Separated worlds

Designing for commercial print is quite unlike outputting to your local inkjet printer, and to be a successful print designer you need to grasp this fact firmly. A good way to start understanding commercial print is to forget about colour entirely and just think black and white. Just 20 or so years ago, the 1-bit Hercules graphics card was the only way a PC could display high-resolution graphics, and by so doing it opened up the world of onscreen typography to PCs. Just like the Macintosh, the Hercules card represented pages onscreen using black pixels on a white background, simulating black ink on white paper. It wasn’t just computers that were restricted to black and white; the vast majority of commercial print was, too.

Separated worlds

Today, this seems odd – for simple black-and-white output, why not just run off copies on your own high-speed laser? Back then, though, quality printers were typewriter-style daisy wheels, restricted to a couple of typefaces at fixed sizes, while draft-quality dot-matrix printers produced a wider range of faces and sizes, but at a horrendously low resolution. To produce high-quality display type for, say, a poster, you resorted to sheets of Letraset transfer lettering rubbed onto your artwork one letter at a time (which was infuriating when you ran out of As or Es). Once assembled, you then took your artwork to the nearest photocopy shop to have it reproduced.

The arrival of the Apple LaserWriter changed this world forever, printing scalable typefaces at a reasonable 8ppm and 300dpi. Even better, the LaserWriter was designed to work with the first DTP application, PageMaker, enabling design-rich, multifont, multicolumn layouts with pictures to be output in a single pass. The LaserWriter revolutionised professional design because, unlike rival laser printers, it was driven by a PostScript processor. Adobe’s PostScript defined pages and fonts programmatically, so they were independent from both resolution and device, producing exactly the same output from any PostScript device. You could accurately proof your layout on the 300dpi LaserWriter, then output the finished job at up to 1,448dpi on a Linotronic 100 (the first PostScript-based imagesetter) at more than two million dots per square inch or more than 20 times finer than the LaserWriter’s 90,000. Moreover, where earlier phototypesetters had produced galleys (columns of text) that had to be cut and pasted into the desired layout, the imagesetter produced a whole high-resolution layout as a ready-to-print composite film.

High-resolution imageset output also made possible the accurate reproduction of greyscale tints rather than just solid black: this halftoning process is a crucial capability, but again one that’s little understood. Halftoning creates grey tints by treating the imageset output as a grid, each square of which contains a halftone spot of variable size and shape: to produce a 50% grey, the spots fill half of each square and so on. Using a fine screen makes the tiny individual spots so hard to distinguish that the optical illusion of a smooth grey is produced.

An extension of this idea enables continuous-tone greyscale images to be printed, with each pixel’s brightness value controlling the size of the corresponding halftone spot. PostScript Level 1 and 2 were restricted to 256 grey levels, which required a grid of 16 x 16 device dots to reproduce (16 x 16 = 256). Whether the halftone grid used actually supports this many greyscale levels depends on the interaction of the output device’s resolution in dpi with the grid’s spacing in lines per inch (lpi). For example, outputting at 1,200dpi through a 100lpi grid means each halftone grid square is 12 x 12 (1,200/100 = 12) device dots, which reduces the available grey levels to 144: in this case, you might want to trade off fewer lpi for more tonal range.

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