How to convert a set of images into a font `.ttf` file with python?

Question

Scenario

I have a set of images named 32.png,..,126.png of handwritten letters pertaining to the ASCII printable characters of the number in the filenames, and I intend to convert these into a font file, like .ttf such that I can type (basic) latex letters with it.

After going through the documentation of project description and documentation of fonttools I have not yet been able to determine how to convert these images into a .ttf font file in python.

It appears I could convert the .png images into .svg format as the fonttools is normally used for font vectors, but I did not find a method that outputs a font file. Hence I wike to ask:

Question

How can I convert a set of images (either .png or .svg) into a .ttf font in python?

Attempts

0. After installing fontforge on windows and adding the ../FontForgeBuilds/bin folder to path, Anaconda does not recognize the fontforge module as it throws error:

ModuleNotFoundError: No module named 'fontforge' in a script that converts .svg files into .ttf files. The script named svgs2ttf is called with command:python svgs2ttf.py examples/example.json.

import sys
import os.path
import json
import fontforge
#python svgs2ttf.py examples/example.json

IMPORT_OPTIONS = ('removeoverlap', 'correctdir')

try:
    unicode
except NameError:
    unicode = str

def loadConfig(filename='font.json'):
    with open(filename) as f:
        return json.load(f)

def setProperties(font, config):
    props = config['props']
    lang = props.pop('lang', 'English (US)')
    family = props.pop('family', None)
    style = props.pop('style', 'Regular')
    props['encoding'] = props.get('encoding', 'UnicodeFull')
    if family is not None:
        font.familyname = family
        font.fontname = family + '-' + style
        font.fullname = family + ' ' + style
    for k, v in config['props'].items():
        if hasattr(font, k):
            if isinstance(v, list):
                v = tuple(v)
            setattr(font, k, v)
        else:
            font.appendSFNTName(lang, k, v)
    for t in config.get('sfnt_names', []):
        font.appendSFNTName(str(t[0]), str(t[1]), unicode(t[2]))

def addGlyphs(font, config):
    for k, v in config['glyphs'].items():
        g = font.createMappedChar(int(k, 0))
        # Get outlines
        src = '%s.svg' % k
        if not isinstance(v, dict):
            v = {'src': v or src}
        src = '%s%s%s' % (config.get('input', '.'), os.path.sep, v.pop('src', src))
        g.importOutlines(src, IMPORT_OPTIONS)
        g.removeOverlap()
        # Copy attributes
        for k2, v2 in v.items():
            if hasattr(g, k2):
                if isinstance(v2, list):
                    v2 = tuple(v2)
                setattr(g, k2, v2)

def main(config_file):
    config = loadConfig(config_file)
    os.chdir(os.path.dirname(config_file) or '.')
    font = fontforge.font()
    setProperties(font, config)
    addGlyphs(font, config)
    for outfile in config['output']:
        sys.stderr.write('Generating %s...\n' % outfile)
        font.generate(outfile)

if __name__ == '__main__':
    if len(sys.argv) > 1:
        main(sys.argv[1])
    else:
        sys.stderr.write("\nUsage: %s something.json\n" % sys.argv[0] )

Answer 1

FontForge is not a python module but separate software. Hence, instead of calling FontForge from a python script, one can call python from a fontforge executable. Since I wanted to create the font in .ttf format from a python script, I wrote an additional python script named execute.py which executes a cmd command that executes fontforge that executes the python svgs2ttf script.

The execute.py contains:

import os
os.system('cmd /k "fontforge -lang=py -script svgs2ttf examples/example.json"')

The svgs2ttf script is modified from this repository contains:

# one can run  this script with:
#fontforge -lang=py -script svgs2ttf examples/example.json

import sys
import os.path
import json
import fontforge

IMPORT_OPTIONS = ('removeoverlap', 'correctdir')

try:
    unicode
except NameError:
    unicode = str

def loadConfig(filename='font.json'):
    with open(filename) as f:
        return json.load(f)

def setProperties(font, config):
    props = config['props']
    lang = props.pop('lang', 'English (US)')
    family = props.pop('family', None)
    style = props.pop('style', 'Regular')
    props['encoding'] = props.get('encoding', 'UnicodeFull')
    if family is not None:
        font.familyname = family
        font.fontname = family + '-' + style
        font.fullname = family + ' ' + style
    for k, v in config['props'].items():
        if hasattr(font, k):
            if isinstance(v, list):
                v = tuple(v)
            setattr(font, k, v)
        else:
            font.appendSFNTName(lang, k, v)
    for t in config.get('sfnt_names', []):
        font.appendSFNTName(str(t[0]), str(t[1]), unicode(t[2]))

def addGlyphs(font, config):
    for k, v in config['glyphs'].items():
        g = font.createMappedChar(int(k, 0))
        # Get outlines
        src = '%s.svg' % k
        if not isinstance(v, dict):
            v = {'src': v or src}
        src = '%s%s%s' % (config.get('input', '.'), os.path.sep, v.pop('src', src))
        g.importOutlines(src, IMPORT_OPTIONS)
        g.removeOverlap()
        # Copy attributes
        for k2, v2 in v.items():
            if hasattr(g, k2):
                if isinstance(v2, list):
                    v2 = tuple(v2)
                setattr(g, k2, v2)

def main(config_file):
    config = loadConfig(config_file)
    os.chdir(os.path.dirname(config_file) or '.')
    font = fontforge.font()
    setProperties(font, config)
    addGlyphs(font, config)
    for outfile in config['output']:
        sys.stderr.write('Generating %s...\n' % outfile)
        font.generate(outfile)

if __name__ == '__main__':
    if len(sys.argv) > 1:
        main(sys.argv[1])
    else:
        sys.stderr.write("\nUsage: %s something.json\n" % sys.argv[0] )

It can be enhanced to convert convert more symbols than the a and b symbol in the examples folder of the repository to include all the symbol images to generate the font.

In response to the comments, here is the contents of the example.json:

{ "props":
  { "ascent": 96
  , "descent": 32
  , "em": 128
  , "encoding": "UnicodeFull"
  , "lang": "English (US)"
  , "family": "Example"
  , "style": "Regular"
  , "familyname": "Example"
  , "fontname": "Example-Regular"
  , "fullname": "Example Regular"
  }
, "glyphs":
  { "0x3f": { "src": "question.svg", "width": 128 }
  , "0xab": { "src": "back.svg", "width": 128 }
  , "0x263a": ""
  , "0x2723": "overlap-test.svg"
  , "0x1f304": "outline-test.svg"
  }
, "sfnt_names":
  [ ["English (US)", "Copyright", "Copyright (c) 2014 by Nobody"]
  , ["English (US)", "Family", "Example"]
  , ["English (US)", "SubFamily", "Regular"]
  , ["English (US)", "UniqueID", "Example 2014-12-04"]
  , ["English (US)", "Fullname", "Example Regular"]
  , ["English (US)", "Version", "Version 001.000"]
  , ["English (US)", "PostScriptName", "Example-Regular"]
  ]
, "input": "."
, "output": [ "example.ttf", "example.svg", "example.woff" ]
, "# vim: set et sw=2 ts=2 sts=2:": false
}