Krux uses a custom fork of the Terminus font for its glyphs that includes Vietnamese characters, the Bitcoin currency symbol (₿) and custom icons.
To rebuild the font for all devices, run:
./bdftokff.sh ter-u14n 8 14 > m5stickv.kff
./bdftokff.sh ter-u16n 8 16 > bit_dock_yahboom.kff
./bdftokff.sh ter-u24b 12 24 > amigo.kff
Once you have .kff files, for each project that you want to use the updated fonts, edit ../MaixPy/projects/*/compile/overrides/components/micropython/port/src/omv/img/font.c (substituting maixpy_amigo_tft for * if only for an amigo) and replace the array contents in the unicode variable with the byte array found within the appropriate .kff file, then rebuild the firmware.
Krux uses bitmap fonts that are custom-built for each device it runs on. The format that the firmware expects fonts to be in is a custom format referred to as "krux font format," or .kff.
Krux font format is a sparse bitmap font format that is intended to minimize the amount of memory used to store a subset of unicode glyphs. In this format, all glyphs are encoded as a 2-byte header integer containing their unicode codepoint followed by a fixed-size array of bytes describing how to draw the glyph. These glyphs are stored contiguously in a one-dimensional byte array sorted by codepoint. The header of this array is a 2-byte integer containing the total number of glyphs in the file. At runtime, each glyph is looked up using binary search.
To create a .kff file, you first need to find a font in .bdf format. You can edit and generate a .bdf file using FontForge
From there, you can run the bdftohex.py script which converts the bdf file to an intermediary .hex format mapping each character's unicode codepoint to a series of hex digits describing how to draw it.
From there, you can run the hexfill.py script which takes in a .hex file and font width and height and post-processes the file to fill in any gaps in the codepoint sequence, adding zero'd lines for missing glyphs. If the specified font size is larger than the hex for a glyph, leading zero padding will be added to the glyph to make it the right size, and if the hex for a glyph is larger than the font size it will be zero'd out.
The hexmerge.py script takes a list of .hex font files and merges them into one combined .hex file sorted by unicode codepoint. If multiple files refer to the same character, the earliest non-zero glyph will be used.
Finally, the hextokff.py script takes a .hex font file and font width and height and converts it into the 1D sparse bitmap array format that Krux needs. Importantly, this script scans the i18n/translations folder and builds the set of unique codepoints used across all translations to drastically reduce the number of glyphs stored.
To simplify this process, a bdftokff.sh bash script has been added that invokes the aforementioned scripts as needed to compile one .kff file given a list of fonts.