Introduced by Adobe in 1985, the page description language known as PostScript has undergone improvements and changes over the years. Just as new versions of software contain new features not available in earlier versions, subsequent PostScript levels have added support for new features and the way the underlying code is written has undergone modifications. 

The key differences in the primary PostScript levels are:

1. Adobe PostScript Level 1 was the original, basic language.
2. Adobe PostScript Level 2 added support for different page sizes and better color printing.
3. Adobe PostScript Level 3, released in 1997, has better graphics handling, supports more fonts, and speeds up printing.

In terms of desktop publishing, the PostScript level used for creating PostScript (and PDF) files is partially dependent on the PostScript levels supported by the printer and the printer driver. Older printer drivers and printers cannot interpret some of the features found in PostScript Level 3, for example.

 A page description language (PDL) developed by Adobe Systems. PostScript is primarily a language for printing documents on laser printers, but it can be adapted to produce images on other types of devices. PostScript is the standard for desktop publishing because it is supported by imagesetters, the very high-resolution printers used by service bureaus to produce camera-ready copy.

 PostScript is an object-oriented language, meaning that it treats images, including fonts, as collections of geometrical objects rather than as bit maps. PostScript fonts are called outline fonts because the outline of each character is defined. They are also called scalable fonts because their size can be changed with PostScript commands. Given a single typeface definition, a PostScript printer can thus produce a multitude of fonts. In contrast, many non-PostScript printers represent fonts with bit maps. To print a bit-mapped typeface with different sizes, these printers require a complete set of bit maps for each size.

 The principal advantage of object-oriented (vector) graphics over bit-mapped graphics is that object-oriented images take advantage of high-resolution output devices whereas bit-mapped images do not. A PostScript drawing looks much better when printed on a 600-dpi printer than on a 300-dpi printer. A bit-mapped image looks the same on both printers.

 Every PostScript printer contains a built-in interpreter that executes PostScript instructions. If your laser printer does not come with PostScript support, you may be able to purchase a cartridge that contains PostScript.

 There are three basic versions of PostScript: Level 1, Level 2 and PostScript 3. Level 2 PostScript, which was released in 1992, has better support for color printing. PostScript 3, release in 1997, supports more fonts, better graphics handling, and includes several features to speed up PostScript printing.

Sample PostScript files and fragments

These files are not necessarily good examples of PostScript coding. They are included because I have found them useful at times.

• A color cube to test color printers.
• A PostScript Error handler.
• The Fermilab logo as an EPS file.
• A `blessed' description of the Fermilab Logo.
• How to obtain interpreter's font list.
• A PostScript program with random behavior. This file takes several minutes to execute. It is a 300 000 dots scattergram.
• A PostScript program which makes random eyecharts.
• A graph paper generator.
• An attempt to obtain a printer's PPD.
• A printer resolution test.
• Tiger, another color printer test.
• Several ways to draw 2 parallel lines
• Why `White Out(tm)' does not work!
• 90 degree rotation of an American letter sized page
  90 rotate 0 -612 translate
• A4 to American letter paper
  26 0 translate .9397 dup scale
• American letter to A4 paper
  0 36 translate .9727 dup scale

How to test an Adobe PostScript print file.

A minimal, useful, but inadequate test is:

1. Print it on a PostScript language level 2 (a.k.a. Red and White Book) interpreter such as a QMS 1700 or HP 4si or 5si.
2. Ghostview. Check for the following:
• Eyeball hangs do not occur (i.e., that the cursor changes to a cross).
• All EPS images show up and are properly sized, located and oriented.
• The file shows all pages in the proper order with no extra pages.
• The page number bar appears in the Ghostview frame.

