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1Dec/160

The flavours of TeX—a guide for publishing staff: LaTeX, pdfTeX, pdfLaTeX, XeTeX, XeLaTeX, LuaTeX et al

Summary

This is not a technical article on using TeX (i.e, TeX installation or programming). Instead, it offers some background information for people who work in STM (scientific, technical and medical) publishing and aims to provide an easy-to-follow explanation by addressing the question “what is TeX?”—and, hopefully, demystifies some confusing terminology. My objective is, quite simply, to offer an introduction to TeX-based software for new, or early-career, STM publishing staff—especially those working in production (print or digital). Just by way of a very brief bio, as in “am I qualified to write this”: I’m writing this piece based on my 20+ years of experience of STM publishing, having worked in senior editorial positions through to technical production and programming roles. In addition, over the last few years I have spent a great deal of time building and compiling practically every TeX engine from its original source code, together with creating my own custom TeX installation to explore the potential of production automation through modern TeX-based software.

Introduction

If you work in STM (scientific, technical and medical) publishing, especially within mathematics and physics, chances are that you’ve heard of something called “TeX” (usually pronounced “tech”)—you might also have encountered, or read about, authors using tools called LaTeX, pdfTeX, pdfLaTeX, XeTeX, XeLaTeX, LuaTeX, LuaLateX etc. Unless you are a TeX user, or familiar with the peculiarities of the TeX ecosystem, you may be forgiven for feeling somewhat confused as to what those terms actually mean. If you are considering working in STM publishing and have never heard of TeX, then I should just note that it is software which excels at typesetting advanced mathematics and is widely used by mathematicians, physicists, computer scientists to write and prepare their journal articles, books, PhD theses and so forth. TeX’s roots date back to late 1970s but over the intervening decades new versions have evolved to provide considerable enhancements and additional functionality. Those new to STM publishing, or considering it as a career, may be surprised to learn that a piece of software dating back to the late 1970s is still in widespread use by technical authors—and publishing workflows.

NOTE: TeX is not just for mathematics. It is a common misconception that the use of TeX is restricted to scientific and technical disciplines—typesetting of complex mathematics. Whilst it finds most users in those domains, TeX is widely used for the production of non-mathematical content. In addition to typesetting mathematics, modern TeX engines (XeTeX and LuaTeX) provide exquisite handling of typeset text, support for OpenType font technologies, Unicode support, OpenType math fonts (as pioneered by Microsoft Word), multilingual typesetting (including Arabic and other complex scripts) and output directly to PDF. LuaTeX, in particular, is incredibly powerful because it also has the Lua scripting language built into its typesetting engine, offering (for example) almost unlimited scope for the automated production/typesetting of highly complex or bespoke documentation, books and so forth. LuaTeX also provides you with the ability to write plugins to extend its capabilities. Those plugins are usually written in C/C++ to perform specialist tasks—for example: graphics processing, parsing XML, specialist text manipulation, on-the-fly database queries or, indeed, pretty much anything you might need to do as part of your document production processes. If you don’t want the complexities of writing plugins, chances are you can simply use the Lua scripting language to perform many of your more complex processing tasks.

Irrespective of the tools used by authors to write/prepare their work, the lingua franca of today’s digital publishing workflows—especially journals—is XML, which is generated from the collection of text and graphics files submitted by authors. Most publishers now outsource the generation of XML to offshore companies usually based in countries such as India, China or the Philippines. Many production staff usually do not have to worry (too much) about the messy details of conversion—provided the XML passes quality control procedures and is a correct and faithful representation of the authors’ work. The future is, of course, online authorship platforms which remove the need for this expensive conversion of authors’ work into XML—but we’re still some way from that being standard practice: old habits die hard, so Microsoft Word and TeX will be around for some time, as will the need for conversion into XML.

And so to TeX: A brief history in time

My all-time favourite quote comes from the American historian Daniel J. Boorstin who once noted that:

“Trying to plan for the future without a sense of the past is like trying to plant cut flowers.”

