PostScript vs. PDF

WHY DO WE OFFER TWO PRINTING TECHNOLOGIES? HOW DO THEY DIFFER?

By David Evans

For individuals who do not know everything about high-end digital imaging, it might be somewhat difficult to discern differences between Portable Document Format (commonly known as PDF) and Adobe® PostScript®. Actually, you have probably heard many people saying that PDF is a potential replacement for Adobe PostScript. If you have ever wondered why this is the case, it helps to first know exactly what PDF and PostScript are at the most basic of levels. The mystique will disappear, and you will finally understand, after this quasi-technical explanation of both of these types of software.

A matter of language

The first thing we should do is take a look at PostScript. This is a page description language; essentially, it is a programming language similar to the languages that software engineers work with to form apps. You can actually experiment with this so that you have firsthand experienced. Open up Adobe Illustrator® and create a new document. Draw a box, save the file, and then reopen it in a word processing program. You will see a program, coded in the PostScript language. This program will define the dimensions of a page and draw a box on it.

When PostScript first came out, it was only possible to create drawings by manually typing in that language. What programmers would do was peruse the PostScript Language Reference Manual, type the appropriate PostScript “code” into a text file, and then export the file to the printer to be “processed” (this will be explained shortly). Illustrator was the original “graphical PostScript interface;” it is comparable to Microsoft® Windows 1.0, which placed a graphical user interface on MS/DOS. With the Illustrator program, a designer can draw with graphic tools on the computer while the program is working in the background to automatically write a PostScript program.

It has now been firmly established that PostScript is a program, like Fortran, BASIC, Fortran, and C++. However, PostScript differs from these other languages in that it is only meant to perform one task. It is designed to give a very accurate description of what a page looks like.

Ends here

A processor is needed with any programming language to run the code. For PostScript, this processor is an RIP or Raster Image Processor; it is a combination of hardware and software which is typically inside a printer. The RIP will take in code and render it into a set of dots on a page. A PostScript printer reads and comprehends PostScript programs, and then it yields graphical information that is converted to images. As a side note, the Adobe Graphics Model (AGM) used by InDesign® is a RIP as well. It processes instructions from PostScript and shows the results on the computer screen. This is far more press-accurate than other vendors’ page layout applications and their bit mapped previews; what you see on the screen accurately reflects how the file will look printed out.

This leads to the EPS or Encapsulated PostScript File. This is a PostScript program saved as a single file that permits some programs to show a preview on the screen, due to a low-resolution preview that is “encapsulated” on the inside. This preview is not necessary with InDesign, due to its built-in RIP that enables its opening of files in a native manner.

One way that you can utilize PostScript is the following: you can print a file to disk and save it as a single PostScript file. You can then send it to a print service provider. Alternatively, you can create EPS files and use them to save and distribute graphics. This is where a lot of people become confused about the matter of PostScript versus PDF. When people talk about how PDF is a “replacement” for PostScript, what they are probably saying is that PDF can replace saved EPS and PostScript files. It cannot actually replace the PostScript language itself, nor can it replace PostScript processors within devices.

A smarter format

Now that we have a better understanding of PostScript, we can go into PDF. Also, for more information on PDF, read the article “ISSUU Alternative” or my article “Issuu Review“. This is a particular file format that happens to be based heavily on the PostScript language. However, while PostScript was meant to describe pages, PDF goes a few steps further and can tell you not only how the page looks, but also how it behaves and what types of information are in the files. PDF is thus more “intelligent” than EPS as a file format; it contains detailed information such as fonts, printing instructions, images, job tickets, keywords for searching and indexing, movies, interactive hyperlinks, and much more.

An added benefit

PDF is considered more advanced than PostScript; a PDF file is actually a PostScript file that has been taken a little further. A RIP has interpreted the file and made it into clearly defined objects that you can see on the screen as visual objects, rather than just code. These files are more reliable when they are printed out that. PS or EP files, having already been interpreted by the RIP. Also, print service providers can see the file after it has been interpreted, before actually sending it to printers, due to the fact that. PS and EPS files can easily be converted to PDF files and viewed on a computer screen. This way, they can see errors before printing them and wasting resources, which is especially helpful for people who operate printers or run service bureaus. The net outcome is quicker and more accurate printed files.

However, to print a PDF file, the printer must render the PDF objects to the page; the best way to do this is using a PostScript printer. Some of these printers understand PDF files natively, in addition to the PostScript language itself. Additionally, there are printers that convert every single job into a PDF file before printing; this is done using a technology that is called “Extreme.” Creo, Agfa, Scitex, and Heidelberg have all declared print workflows that are based on this technology.

Although professionals use PostScript processors for the best results possible (because Adobe PostScript is the most accurate and reliable language for this purpose), non-PostScript printers can be used as well. In fact, Adobe Acrobat®, which is the major tool used for modifying and enhancing PDF files, can print PDF files to printers that are not PostScript processors.

Happily ever after

The main idea to take away here is that PDF can be utilized as a file format that replaces EPS. Additionally, PDF can be used as a delivery format when one wants to print out complete publications. Being entirely self-contained, PDF is also appropriate for file archiving, distribution on the Internet, and soft-proofing. However, if you want the best possible PDF printouts, you should print your files to a device that has true Adobe PostScript abilities.

The majority of PDF friendly service bureaus have third party applications that can work with PDF files. They take these files; then they will impose them, trap them, preflight them, and finally send them to the RIP. These tools are becoming more potent as technology advances, and as they get better and better, there will be more people who deliver PDF files for printing. Adobe applications will be continuing to support EPS files as well, in order to support a professional publishing workflow; however, next time you are about to send a file to a prepress operator or export a file out of whatever drawing program you enjoy using, consider sending a PDF file in lieu of that. If you plan well and use your printer correctly, you will probably love the outcome.

D. Evans works for Adobe InDesign as a Product Manager. Before becoming part of the Adobe team, he was a print workflow consultant and freelance designer.