The print file should be submitted to each of the following and the outputs checked for proper behavior:

1. A PostScript language level 1 (a.k.a. Red Book) interpreter such as an Apple LaserWriter.
2. A PostScript language level 2 (a.k.a. Red and White Book) interpreter such as an QMS 1700 or a HP 4si or 5si.
3. Ghostview. Check for the following:
• Eyeball hangs (the cursor does not change to a cross) do not occur.
• All EPS images show up and are properly sized, located and oriented. If an EPS image does not have appropriate amounts of white space around it, then check the image itself separately for a proper BoundingBox DSC comment.
• The file shows all pages in the proper order with no extra pages.
• The page number bar appears in the Ghostview frame.
• Skipping around in the file works between arbitary pages.
4. Ghostscript itself (somestimes the program name is gs). Check for the following:
• All EPS images show up and are properly sized, located and oriented. If an EPS image does not have appropriate amounts of white space around it, then check the image itself separately for a proper BoundingBox DSC comment.
• The file shows all pages in the proper order with no extra pages.

How to test an Adobe EPS file.

Note that an EPS file may itself contain other EPS files.

A minimal, useful, but inadequate test is:

1. After being encapsulated in a test harness, the resultant test file should be submitted to a PostScript language level 2 Red and White Book) interpreter such as an QMS 1700 or a HP 4si or 5si.
2. After being encapsulated in a test harness, the resultant test file should be submitted to Ghostview. Check for the following:
• Eyeball hangs (the cursor does not change to a cross) do not occur.
• All EPS images show up and are properly sized, located and oriented. If an EPS image does not have appropriate amounts of white space around it, then check the image itself separately for a proper BoundingBox DSC comment.
• The page number bar appears in the Ghostview frame.
3. Give the EPS file by itself to Ghostview. Check for the following:
• Eyeball hangs (the cursor does not change to a cross) do not occur.
• All EPS images show up and are properly sized, located and oriented. If an EPS image does not have appropriate amounts of white space around it, then check it for a proper BoundingBox DSC comment.

The EPS file should be submitted to each of the following tests and the outputs checked for proper behavior:

1. After being encapsulated in a test harness, the resultant test file should be submitted to a PostScript language level 1 (a.k.a. Red Book) interpreter such as an Apple LaserWriter.
2. After being encapsulated in a test harness, the resultant test file should be submitted to a PostScript language level 2 Red and White Book) interpreter such as an QMS 1700 or a HP 4si or 5si.
3. After being encapsulated in a test harness, the resultant test file should be submitted to Ghostview. Check for the following:
• Eyeball hangs (the cursor does not change to a cross) do not occur.
• All EPS images show up and are properly sized, located and oriented. If an EPS image does not have appropriate amounts of white space around it, then check the image itself separately for a proper BoundingBox DSC comment.
• The file shows all pages in the proper order with no extra pages.
• The page number bar appears in the Ghostview frame.
• Skipping around in the file works between arbitary pages.
4. After being encapsulated in a test harness, the resultant test file should be submitted to ghostscript itself (somestimes the program name is gs). Check for the following:
• All EPS images show up and are properly sized, located and oriented. If an EPS image does not have appropriate amounts of white space around it, then check the image itself separately for a proper BoundingBox DSC comment.
• The file shows all pages in the proper order with no extra pages.
5. The EPS file by itself to Ghostview. Check for the following:
• Eyeball hangs (the cursor does not change to a cross) do not occur.
• All EPS images show up and are properly sized, located and oriented. If an EPS image does not have appropriate amounts of white space around it, then check it for a proper BoundingBox DSC comment.
6. The EPS file by itself ghostscript itself (somestimes the program name is gs). Check for the following:
• The EPS image shows up and is properly sized, located and oriented. If an EPS image does not have appropriate amounts of white space around it, then check for a proper BoundingBox DSC comment. pages.

About extracting illustrations from print files

Admittedly, this is a very tempting operation. It can be done.

It can not be done properly by extracting the pertinent page from the print file, for example by using Ghostview page selection; using an illustration modifier program to `White Out(tm)' the undesired material on the page; adding EPS DSC material to the resultant file and using the result as an EPS file.

With generally poor results and considerable effort, it can be done by extracting the pertinent page from the print file, converting the image to a bit-map image, doing a bit-map edit on the image, selecting the desired portion of the bitmap image and generating a PostScript bitmap image. There generally is a major loss of resolution and fine detail when this is done. The file size generally grows significantly--10 000 byte files can easily become 10 000 000 byte files.

