Postscript is a file format supported by almost all high-end printers and many business-class laser printers. With these printers, you can simply send a postscript file to them over USB—no drivers required—and they’ll print it perfectly.
What makes postscript so special that these printers all support it natively?
Postscript And Vector Graphics
Printers print using dots to create images the same basic way the pixels in your computer monitor create images. Some image file formats reflect this by creating a bitmap, a map which describes what color to use for each bit (pixel or dot) in the image. This works well if your monitor or printer has the same number of pixels or dots as the image.
But when the bitmap has 72 pixels per inch (a common computer monitor resolution prior to high definition) how should your printer print it at 300 or 600 dots per inch (common printer resolutions)?
The answer is vector images. Instead of describing where each pixel or dot goes, vector images use constructs which look a lot like your high school geometry. To draw a line, a vector image doesn’t define which pixels to fill; it simply specifies the start point of the line and the end point of the line.
When your monitor or printer reads the vector image, it can turn the line definition into a specific series of pixels or dots, maintaining near-perfect quality at any resolution.
Postscript is a file format created to make it easy for computers to create vector images. To help computers quickly define complex operations, it also included a computer programming language which could be used to move or repeat objects without having to redefine them.
How Postscript Became Popular
To develop and sell postscript, its two lead programmers founded a company, Adobe Systems. You may have heard of them. But postscript’s real breakthrough came thanks to a suggestion made by someone else you may have heard of: Steve Jobs, co-founder of Apple Computer.
Around the time Adobe began to sell Postscript for describing computer graphics, Apple was about to produce its first laser printer, the LaserWriter. Jobs suggested to Adobe that postscript would work very well as a printer language, so Adobe made a few changes and the LaserWriter became the first printer to use postscript.
Because the LaserWriter was powered by postscript, the LaserWriter could print higher quality documents than all of its competition, making the LaserWriter the most successful printer of its day by a long shot and solidifying postscript’s reputation as necessary for high-quality graphics. Most laser printers which followed the LaserWriter used postscript as well, until cheap laser printers dropped in price so much that it became too expensive to pay Adobe for postscript licensing fees. Cheap printers instead used Windows or Mac to convert postscript files into bitmap images and then print the images.
High-end printers, however, still use postscript directly to help ensure maximum quality and because many high-end desktop publishing programs still expect to print postscript.
As a complete programming language, postscript has some flaws. One of them is that programs aren’t always safe—they can contain viruses and bugs. Postscript viruses can’t infect your computer, but they can use up all of the CPU and memory on your computer (or your printer) until you close the postscript file or turn off your printer. (This is annoying but not really dangerous.)
Another problem with programs is that you have to run the whole program in order to get to the parts at the end. So to display the last page of a 300-page document, a computer needs to internally display all 299 preceding pages, which makes postscript files seem slow.
Adobe addressed these problems with the Portable Document Format (PDF) which is very similar to postscript but removes the programming parts and adds in some other features. Most programs today which create PDFs actually generate postscript and then convert it into a PDF. Conversely, many printers which directly print PDFs actually translate the PDFs back into postscript and print the postscript.
Although postscript will celebrate its 30th anniversary in 2014, it’s still the predominant language for high-end printers and shows no signs of being replaced.