PostScript, The Forgotten Art of Programming

A tutorial for beginners is presented on writing PostScript files to display data.
Action

Time to do something more interesting. To draw a line, enter:

newpath 100
400 moveto 300 200 lineto
        500 300 lineto stroke

This instruction starts a new path by moving to point (100, 400), drawing a line to point (300, 200), then to (500, 300) and, finally, painting the current path. Without the stroke you wouldn't see a thing. Instead of absolute positions you can also use relative movements with rmoveto and rlineto. The thickness of the lines can be controlled by the setlinewidth option, e.g., to draw a hair line:

0.01 setlinewidth
Filling shapes is also easy. Replace stroke by closepath fill; the command closepath connects the last point with the first point to form the shape and fill fills the shape with the current color or grayscale. In this case we get a black triangle. If you do 0.9 setgray, the fill color will be a light gray (0 is black and 1 is white). You can also select colors with sethsbcolor or setrgbcolor, but these options are a bit more complicated. (For further information, see the Red Book in the References.)

Placing text takes some initial preparation: First select the proper font:

/Times-Roman findfont 10 scalefont setfont

selects a 10-point Times-Roman font. Other well-known fonts are Helvetica and Courier. Placing text is easy; you move to the position where you want the text, add parentheses to enclose the text and add the command show. In the case where the text contains parentheses or a backslash you must “escape” them by inserting a backslash before the character to be escaped. Sounds familiar, right? So the line:

400 400 moveto (Hello World) show
prints a greeting you have seen many times before.

There is one final command that is crucial if you want to see anything rolling out of the printer: showpage. This command transfers the picture, made by the PostScript interpreter of your printer, to paper and clears the memory afterwards so that you can start a new page. The command run can be used to load your file:(file.ps) run executes the file named file.ps.

Compliant PostScript

Compliant PostScript is nothing more than PostScript with some special comments and layout instructions for your PostScript program, so that other programs can read your PostScript program and perform certain operations on it. There are numerous examples of available operations: reversing the pages, scaling the pages, rotating the pages, placing two or four pages on one page, etc. Even the program Ghostview uses compliant PostScript, the title and page numbers that you see are retrieved from the PostScript comments.

Since this is all done through comments, it does not matter to your printer if the PostScript is compliantor not—the printer skips all comments. However, in a Unix environment it is common to use the output of one program as the input for another. Thus, it is quite natural not to consider a PostScript file as an end station. In a Unix environment compliant PostScript files are a must; otherwise, the print filters will not be able to process the files.

In a Microsoft environment with Microsoft's well-known word processor, PostScript is an end station. The generated output suggests that it is level-2 compliant PostScript, but, alas, it is not. There are so few rules to make your document level-1 compliant (see Red Book) or even level- 2 compliant (see DSC in the References), that you might wonder how it ever wouldn't be.

I will stick to level-1 compliant (it is a bit easier as level-2 has more overhead). In the Red Book, level-1 compliant PostScript is described in just eight pages; so if you want to know all the ins and outs, it is the right source.

There are three types of comments:

  • Header comments go before any PostScript code.

  • Body comments primarily serve to mark the boundaries of pages.

  • Trailer comments follow all PostScript code and are normally used to describe certain features that were not yet known from the header comments, so they were deferred (e.g., number of pages, fonts to be used and bounding box).

Normally, a comment in PostScript starts with a % (percent symbol), but these structuring comments start with %% (or %! if it is the very first line). A %% comment is directly followed by a keyword denoting its type of structuring comment, and if arguments are needed, they follow a :(colon) and are separated by spaces. As an example, let's look at the following template (the number in front of each line is just for reference):

1  %!PS-Adobe-1.0
2  %%DocumentFonts:
3  %%Title:
4  %%Creator:
5  %%CreationDate:
6  %%For:
7  %%Pages:
8  %%BoundingBox:
9  %%EndComments
The first line suggests that these files are going to be level-1 compliant. One common misunderstanding is that people think the 1 in 1.0 denotes the compliance level, but that is not the case. Only level-1 and level-2 compliancy exist, so even if you see %!PS-Adobe-3.2 , it is not level- 3 compliant (it should be level-2 compliant).

The second line contains the fonts to be used in this file. Some programs find it handy to know in the beginning which fonts they should load. However, when you create a program that generates PostScript, you often do not know this at this time. This header comment can be deferred to the trailer comments. In that case you will have to replace the font1 font2... part of the line in the header with (atend).

The third line is easy; text indicates the title of the document. Often this is the file name, but it does not have to be. Spaces in text are no problem. The fourth line is equally easy; here text should be replaced by the author or the application that created this file.

In the fifth line text should be a date and time humans can interpret. Line 6 is optional, and text should be replaced by the intended recipient. If absent, the intended recipient is Creator.

In line 7 number should reflect the number of pages in the document. Since this number is often not known beforehand, it is frequently deferred to the trailer comments. Again, substitute (atend) in this case.

Line 8 contains four arguments: the x and y coordinates for the lower left corner and the upper right corner. In the case of multiple pages you should use the bounding box, so that all pages lie in the bounding box. To fill in the right values, you will find Ghostview very handy. The bounding box can also be deferred to the end; again you would specify (atend).

Finally, line 9 ends the header section. Besides line 1 and 9 the order of the other lines can be chosen as you see fit.

After the header you normally see some PostScript definitions of variables and subroutines. These variables are intended to remain constant throughout the rest of the PostScript program.

Next, it is time for the body comments. The first body comment is %%EndProlog, which ends the “invariant” section of the program. Most print filters leave everything up to this line intact.

Each page is preceded by (again line numbers are only for reference purposes):

1  %%Page:
2  %%PageFonts:
elm

The first line contains label and ordinal. label should be replaced by a string, containing no white space, that indicates which page it is. Sounds a bit weird, but this means that Roman numerals are okay, and if you have two pages on one, then 1,2 or 3,4 is a valid page number as well. In our case, it is just a plain number. The ordinal part indicates the value of the page number. Having two types of page numbers is just for your convenience.

The second line is optional and describes which fonts are to be used in the output. If absent, the fonts specified for DocumentFonts in the header are used.

Although it is not necessary, if you want to create just a single page, the next PostScript command is save, which makes a copy of the environment you have built so far. At the end of the page you will find the line:

restore showpage

This command retrieves the original situation and prints the page. It means that anything you do between save and restore is fair game, i.e., you can't screw up other pages if a filter reorders the pages.

After the final page you have %%Trailer. Most filters leave this line and everything that follows it intact. In some PostScript programs some cleaning up occurs here, but in most PostScript programs the trailer comments follow directly (again, line numbers are for reference purposes):

1  %%DocumentFonts:
2  %%Pages:
3  %%BoundingBox:

Of these three lines, only those that were deferred in the header comments should be included here in the trailer.

If you want to make your PostScript level-2 compliant, you need to read the DSC (see references).

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