# How to use textpath with Metapost

It took me more than a few hours to get this working and as the manual is not at all fully clear on what you need to do (or rather, it assumes more knowledge of LaTeX than I had), I thought it would be useful to set this out.

Textpath allows one to draw text on a path (i.e., not just on a straight line). Here’s an example I prepared earlier:

Decus et tutamen as the coins say.

But using it is not as simple as I hoped it would be and even the very fine The LATEX Graphics Companion does not cover it – this is despite textpath being in the standard free TeX distributions and the alternative they describe – TXP – requiring a manual install.

Here are the key steps you need to get it to work.

Firstly, you have to configure your metapost file as a full LaTeX file and not just a piece of Metapost.

So at the top:

input latexmp;
setupLaTeXMP(preamblefile="mypreamble");
input textpath;
filenametemplate "%j-%c.mps";

And then you need to create your LaTeX preamble – i.e. the file I have called “mypreamble.tex” here but you can call it anything.

Here is mine:

\documentclass{article}
\usepackage[T1]{fontenc}
\usepackage[osf]{mathpazo}
\usepackage{textpathmp}

This is what constrains your file as LaTeX.

This also means you have to end your file with an end after the endfig of the Metapost.

If you get some of this wrong you may find that you get an output but the strings (labels) are messed up – it is quite sensitive.

Here’s the file that generated the above graphic:

input latexmp;
setupLaTeXMP(preamblefile="mypreamble");
input textpath;
filenametemplate "%j-%c.mps";
beginfig(1);
prologues:=3;

%some colours
color lightgrey;
lightgrey:=(4/5, 4/5, 4/5);
color midgrey;
midgrey:=(3/5, 3/5, 3/5);
color darkgrey;
darkgrey=(2/5, 2/5, 2/5);
color gloom;
gloom=(1/5, 1/5, 1/5);

color azure;
azure:=(12/13, 1, 1);
color baby;
baby:=(12/13, 24/25, 1);
color lavender;
lavender=(23/26, 23/26, 49/50);
color ultra;
ultra=(8/13, 23/26, 12/13);
color ukraine;
ukraine=(66/255, 173/255, 222/255);
color turquoise;
turquoise=(0, 191/255, 1);

path hardware, virtlayer, kernel, user;
path hardwareL, virtlayerL, kernelL, userL;

hardware = fullcircle scaled 100;
virtlayer = fullcircle scaled 150;
kernel = fullcircle scaled 250;
user = fullcircle scaled 350;

hardwareL := reverse fullcircle scaled 75 rotated 180;
virtlayerL := reverse fullcircle scaled 125 rotated 180;
kernelL := reverse fullcircle scaled 225 rotated 160;
userL := reverse fullcircle scaled 325 rotated 160;

fill virtlayer withcolor lavender;
draw user;
fill user withcolor midgrey;
draw kernel;
fill kernel withcolor lightgrey;
draw virtlayer;
fill virtlayer withcolor lavender;
draw hardware;
fill hardware withcolor white;
draw textpath("Virtualisation", virtlayerL, 0);
draw textpath("Hardware", hardwareL, 0);
draw textpath("Kernel", kernelL, 0);

label.urt(btex Hardware virtualisation etex, (-200,-200));

endfig;
end;

There are lots of tricks to get the text looking the right way – e.g., inside or outside the path, left to right and so on. But they are described in the manual and I will leave them to readers to work out – getting the output at all is the most fundamental task!

# How do I do this?

I am preparing a computer science slide presentation and (as I am sane) do not want to use Powerpoint.

So I have been using LyX and the Beamer class.

The slides look great. But how do I add notes?

If I add notes “by hand” e.g. \note{Here is a note} – I see nothing in the PDFs.

And if I use the LyX “Noteitem” I get something like this “Note: Here is a note” on screen, but again nothing in the PDFs.

What am I doing wrong? I have looked at the LyX wiki and it is not illuminating.

