (e.g.

with regard to the use of ICC based colors, transparency blend color spaces) to maintain

predictable and reliable file exchange for the next decade, based on proven and solid PDF

standards and emerging technology.

2.3 GWG2012_CMYK and PDF/X

All seven variants found in the GWG2012_CMYK specification are built off the latest

releasePDF/X-4 standard (ISO 15930-7:2010) which was updated in 2010, and

consequently all PDF files generated using these variants must comply with this standard.

2.3.1 Why PDF/X-4?

PDF/X is an international ISO standard that is recognized by most software and hardware

used in file production for the graphic arts. By using PDF/X as the foundation for the

GWG specifications, the validity and consistency of the files created can be better

measured and achieved.

PDF/X-4 offers significant enhancements to earlier renditions of PDF/X. Two very

important attributes of PDF/X-4 worth noting are support for native transparency (no

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Ghent Workgroup PDF Specification in Practice: GWG2012_CMYK

flattening required), and the ability to retain optional content groups, such as PDF

Layers.

By allowing transparency to remain in the PDF file, printers can take advantage of PDF

native interpreters within RIP workflows to reduce potential errors that can occur as a

result of flattening transparency. The ability to maintain optional content in the PDF file

can be advantageous when print runs require versioning, or for specialty printing such as

packaging. By using PDF/X-4 as the foundation for the GWG specifications, these

advantages can be utilized, and the standard can be used to ensure compliancy.

PDF/X-4 is a broad standard that can be used for a variety of applications. It is not

meant to be specific to a singular market segment. Consequently, it is entirely possible to

create a perfectly formed PDF/X-4 file that is not suited for a particular use. For example,

a PDF/X-4 file that allows spot colors would be ill suited for the CMYK only workflow. It is

for this reason that the GWG has created the 2012 variants. These variants further

restrict the PDF/X-4 standard to make sure that files being produced are suitable for the

purpose they are being intended for.

2.4 Comparison with GWG Version 4 Specifications

GWG2012_CMYK specifications are considerably different than the Version 4

specifications due to the support of PDF/X-4. This change was necessary to take full

advantage of changes in RIP technology and software upgrades.

This leads to a situation, where a PDF file that was compliant to the Version 4

specification, may not be compliant when checking against the 2012 specification and

vice versa. Specifically issues may arise when preflighting with regards to transparency,

ICC profiles, and optional content (layers). Therefore the GWG recommends using the

latest versions of the PDF Workflow preflighting and creation tools in place when

upgrading to the 2012 specifications. Additional Information about new requirements can

be found on the homepage in the section relating to the 2012 specifications on

www.gwg.org.

2.5 How can you use the GWG2012_CMYK Specification?

The technical descriptions developed by the GWG for each specification are not of direct

use to most printers, designers, or publishers. The application settings derived from

those specifications are the item they typically want to use. Application settings are

tested configuration files which can be downloaded from the GWG website and imported

in the most commonly used graphic arts applications to make it easier to create and

deliver good (and GWG compliant) PDF files. Examples of application settings found on

the GWG website (http://www.gwg.org) include PDF export settings for common

authoring applications, color management settings, and PDF prefight profiles.

It should be noted that with the adoption of PDF/X-4 as the ISO standard for the GWG

2012 specification, PostScript is no longer a recommended workflow. PostScript has

limitations that prevent it from taking full advantage of the benefits of PDF/X-4, and has

been known to result in conversion discrepancies. To take full advantage of the benefits

of PDF/X-4, only workflows that export and support PDF/X-4 compliant files are

recommended (e.g. workflow system use a native PDF RIP inside for full support of the

PDF 1.4 and higher model).

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Ghent Workgroup PDF Specification in Practice: GWG2012_CMYK

2.6 Which 2012 Variant Should You Use?

Different market segments and output conditions have different needs. As a result, the

GWG has identified 7 different variants of the 2012 specification in order to accommodate

for the variation of requirements between different market segments.

By creating a PDF file that conforms to the variant best suited for the printing conditions

of the job, there is a greater likelihood that the PDF file will pass the preflighting

requirements and not require further modifications.

The following table identifies the 7 GWG2012_CMYK variants and provides a brief

description of the intended market and use. If you are unsure as to which variant is best

suited for your print job, contact the service provider or printer that will be processing

the files.

Commercial Sheet Offset and Digital Printing

GWG_SheetCmyk_2012 For commercial sheetfed offset printing and/or digital printing,

4-colour process printing only (CMYK).