With generally good results, it can be done by extracting the pertinent page from the print file, text editing the file to remove the encapsulating material, reconstructing the frequently deleted EPS DSC material and analyzing for and removing any undesired annotations. The resultant EPS file should be given the same testing as any other EPS file. Here is a basic set of EPS headers:

%!PS-Adobe-2.0 EPSF-1.2 %%BoundingBox: llx lly urx ury %%EndComments     

The BoundingBox numbers are, in pairs, the coordinates of the lower left (ll) and upper right (ur) corners of the box which surrounds the figure. They must be integers. The horizontal (x) coordinate is the first coordinate in a pair; the vertical (y) coordinate is the second coordinate in a pair. The origin is the lower left corner of the interpreter's imaging area. The units are PostScript points (72ths of inches or 352 7/9 micrometers). Select values which are a small nearly uniform amount outside the actual figure.

The difficulty in the last method is mostly in the removal of undesired annotations. With LaTex, the encapsulating material is quite stylized and therefore usually easily removed. Ghostview can be used to reconstruct the BoundingBox data which is the hardest of the required EPS DSC material to reconstruct.

About files

Whereas Adobe PostScript interpreters can process files with any of UNIX, Apple MacIntosh or MSDOS line terminators, the same is not generally true of the application programs that process the Adobe PostScript files. To each other operating system, UNIX and Apple MacOS files appear to be one long line. This can cause problem with line buffer sizes in applications and print spoolers. Adobe PostScript files should be copied between dissimilar operating systems as text files.

Whereas Apple MacOS considers null (zero) bytes to be white space characters, UNIX considers them to character string terminators. Unless an UNIX application is careful coded to handle character streams containing null bytes which are data, ommisions of data may occur. Many Adobe PostScript interpreters will treat null bytes as whitespace.

About Ghostview

Ghostview is dependent on proper Adobe DSC. Ghostview uses these comments to locate the file's prolog which defines procedures and values used by the individual page descriptions, the individual page descriptions themselves and the file's epilog which may do some cleanup. If the application generating the file does not generate proper DSC or obey the constraints of page independence, the individual pages may not be properly handed to ghostscript for display. A common symptom is an eyeball hang. Ghostview presents an eyeball cursor until the underlying ghostscript interpreter signals that it has presented the page to the display. Then, Ghostview switches to a cross cursor. If the showpage command which should be part of the page description is not executed, then the cursor does not switch.

If ghostview erronously includes extra material after the showpage, such as combining multiple page descriptions into a single offering to ghostscript, then the extra material is silently ignored. This can result in a file that appears to display properly in Ghostview which prints extra pages on a printer.

Font Selection

Yes, I know that Palatino looks good. But, it is not present in many interpreters and printers. If you want your document to be presentable on most printers and display programs, then you should only use the basic 13 fonts that the Apple Laserwriter and almost all other printers have:

• Courier
• Courier-Bold
• Courier-BoldOblique
• Courier-Oblique
• Helvetica
• Helvetica-Bold
• Helvetica-BoldOblique
• Helvetica-Oblique
• Times-Roman
• Times-Bold
• Times-BoldItalic
• Times-Italic
• ZapfDingbats

Even the Helvetica-Narrow family is suspect; some interpreters have Helvetica-Condensed instead.

Paper bin, tray, size, feed mode, etc. selection

In a word, don't do this in a document intended for general distribution! What your printer calls bin 0, may mean manual feed or not exist on another printer or interpreter. The entire world does not use either A4 (about 8.28 by 11.7 inches) or American letter (215.9 by 279.4 mm) paper. Instead of selecting a paper size, set your margins so that the print area is not too high for an American letter page nor too wide for A4 paper.

Using an unavailable paper tray, bin, size, feed mode, etc. is a good way to cause the print job to be canceled without being printed!

There are tricks available to resize print files to fit the other paper sizes.

In General ...

PostScript Sins lists a number of failings of a number of Adobe PostScript language generating and handling applications.