By Mark Lewiecki, Senior Product Manager, Adobe
In the near future, the International Organization for standards will publish ISO 32000-2, the first major revision of the PDF specification since 2008, when Adobe officially released the open standard to the world. ISO Technical Committee 171 has been working on the upgrade for over 8 years, and has added several new capabilities that can enhance security, accessibility, 3D, multimedia, geospatial mapping and prepress workflows. Once the PDF 2.0 spec is published, the ISO Committee that maintains Graphic Arts standards (TC-130), will formulate a new version of PDF/X to specify exactly how the new features should be implemented for use in print production. PDF/X-6 will likely be ready sometime in the next 12-18 months. The workflow systems which process PDF, the RIPs which render it, and the applications which generate it, will all need to be upgraded for printers to realize the benefits. Although the industry has some time to figure out how PDF 2.0 will play out, it’s not too early begin considering its impact on prepress production. The main print-related adjustments to the PDF imaging model are: (1) Page-Level Output Intent profiles; (2) Black Point Compensation; and (3) Spectral Values for Device Colourants.
(1) Page-Level Output Intent. Output profiles are essential to modern colour management. The images and graphics in a PDF file may have different origins, encodings and gamuts. But, when rendered for print reproduction, they all need to be converted to the colour space of a standardized printing condition (e.g. SWOP uncoated), or a device-specific colour space. Today, PDF/X files include a single Output Intent profile, which is applied to every page in the job. However, the pages within a job may be printed quite differently. For example, a magazine: the cover is printed on coated stock (full-colour) on a gravure press, while the inside pages, which may include both monochrome and process-colour pages, are offset-printed on uncoated. So, a single Output Profile cannot always cover an entire multi-page file. With the ability in PDF 2.0 to attach multiple Output Profiles, and associate them with specific pages, prepress solutions/operators will have greater flexibility in managing, routing and automating multi-page jobs. (Note: this feature requires the use of PDF/X-6.)
(2) Black Point Compensation. Adobe introduced the concept of BPC in Photoshop 5 (2006). It’s mainly used to convert an image from a larger gamut (e.g. sRGB) to a smaller gamut (e.g. SWOP uncoated). Before BPC, out-of-gamut shadow tones were all converted to solid back. BPC preserves shadow detail by re-mapping the full grayscale range (the “L” in CieLab – for “luminosity”), albeit with some compression. Modern prepress workflow systems and RIPs enable BPC to be turned on for a job, which works for most situations. However, in some cases, BPC produces unsatisfactory results (e.g. when converting from a smaller to larger gamut). With PDF 2.0, it will be possible to specify BPC instructions (on/off/default) for an entire job, or for individual elements within. As with most image conversions, BPC transformations are lossy. So converting at run-time versus upstream is always preferred. Embedding a BPC switch for objects within a PDF 2.0 file will enable late-binding BPC conversion in the RIP, (e.g. when the rendering is performed by the Adobe PDF Print Engine).
(3) Spectral values for device colourants. When a colourant is printed on a substrate, it reflects/absorbs different amounts of light at different points on the visible spectrum (400-700 nm). Communicating a brand colour by name, or printed swatch, or CieLab value, is the norm today – but can be problematical. Even worse is the common practice of communicating a colour by “equivalent” RGB or CMYK values. Spectral values are absolute and definitive. Their meaning is not open to misinterpretation, nor dilution by tolerances (acceptable deviations). Neither are they dependent upon lighting conditions. The Color eXchange Format (CxF), an ISO standard, defines a colour using a set of reflectance values for different points on the spectrum. One or more CxFs can be attached to the Output Intent dictionary within a PDF 2.0 file. This ability will enable greater accuracy in the reproduction of spot colours (critical to brand owners). Even more importantly, spectral colours will enable more accurate emulation of specialty colours on proofing devices, including when they’re used in a process fashion (i.e. screened and overprinted). Future workflows in package printing (both flexo and inkjet), industrial printing and textile printing, will benefit from superior Quality Control, made possible by PDF 2.0’s ability to convey spot colours with spectral data. (Note: this feature requires use of PDF/X-6).