This week, Mako™ product manager David Stevenson explains vector flattening:
When you print PDF content or save or export it to other formats that do not support transparency, it will need to undergo a process called flattening. Flattening usually involves rasterizing areas of the page that are subject to transparency effects, which could mean replacing sharp-edged vector content with a jagged-edged bitmap. Of course, increasing the resolution of the rasterization can mitigate that problem, but doing so takes longer and adds to file size.
The alternative is to retain vector geometry, including text, as vector objects. This requires dividing the artwork down into smaller parts that no longer overlap, then tracing the edges of the new shapes with a vector path. In the latest release, Global Graphics Software’s Mako Core SDK (v6.2.0) adds this capability to its raster-based transparency flattening API. Using existing APIs that apply De Casteljau’s algorithm to decompose Bézier curves and a new method to trace around shapes, flattened content can retain its device independence and printing quality.
In this example, two partially transparent shapes overlap, and set to use a multiply blend.
The rectangle indicates the zoom area for the next two images.
The result of regular raster-based flattening. The shapes are rasterized (at somewhat low resolution for the purposes of illustration) and you can see the jagged edges that result.The result of vector-based flattening. The edges are smooth.
This image shows how the vector flattener has created three new vector paths that no longer overlap (moved apart for the purposes of illustration), with the color of 2 representing the blend evident in the original artwork.
I’ve included a short demo of the vector-based transparency flattening feature using Mako here:
In the middle of 2017 ISO 32000-2 will be published, defining PDF 2.0. It’s eight years since there’s been a revision to the standard. In his next blog post about the changes afoot, Martin Bailey, the primary UK expert to the ISO committee developing PDF 2.0, looks at halftones, an area where the new specification will offer significant benefits for flexo jobs.
Lists of spot functions in halftones PDF allows a PDF file to specify the halftone to be used for screening output in a variety of ways. The simplest is to identify a spot function by name, but that method was constrained in versions of the PDF standard up to PDF 1.7 to use only names that were explicitly listed in the specification itself. This has been a significant limitation in print sectors where custom halftones are common, such as flexography, gravure … and pretty much everywhere apart from offset plate-making!
PDF 2.0 allows the PDF file to specify the halftone dot shape as a list of spot function names, and those names no longer need to be picked from the ones specified in the standard. The renderer should use the first named spot function in the list that it supports. This allows a single file to be created that can be used in a variety of RIPs that support different sets of proprietary halftones and to select the best one available in each RIP for that specific object.
This functionality is expected to be used mainly for high-quality flexo press work, where it’s a key part of the workflow to specify which halftone should be used for each graphical element.
A PDF 1.7 reader will probably either error or completely ignore the screening information embedded in the PDF if a file using the new list form is encountered. In the flexo space that could easily cause problems on-press, so take care that you’ve upgraded your RIPs before you start to try rendering PDF files using this new capability.
Halftone Origin (HTO) Very old versions of PDF (up to PDF 1.3) included a partial definition of an entry named HTP, which was intended to allow the location of the origin or phase of a halftone to be specified. That entry was unfortunately useless because it did not specify the coordinate system to apply and it was removed many years ago.
PDF 2.0 adds a new entry called HTO to achieve the same goal, but this time fully specified. The use case is anywhere where precise specification of the halftone phase is valuable. Examples include pre-imposed sheets for VLF plate-setters, where specifying the halftone phase for each imposed page can reduce the misalignment of halftones that can occur over very long distances, or setting the halftone phase of each of a set of step-and-repeat labels to ensure that the halftone dots are placed in exactly the same position relative to the design in each instance.
A PDF 1.7 reader will simply ignore the new key, so there’s no danger of new files causing problems in an older workflow. On the other hand, those older RIPs will render as they always have, which would be a missed opportunity for the target use cases.
Halftone selection in transparent areas Up to PDF 1.7 there has been a requirement to apply the “default halftone” in all areas where transparency compositing has been applied. This was problematic for those print technologies where different halftones must be used for different object types to achieve maximum quality, e.g. for flexo. Transparency is used in these jobs most commonly for drop shadows, so that’s where you’re most likely to have encountered problems.
PDF 2.0 effectively gives complete freedom to renderers to apply the supplied screening parameters in whatever way they see fit, but two example implementations are provided to encourage similarity between implementations. One of those matches the requirements from PDF 1.7, while the other applies the screen defined for the top-most graphical element in areas where transparency was applied. The second one means that the screening selected for the drop shadow will now be used, matching requirements for the flexo market.
The background The last few years have been pretty stable for PDF; PDF 1.7 was published in 2006, and the first ISO PDF standard (ISO 32000-1), published in 2008, was very similar to PDF 1.7. In the same way, PDF/X‑4 and PDF/X‑5, the most recent PDF/X standards, were both published in 2010, six years ago.
In the middle of 2017 ISO 32000-2 will be published, defining PDF 2.0. Much of the new work in this version is related to tagging for content re-use and accessibility, but there are also several areas that affect print production. Among them are some changes to the rendering of PDF transparency, ways to include additional data about spot colors and about how color management should be applied.