For line screens up to 175 lpi May also be used for FM

(Stochastic) screening

GWG_SheetSpot_2012

For commercial sheetfed offset printing and/or digital printing,

4-colour process and spot color printing

For line screens up to 175 lpi May also be used for FM

(Stochastic) screening

Commercial Web Offset Printing (Heat Set)

GWG_WebCmyk_2012

For commercial web offset printing, 4-colour process printing

only (CMYK)

For line screens up to 175 lpi May also be used for FM

(Stochastic) screening

GWG_WebSpot_2012

For commercial web offset printing, 4-colour process and spot

color printing

For line screens up to 175 lpi May also be used for FM

(Stochastic) screening

Newspaper Printing (Cold Set)

GWG_WebCmykNews_2012 For commercial coldset web offset printing, 4-colour process

only (CMYK)

For line screens up to 133 lpi

Advertising

GWG_MagazineAds_2012 For magazine advertisements, 4-colour printing only (CMYK)

GWG_NewspaperAds_2012 For newspaper advertisements, 4-colour process and spot

color printing

I some cases, such as newspaper and magazine advertisements, the choice of variant is

quite simple, as there is only one variant to choose from. In cases of general commercial

printing (sheet or web) the choice may not be so evident. In cases where there is more

than one possible variant to choose from, it is helpful to consider four variables:

1. The type of job it is

2. The type of printing that will be used

3. The number of colors the job will have

4. The quality (resolution) of the final print

For example, lets consider a brochure that is being printed using commercial offset

printing and a linescreen of 150 lpi. The brochure will have 5 colors on one side and 4

colors on the other side. Which GWG variant would be best to choose?

1. The type of job: commercial printing (brochure)

2. The type of printing: sheetfed offset

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Ghent Workgroup PDF Specification in Practice: GWG2012_CMYK

3. The number of colors: CMYK + spot

4. The quality (resolution) of the final print: 150 lpi

Based on the variables of the job, the table above indicates that the best variant to be

used would be the GWG_SheetSpot_2012 variant. By applying this methodology to a print

job, it is easy to determine which variant to use.

For more information regarding the GWG2012_CMYK variants, please refer to the

GWG2012_CMYK specification found at www.gwg.org.

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Ghent Workgroup PDF Specification in Practice: GWG2012_CMYK

3 Significant Changes from v4

Due to the differences between PDF/X-1 and PDF/X-4, there are a few significant

changes that have occurred between the GWG Specification v4 and the GWG 2012

specification. This section will highlight some of the more significant changes.

3.1 Transparency

The GWG 2012 standard is based on the ISO Standard PDF/X-4. PDF/X-4 uses PDF 1.6

as its basis for generating PDF files. As a result, flattening (rasterizing) of vector

transparency is not required as it was for PDF/X-1a, which was based on PDF 1.3. The

advantage of non-flattened PDF files is that they can take full advantage of native PDF

interpreters, such as Adobe’s PDF Print Engine and other, found in many of today’s

advanced RIPs. The result is a reduced likelihood of artifacts and improper rendering that

can result when native transparency is flattened.

3.2 Optional Content

PDF/X-4 allows for Acrobat Layers (technically known as Optional Content Groups or

OCG’s) to be present in PDF files. This feature, when combined with a native PDF RIP,

can be useful for certain market segments. For example, layers can be useful in

commercial brochures and catalog publishing for versioning purposes. Additionally, layers

can be of significant importance in packaging applications. Because layers are supported,

one PDF file can be sent to the RIP, and layers can be selected or deselected at the RIP,

thus increasing productivity and reducing the margin of error that results by sending

multiple files to the RIP.

3.3 Color Management

The GWG Specification 2012 is based on the PDF/X-4 standard that has full ICC color

management capabilities.

The GWG2012_CMYK Specification does not support device independent workflows

though. The reason for this has to do with the blending color space that is required when

working with non-opaque objects. PDF 1.6 (and thus PDF/X-4) does support non-CMYK

blending color spaces such as CalRGB and ICCBased; however there is some ambiguity

with the PDF Reference Guide as to how to use these color spaces, and as a result there

are differences in implementations. To ensure more consistent results with transparency

and the transparency blending color space, the GWG decided that the 2012 specification

would only support DeviceCMYK as a blending color space.

The GWG is in full agreement that a Best Practices document for device independent

workflows needs to be published, and continues to investigate this.

PDF/X-4 requires that an ICC profile must be embedded in the PDF/X Output Intent. The

GWG 2012 specification provides a list of recommended ICC Profiles that can be used

with each GWG2012_CMYK variant (gwg.org). When in doubt about which profile to use,

be sure to contact your printer/service provider for direction. It should be noted that

currently only up to ICC v2 profiles are supported. Care should be taken not to include

ICC v4 profiles as Output Intent Profiles in the PDF file.

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Ghent Workgroup PDF Specification in Practice: GWG2012_CMYK

4 Conclusion and Further Information

4.1 Conclusion

The GWG 2012 specification is a major update as it has replaced PDF/X-1a with PDF/X-4

as its basis. By adopting PDF/X-4 as its foundation, the GWG2012_CMYK specification

can accommodate significant improvements that take advantage of modern-day

workflows and hardware. The specification identifies 7 unique variants that can be used

to meet the requirements of different market segments, and also provides a

comprehensive lists of ICC Profiles, by specification variant, to assist in creating a proper

PDF file.

4.2 Further Information

You can find much more information on the work of the Ghent Workgroup and on

delivering print ready PDF/X files that comply with the Ghent Workgroup specifications on

the Ghent Workgroup website: www.gwg.org. You can also get in touch with someone at

the Ghent Workgroup via the online form found under the contact area of the website.

Here you can ask specific questions and get feedback.

For more information on color management, you can visit the websites of the following

organizations:

International Color Consortium

European Color Initiative

IDEAlliance

SWOP

GRACoL

www.color.org

www.eci.org

www.idealliance.org/specifications/swop

www.idealliance.org/specifications/gracol

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