Monthly Archives: November 2013

What does a TrueType font look like on the inside? And how do you make one? (Part I)

So, here the other day (actually a few months ago) I was looking for some retro-looking 8×8 pixel bitmap fonts for a project involving an old flip-dot display at My Friendly Local Hackerspace™. During this search I came across this website with more than 600 character sets from various old Commodore 64 games, applications, and demos. However, they are all in a very raw format where they are basically just partial memory dumps from running C64 games/demos. The author even mentions that he hasn’t found any easy way to convert them to a more useful format like, for example, TrueType. In fact, a Google search indicates that only very few C64 character sets are available for download as TrueType files. Most notable are the ones by Style64. However, they all seem to be more or less converted by hand using various font authoring tools. Well that’s not very elegant…

Somebody ought to do something!

Let’s explore what a TrueType font file looks like on the inside so we can create one ourselves. As I’ll cover later, it turns out to be quite possible to do with a bit of coding effort. In fact we don’t even have to do all the dirty work moving bits around and calculate checksums in a binary Truetype font file ourselves. There is (of course) a nice Python package called TTX/FontTools that can help us out. It encapsulates all the relevant data tables, as they are called, but it’s still fairly close to “the metal” and rather difficult to get started with.

The main use-case for TTX/FontTools seems to be TTX, which is a command-line tool for converting TrueType files into a corresponding XML-format and vice versa. Users can then manipulate a font in various ways using a basic text editor rather than sophisticated font editor programs, where it can be difficult to see what actually happens to the font behind the scene. But for our particular use-case we are not interested in the tool itself, but rather what makes it tick. It is really difficult to find any documentation or tutorials for this package. Especially if one wants to create a new font from scratch instead of just editing an existing one. I’ll get back to this later.

This post is meant as an introduction to the subject. All the dirty details will be fleshed out in upcoming posts. I have implemented a conversion tool in Python and if you can’t wait for the details, you can have a sneak-peek of the code on GitHub.

The problem in a nutshell

OK, so how do we get from a C64 memory dump containing some 8×8 pixel bitmapped characters to a usable TrueType file? The problem can be split into the following sub-problems:

  1. Vectorizing the 8×8 pixel bitmaps into glyph into a set of contours.
  2. Mapping from the native C64 format into ASCII (and Unicode).
  3. Generating the actual TrueType file using TTX/FontTools.

I’ll address these three sub-problems across the next three blog posts.

Useful software:

  • Python – A very nice programming language. Like Perl but less painful and much prettier.
  • TTX/FontTools – Without this package I probably wouldn’t have bothered with TrueType.
  • FontForge – A TrueType font editor. Nice for checking the sanity of font files.

If your operating system is a derivative of Debian Linux (like Ubuntu or Linux Mint) then Python is probably pre-installed and the other two packages are easily installable through the built-in package system. It’s also pretty easy to get it to work in Mac OS X. It probably also works in Windows, but then you’re on your own..

Useful links:

The TrueType file format at a glance

It seems that the file format specification varies a bit depending on who you ask. Traditionally, there are three big players in this field, Microsoft, Apple, and Adobe, and they don’t agree 100% on what a proper TrueType file should look like. I will primarily be using Microsoft’s definition, as Apple’s specification on required tables in the file lacks the “OS/2” table, which is used by Microsoft Windows itself as well as Microsoft Internet Explorer when loading fonts from a website. There are some licensing bits in here that tells Internet Explorer if it’s OK to download and use this font.

Basically, a TrueType file is a set of tables. Each of these tables describe a certain aspect of the font. For my particular purpose I can leave out features like kerning, hinting, ligatures, and all other fancyness that’s useful for working with proportional fonts. My goal here is to convert monospaced 8×8 pixel bitmaps into proper TrueType glyphs. The required tables for a bare-bones font are as follows:

Table nameTable description
cmapCharacter to glyph mapping
glyfGlyph data
headFont header
hheaHorizontal header
hmtxHorizontal metrics
locaIndex to location
maxpMaximum profile
nameNaming table
postPostscript information
OS/2OS/2 and Windows specific metrics

A detailed description of these tables are outside the scope of this post, but it’s all there in the specifications I linked above. As mentioned earlier, I didn’t succeed in finding a tutorial for creating font files from scratch using TTX/FontTools. It requires a deep knowledge of the TrueType format, but it can be done by reading the FontTools code (as well as the specifications) and doing some trial-and-error using the XML output feature. Basically you just need to fill in all the data mentioned in the specification. There are very few shortcuts in the process. But more on this in a later blog post…

In my next post, I’ll discuss the vectorization algorithm for converting a 8×8 pixel bitmap into a set of polygon contours. Stay tuned…