I have the same font in two files, both without file extensions.
Both files begin with the type code OTTO, so the fonts must be OpenType.
When I add the file extension .otf and open the files in a font editor, they both look the same and contain the same glyphs.
But when I diff the files in Vim, some parts of the files differ, but since the code is unintelligible to me, I cannot understand what aspect of the font these parts of the files refer to.
How can I find out what the difference between these two font files is?
You should locate the tool ttx [1].
That's a command line tool to convert a font file from OpenType format to an XML representation and back again.
The XML representation is very easy to edit in any text editor (for minor changes to a font, for example). It should also be easy to (ab)use it as a format where you can do your comparison.
So these commands should give you a starting point:
ttx -l otf1file
ttx -l otf2file
The -l option does not yet dissect (and convert to XML) the font files. Instead, it prints an overview of tables like the following:
Listing table info for "/Library/Fonts/WeidemannStd-Medium.otf":
tag checksum length offset
---- ---------- ------- -------
BASE 0x3f624fba 52 24836
CFF 0xec0764e1 18801 2752
DSIG 0xa4d90535 5180 24888
GPOS 0xffb60926 1456 23380
GSUB 0x6de87013 812 22568
OS/2 0x7c681439 96 320
cmap 0x79e54a16 932 1788
head 0xd391fc8f 54 220
hhea 0x06fd0364 36 276
hmtx 0xf9581b97 1012 21556
maxp 0x00fd5000 6 312
name 0x987b2db3 1370 416
post 0xffb80032 32 2720
Now that you are familiar with some of the table specifics, you may want to just look at these tables which are different. Assuming it's only the cmap and name tables:
ttx -o otf1-cmap+name-tables.ttx -t cmap -t name otf1file
ttx -o otf2-cmap+name-tables.ttx -t cmap -t name otf2file
vimdiff otf1-cmap+name-tables.ttx otf2-cmap+name-tables.ttx
In case all tables were different, you can create the complete fontfile's TTX dump by skipping the -t parameters:
ttx -o otf1.ttx otf1file
ttx -o otf2.ttx otf2file
vimdiff otf1.ttx otf2.ttx
Update: Another useful tool to look into (also from the Adobe Type Tools repository) may be sfntdiff.
But possibly the ttxn tool is what serves you best. Here is its self-description:
"This tool is used to test if two fonts are functionally the same. It sorts and modifies the output from the
ttxandtxtools to build a normalized text dump that eliminates differences due to issues such as OTL table record order, glyph order and subroutinzation differences. It writes one file for each table in the font. A good difference editor, such as BBEdit on the Mac, or UltraEdit on the PC, can then be used to compare the output files from two different fonts. It is particularly useful in comparing older and newer versions of the same font."
[1]: TTX was originally developed by Just van Rossum in 1999 and maintained as an Open Source project on SourceForge (sf.net/projects/fonttools/). It is written in Python: