Fine line rendering: New in version 5.5 of the PDF Print Engine
Image source: Adobe Stock/Taiga.
By Mark Lewiecki
Posted on 12-08-2020
Ink meets paper: That’s how a beautiful design becomes a physical object, something that thousands, or even millions of people can touch and react to in the real world, a.k.a. printing. In the prepress cycle, lines and curves in the design are converted into instructions for a laser beam or inkjet head. Shapes and contours, filled with color, are transformed into pixels, each one turned “on” or “off.” Over 2 billion pixels are needed to print a 4-color letter-size sheet on an offset press. A laser beam exposes a metal plate for each “on” pixel — designating where the ink will be deposited after the plate is mounted on an offset drum, and the press starts rolling. For a digital press, approximately 33 million pixels are needed to print the same job. An inkjet nozzle will squirt out a few picoliters of ink for each pixel (1 picoliter = 1 trillionth of a liter). Voilà! Ink-on-paper becomes information, an image, a message, a story, a burst of color.
Converting vector shapes into pixels is the job of the Raster Image Processor, known as a RIP. PostScript was the world’s first device-independent RIP technology, the product that launched Adobe over 35 years ago. Device independence was a game-changer: For the first time, any design could be printed on any printer or press, with the graphic elements appearing as sharply as possible. The greater the resolution, the sharper the edges. The Apple Laser Writer was the first PostScript-powered printer, with a resolution of 300 dots per inch (dpi). Most desktop laser printers today support at least 4x more pixels, and platesetters typically have resolutions of at least 2400 dpi.
But before a design could be RIP’ed, the graphics must first be described mathematically. PostScript and PDF employs the language of Bézier curves, familiar to Adobe Illustrator users as the points and handles that define a contour. The shapes could be outlined and/or filled with any combination of colors. What about text? In the 1980s, Adobe took the initiative to port popular fonts into mathematically-described shapes, converting each letter in a font (a.k.a. “glyph”) to a set of Bézier curves.
Just as graphics are mapped to the pixels in a computer monitor, a RIP maps graphics to the pixel grid of the platesetter or digital press, deciding which pixels to turn on or off. For the interior of a shape, it’s a no-brainer. But things can get tricky at the edges, and there are trade-offs to any decision. Different algorithms may be used for different types of graphics. For example, the Adobe PDF Print Engine employs Adobe CoolType, the same font technology used by Creative Cloud applications like Illustrator or InDesign or Photoshop. This ensures consistency in the look and feel of text elements. But the Print Engine uses a different scan conversion rule (vector-to-pixel algorithm) for the graphic elements, which errs on the side of “strong around the edges,” a slight boldening effect. That works well for almost all graphics. But there are some exceptions.
Paper currency usually includes fine wavy lines, a commonly used feature in many types of security printing. It is a delicate balancing act. If a line is too thick, its effectiveness in foiling counterfeiters is reduced. Likewise, if a line thins out to the point where it drops out. Fine lines can be used in maps, architectural diagrams, and other types of print jobs.
Another important exception: Text that has been converted to outlines. This is a feature in Illustrator which is sometimes used by packaging designers and prepress operators to ensure that copy is locked down, preventing inadvertent errors, which could be very serious. (“The box for your prescription medicine should have read 2 pills/day.”) But converting text to outlines results in words appearing slightly heavier than if the text had remained live (i.e. using a font), since different algorithms are used for text and graphics. The effect is not noticeable for typical pointsizes, or even small pointsizes on a high-resolution press. But printing small outlined text on an inkjet press can cause white areas within a glyph to fill in. This fill-in effect is especially noticeable in Chinese or Japanese characters, which contain more fine detailing than Roman characters.
The PDF Print Engine is the rendering technology used in most prepress solutions around the world. Over the past 14 years, it has brought the latest advancements in imaging science to the print industry. Fine Line Rendering is a new feature in PDF Print Engine version 5.5, recently made available to our solution partners. It addresses some of the special cases described in this post. When built into products, Fine Line Rendering will give printers the power and freedom to crisply reproduce the finest lines and the smallest type.
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