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Graphical Printers
The development of GUIs led to major changes in printer technology. As computers moved to more picture-based displays, printing moved from character-based to graphical techniques. This was facilitated by the advent of the low-cost laser printer which, instead of printing fixed characters, could print tiny dots anywhere in the printable area of the page. This made printing proportional fonts (like those used by typesetters), and even photographs and high-quality diagrams, possible.
However, moving from a character-based scheme to a graphical scheme presented a for- midable technical challenge. Here’s why: The number of bytes needed to fill a page using a character-based printer can be calculated this way (assuming 60 lines per page each containing 80 characters):
60 X 80 = 4800 bytes
In comparison, a 300 dot per inch (DPI) laser printer (assuming an 8 by 10 inch print area per page) requires:
(8 X 300) X (10 X 300) / 8 = 900000 bytes
Many of the slow PC networks simply could not handle the nearly one megabyte of data required to print a full page on a laser printer, so it was clear that a clever invention was needed.
That invention turned out to be the page description language (PDL). A page description language is a programming language that describes the contents of a page. Basically it says, “go to this position, draw the character ‘a’ in 10 point Helvetica, go to this position...” until everything on the page is described. The first major PDL was PostScript from Adobe Systems, which is still in wide use today. The PostScript language is a com - plete programming language tailored for typography and other kinds of graphics and imaging. It includes built-in support for 35 standard, high-quality fonts, plus the ability to accept additional font definitions at run time. At first, support for PostScript was built into the printers themselves. This solved the data transmission problem. While the typical PostScript program was very verbose in comparison to the simple byte stream of charac- ter-based printers, it was much smaller than the number of bytes required to represent the entire printed page.
A PostScript printer accepted a PostScript program as input. The printer contained its
own processor and memory (oftentimes making the printer a more powerful computer than the computer to which it was attached) and executed a special program called a PostScript interpreter, which read the incoming PostScript program and rendered the re- sults into the printer’s internal memory, thus forming the pattern of bits (dots) that would be transferred to the paper. The generic name for this process of rendering something into a large bit pattern (called a bitmap) is raster image processor or RIP.
As the years went by, both computers and networks became much faster. This allowed the RIP to move from the printer to the host computer, which, in turn, permitted high-quality printers to be much less expensive.
Many printers today still accept character-based streams, but many low-cost printers do not. They rely on the host computer’s RIP to provide a stream of bits to print as dots. There are still some PostScript printers, too.