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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. Page 8
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. Page 9
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