In keeping with the ethos of that quote I’ll start with a very brief history of TeX.

On 30 March 1977 the diary of Professor Donald Knuth, a computer scientist at Stanford University, recorded the following note:

“Galley proofs for vol. 2 finally arrive, they look awful (typographically)… I decide I have to solve the problem myself”.

That small entry in Professor Knuth’s diary was the catalyst for a programming journey which lasted several years and the outcome of that epic project was a piece of typesetting software capable of producing exquisitely typeset mathematics and, of course, text: that program was called TeX. Along the way, Knuth, and his colleagues, designed new and sophisticated algorithms to solve some very complex typesetting problems: including automatic line breaking, hyphenation and, of course, mathematical typesetting. As part of the development, Knuth needed to fonts to use with his typesetting software so he also developed his own font technology called METAFONT, although we won’t discuss that in any detail here.

To cut short a very long story, TeX proved to be a huge success—in no small part because Knuth took the decision to make TeX’s source code (i.e., program code) freely available, meaning that it could be built/ported, for free, to work on a wide range of computer systems. TeX enabled mathematicians, physicists, computer scientists and authors from many other technical disciplines to have exquisite control over typesetting their own work, producing beautifully typeset material containing highly complex mathematical content. Authors could use TeX to write and prepare their books and papers, and submit their “TeX code” to publishers—usually assured of a greater degree of certainty that their final proofs would not suffer the same fate as Knuth’s.

TeX: Knuth maintains his version, but others have evolved

Even today, nearly 4 decades after that fateful genesis of TeX, Professor Knuth continues to make periodic bug fixes to the master source code of his version of TeX—which is archived at ftp://ftp.cs.stanford.edu/tex/ and available from other sources, such as CTAN (Comprehensive TeX Archive Network). Those updates take place every few years with the latest being “The TeX tuneup of 2014” as reported in the journal TUGboat 35:1, 2014. During those “tuneups” Knuth does not add any new features to TeX, they really are just bug fixes. In the 1980s Knuth decided that in the interest of achieving long-term stability he would freeze the development of TeX; i.e., that no new features would be added to his version of  TeX. I specifically mentioned “his version of TeX” because Knuth did not exclude or prevent others from using his code to create “new versions of TeX” which have additional features and functionality. Those “new versions” are usually given names to indicate that whilst they are based on Knuth’s original they have additional functionality—hence the addition of prefixes to give program names such as pdfTeX, XeTeX and LuaTeX.

Huh—what about LaTeX? At this point you might be wondering why I have not mentioned LaTeX, and it is a good question. Just to jump ahead slightly, the reason I am not mentioning LaTeX (at this point) is because LateX is not a version of the executable TeX typesetting program—it is a collection of TeX macros, a topic which I will discuss in more detail below.

At this point, I’ll just use the term “TeX” (in quotes) to refer to the Knuth’s original version and all its later descendants (pdfTeX, XeTeX, LuaTeX).

So, what does “TeX” actually do?

As noted, “TeX” is a typesetting program—but if you have formed a mental image of a graphical user interface (GUI), such as Adobe InDesign, then think again. At the time of TeX’s genesis, in the late 1970s, today’s sophisticated graphical interfaces and operating systems were still some way into the future and TeX’s modus operandi still reflects its heritage—even for the new modern variants of TeX. Those accustomed to using modern page layout applications, such as Adobe InDesign, may be surprised to see how TeX works. Suppose someone gives you a copy of a “TeX” executable program and you want to use it to do something, how do you do that? “TeX” uses a so-called command-line interface: it has no fancy graphical screen into which you type your text to be typeset or point, click, tap to set options or configurations. If you run the “TeX” program you see a simple screen with a blinking cursor. Just by way of example, here’s the screen I see when I run LuaTeX (luatex.exe on Windows):

