This page describes sfntly’s implementation of MicroType Express compression.
The MicroType Express format is documented, and is recently available under license terms we believe are friendly to open source (as well as proprietary) implementation. The most popular implementation of a decoder is Internet Explorer, as a compression option in its Embedded OpenType (EOT) format. Since MicroType Express gives an approximate 15% gain in compression over gzip, a web font service striving for maximum performance can benefit from implementing EOT compression.
The current codebase in sfntly is for compression only (this is by far the most useful for web serving). The easiest way to get started is with the command line tool, sfnttool:
sfnttool -e -x source.ttf source.eot
The code has been tested extensively against IE, and is currently in production in Google Web Fonts, both for full fonts and for subsetting (using the text= parameter). This serving path also uses sfntly to compute the subsets, and is essentially the same as the -s parameter to sfnttool.
If you’re interested in the code and details of the compression algorithm, here’s a bit of a guide:
The top-level MicroType Express compression code is in MtxWriter.java (in the tools/conversion/eot directory). This code implements almost all of the MicroType Express format, including the hdmx tables, push sequences, and jump coding. The main feature missing is the VDMX table (vertical device metrics), which is very rarely used in web fonts.
Patches are welcome -- possible areas include: implementing the VDMX table, speeding up the LZCOMP entropy coder (the match finding code is a straightforward adaptation of the algorithm in the format document), implementing a decoder in addition to an encoder, and porting the Java implementation to C++.