# Anyone got any thoughts on the LaTeX companion?

Should I shell out £23 for The Latex Companion to ensure I can most effectively write my documents and design my slides for university? I have The LATEX Graphics Companion and there is no doubt it is a good book, but the number of books I could buy increases exponentially the more I think about the work I need to do. So, anyone have any practical experience with the book’s usefulness to a computer science research student with a middling level of LaTeX experience, who is likely to use LyX for a lot of his work?

# I bought a book

Now I just have to decide what I want to do with it! I had thought about a LaTeX based application, as both LaTeX and LyX are thinly supported on Android. But that looks very ambitious at the moment.

We’ll see.

# LaTeX and O’Reilly

I have been in touch with Anslem Lingnau, the author of the (German-language) LaTeX Hacks: Tipps und Techniken für professionellen Textsatz – and he tells me that while he would love to see an English translation of his book there seems to be no sign of one forthcoming.

Indeed a quick scan seems to suggest that O’Reilly are not so great at publishing books on LaTeX.

Anslem is also a Scottish country dancing enthusiast of note, if Dashing White Sergeants are your thing…

# LaTeX w00t!

I owe Professor Paul A. Rubin another apology – turns out I can display LaTeX natively in a wordpress.com blog after all – as the following wave equation shows:

$i\hbar\frac{\partial}{\partial t}\left|\Psi(t)\right>=H\left|\Psi(t)\right>$

# Red-black trees

Binary trees are seen and used frequently in computing science and computing. They are a good abstraction for many naturally occurring relationships (most of our mathematics is based on binary operations, for instance) and have O(log n) complexity (ie if you went from searching a tree of 1000 elements to a tree of 100,000 elements then the search should not take 100 times longer but about 10.)

Of course that log n goodness requires the tree to be “balanced” ie for any given node there should be roughly equal numbers to the left and the right. One way of doing this is through a “red black tree” – here nodes in the tree are assigned a colour: the root is always black. The rule is that any traversal from the root to the leaves should always go through an equal number of black nodes and to ensure this is possible red nodes may be inserted in the tree, but no red node may have another red node as an immediate descendant. (A full explanation is in Introduction to Algorithms though one can also work off various explanations on the internet, though they tend to be less than complete.)

The Linux kernel natively implements a red black tree (in C) and the first bit of work I started on my MSc project was to write my own, userland, implementation so I could see processes in the same way the kernel did.

As I had got a less than glorious mark (still a pass, so that’s what counts) in the C++ exam last year I also decided that I would write this in C++ using templates. A few days after I started I discovered that actually the writers of the STL had got there before me, but as this was an academic exercise I ploughed on with my own implementation.

Essentially this is what I did on my two week summer holiday in Scotland last year! When I was there I also started (though completed when I got home) a couple of helper applications to position the tree according to Reingold and Tilford’s algorithm (which I had to translate from PASCAL) for “better drawing of trees” and a Qt application to display it all.

In fact I had a nagging problem with the Reingold-Tilford algorithm which I finally got around to fixing last night.

(Interestingly the code also allows you to use the Turing-complete capabilities of LaTeX by specifying a TeX output that uses LaTeX’s own positioning algorithm – something I picked up from The LATEX Graphics Companion – that is what the example shown above uses, though unfortunately for even moderately loaded systems the LaTeX processor objects to the width of the output).

Fancy trying it? I could do with someone giving it a bash on a BSD system – not needed for my course but interesting none the less.

The code is all at GitHubhttp://github.com/mcmenaminadrian: memball gives the basic GraphML or TeX or plaintext output, treedraw will convert the GraphML to an SVG or serialiszed stream using Reingold and Tilford’s algorithm and treeqt will use Qt to display the tree using the serialized class. You may have to download various libraries to get it to work (certainly the libproc-dev package on Ubuntu/Debian) – I couldn’t get memball to work on a Windows machine using Cygwin but maybe even that is fixable.

There is a script in the treeqt repo to make it easier: download the sources form all three repos, build them and then run:

./setup | ./treeqt --r 1