luatex

Clearly, if you want a piece of software to typeset something, you will need to provide some form of input (material to typeset) in order to get some form of output (your typeset material). Your input to the typesetting program will not only need to contain the material to be typeset but will also require some instructions to tell a typesetting program which fonts to use, the page size and a myriad of other details controlling the appearance of the typeset results. To typeset anything with “TeX” you provide it with an input text file containing your to-be-typeset material interspersed with “typesetting instructions” telling “TeX” how to format/typeset the material you have provided: i.e., what you want it to achieve. And here is where “TeX” achieves its legendary power and flexibility. The “typesetting instructions” that control “TeX’s” typesetting process are written using a very powerful programming language—one that Professor Knuth designed specifically to provide users with enormous flexibility and detailed control of “TeX’s” typesetting capabilities. So we can now start to see that “TeX” is, in fact, a piece of typesetting software that users can direct and control by providing it with instructions written in a programming language. You should think of “TeX” as an executable program (“typesetting engine”) which understands the TeX typesetting language.

A tiny example

Just to make it clear, here is a tiny example of some input to “TeX”—please do not worry about the meaning of the strange-looking markup (“TeX” commands that start with a “\”). The purpose here is simply to show you what input to “TeX” looks like:

$$\left| 4 x^3 + \left( x + {42 \over 1+x^4} \right) \right|.$$

And here is the output (as displayed in this WordPress blog using the MathJax-LaTeX plugin):

\[\left| 4 x^3 + \left( x + {42 \over 1+x^4} \right) \right|.\]

So, in order to produce your magnum opus you would write and prepare a text file containing your material interspersed with “TeX” commands and save that to a file called, say, myopus.tex and then tell your “TeX” engine to process that file. If all goes well, and there are no bugs in your “TeX” code (i.e., “TeX” programming instructions) then you should get an output myopus.pdf containing a beautifully typeset version of your work. I have, of course, omitted quite some detail here because, as I said at the start, this is not an article about running/using “TeX”.

“TeX” the program (typesetting “engine”) and “TeX” the typesetting language

So, the word “TeX” refers both to an executable program (the “TeX” typesetting engine) and the set of typesetting instructions that the engine can process: instructions written in the “TeX” language. Understanding that the executable “TeX” engine is programmable is central to truly appreciating the differences between LaTeX, pdfTeX, pdfLaTeX, XeTeX, LuaTeX and so forth.

Each “TeX” engine (program) understands hundreds of so-called primitive commands. Primitive in this sense does not mean “simple” or “unsophisticated”, it means that they are the fundamental building blocks of the TeX language. A simple, though not wholly accurate, analogy is the alphabet of a particular language: the individual characters of the alphabet cannot be reduced to simpler entities; they are the fundamental building blocks from which words, sentences etc are constructed.

And finally: from TeX to pdfTeX, XeTeX and LuaTeX

Just to recap. When Knuth wrote the original version of “TeX” he defined it to have the features and capabilities that he thought were sufficient to meet the needs of sophisticated text and mathematical typesetting based, of course, on the technology environment of that time—including processing and memory of available computers, font technologies and output devices. Knuth’s specification of “TeX” included its internal/programming design (“TeX’s” typesetting algorithms) and, of course, defining the “TeX” language that people can use to “mark up” the material to be typeset. What I mean by “defining the TeX language” is defining the set of several hundred primitive commands that the “TeX” engine can understand, and the action taken by the “TeX” engine whenever it encounters one of those primitives during the processing of your input text.

Naturally, technology environments evolve: computers become faster and have more storage/memory, new font technologies are released (Type 1, TrueType, OpenType),  file output formats evolve (e.g., the move from PostScript to PDF) and Unicode became the dominant way to encode text. Naturally, “TeX” users wanted those new technologies to be supported by “TeX”—in addition to incorporating ideas for, and improvements to, the existing features and capabilities of Knuth’s original TeX program. As noted earlier, in the 1980s Knuth decided to freeze his development of TeX: no more new features in his version—bug fixes only.  With the genuine need to update/modernize Knuth’s original software, TeX programming experts have taken Knuth’s original source code and enhanced it to add new features and provide support for modern typesetting technologies. The four-decade history of TeX’s evolution is quite complex but if you really want the full story then read this article by Frank Mittelbach: TUGboat, Volume 34 (2013), No. 1.

