stripfont is a dotnet cross-platform command line utility to strip unwanted characters or glyphs from a font file.
The characters we want to keep are identified by the usage of the font in an ASS script file, based on style definitions and tags (\fn).

Technically, it's just a wrapper around fontTools. The most annoying part being parsing the ASS file.

While western font files are usually light in terms of size on the disk, that's not the case for CJK (Chinese/Japanese/Korean) fonts.
Embedding such fonts in an MKV file just bloats its size and takes more time to load in memory when playing.

Prerequisites:
You're gonna need the following:

• dotnet 6. Can be downloaded from here
• Python3. Download link here
• fontTools : python3 -m pip install fonttools

Install stripfont:
You can install it by cloning this repo and running dotnet build at the root of the stripfont folder.
The binary file (./stripfont on Linux or stripfont.exe on Windows) will be available in the bin/Debug/net6.0 folder.

git clone https://github.com/nipponsei/stripfont.git
cd stripfont
dotnet build

stripfont -h
Description:
  Strip unwanted glyphs from a font file based on the usage of the font in a ASS subtitle file.

Usage:
  stripfont [options]

Options:
  --ass-file <ass-file> (REQUIRED)    The ASS scritp file to analyze.
  --font-file <font-file> (REQUIRED)  The font used in the ASS script we want to strip.
  --version                           Show version information
  -?, -h, --help                      Show help and usage information

It takes two parameters:

• --font-file: full path of the font file we want to strip
• --ass-file: full path of an ASS file as a source to detect the characters we want to keep

If everything goes right, the newly created font will be available in the directory of the source font file, its name being suffixed with -subset.

Example:

./stripfont --ass-file ~/_github/_wip/niehime/subs/NCOP1/NCOP1.ass --font-file ~/_github/_wip/niehime/fonts/DFHanziPenStdN-W5.otf 
Subset of font 'DFHanziPen StdN W5' successfully created.

Here are a few examples using the following fonts:

• DynaComware DF ハンジペン Std W5
• Fontworks FOT-マティスはつひやまとPro B
• Morisawa A-OTF すずむし Std M

Note: The resulting file size could be smaller, but in my case I decided to keep all OpenType layout features.

For a more visual comparison, you can check this page.

• Because I needed this tool like yesterday, I didn't bother to package it properly at all.That's on my TODO list.
• I didn't check (yet) if there's a good ASS parser in dotnet out there. Maybe I'll give it a go.