These new versions of TeX not only provide additional features (e.g., outputting direct to PDF, supporting OpenType fonts) they also extend and adapt the TeX language too: by adding new primitives to Knuth’s original set, thus providing users with greater programming power and flexibility to control the actions of the typesetting engine. Each new TeX engine is given its own name to distinguish it from Knuth’s original software: hence you now have pdfTeX, XeTeX and LuaTeX. These three TeX engines are not 100% compatible with each other and it is quite easy to prepare input that can be processed with one TeX engine but fail to work with others—simply because a particular TeX engine may support primitive commands that the others do not. But all is not lost: enter the world of TeX macros!

Primitives are not the whole story: macros and LaTeX

I have mentioned that each TeX engine supports a particular set of low-level commands called primitives—but this is not the full story. Of course, many of the same primitives are supported by all engines but some are specific to a particular engine. “TeX” achieves its true power and sophistication through so-called TeX macros. The primitive commands of an engine’s TeX language can be combined together to define new commands, or macros, built from low-level primitive instructions—and/or other macros. TeX macros allow you to define new commands that are capable of performing complex typesetting operations, saving a great deal of time, typing and programming errors. In addition, TeX engines provide primitives that you can use to detect which TeX engine is being used to typeset a document—so that a TeX engine can, on-the-fly, adapt its behaviour depending on whether or not it supports a particular primitive it might encounter. If a particular primitive is not supported directly but can be “mimicked” (using combinations of other primitives) then all is usually well—but if the chosen TeX engine really cannot cope with a particular primitive then typesetting will fail and an error will be reported.

The TeX language is, after all, a programming language—albeit one designed to solve typesetting problems; but as a programming language TeX is extremely arcane and works very differently to most programming languages you are likely to encounter today.

So, finally, what is LaTeX?

We’ve talked about various versions of the TeX engine—from Knuth's original TeX to its descendants of pdfTeX, XeTeX and LuaTeX—and briefly discussed TeX as a typesetting language: primitives, programming and the ability to write macros. Finally, we are in a position to discuss LaTeX. The logical extension to writing individual TeX macros for some specific task you want to solve, as an individual, is to prepare a collection of macros that others can also use—a package of macros that collectively provide some useful tools and commands that others can benefit from. And that is precisely what LaTeX is: it is a very large collection of complex and sophisticated macros designed to help you typeset books, journal papers and so forth. It provides a wealth of features to control things like page layout, fonts and a myriad of other typesetting details. Not only that but LaTeX was designed to be extensible: you can plug-in additional, more specialist, macro packages written to solve specific typesetting problems—e.g., producing nicely typeset tables or typesetting  particularly complex forms of mathematics. If you visit the Comprehensive TeX Archive Network you can choose from hundreds, if not thousands, of macro packages that have been written and contributed by users worldwide.

So, if someone says they are typesetting their work with LaTeX then they are only telling you part of the story. What they really mean is that they are using the LaTeX macro package with a particular TeX engine—usually pdfTeX but maybe  XeTeX (for multilingual work) or LuaTeX (perhaps for advanced customized document production). Sometimes you will see terms such as pdfLaTeX, XeLaTeX or even LuaLaTeX: but these are not actually the names of TeX engines, all they signify is which TeX engine is being used to run LaTeX. For example, if someone says I am “using pdfLaTeX” what that really means is “I am preparing my typeset documents using the LaTeX macro package and processing it with the pdfTeX engine”. Equally, if anyone says to you that they are “using TeX” then, I hope, you now see that statement does not actually tell you the whole story.

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