ImageFont detect missing glyph (Python Pillow)

Question

Here is a short example.

from PIL import ImageFont, ImageDraw

draw = ImageDraw.Draw(image)

# use a bitmap font
font = ImageFont.load("arial.pil")

draw.text((10, 10), "hello", font=font)

# use a truetype font
font = ImageFont.truetype("arial.ttf", 15)

draw.text((10, 25), "world", font=font)

I want to know if the font is missing any glyphs from the rendered text.

When I try to render a glyph that is missing I get an empty square.

draw.text((10, 10), chr(1234), font=font)

How to programmatically determine missing glyphs?
How to list available glyphs in a ttf file?

The two questions are almost the same.

I would prefer using Pillow to determine what I want. Other modules from PyPI are welcome as well.

Related http://stackoverflow.com/questions/35436932/is-there-a-way-to-fallback-for-missing-glyphs-with-pil (But the solution there is to edit the font) — James K, Sep 04 '16 at 11:45
thanks, the best would be if I could list the available glyphs. — Szabolcs Dombi, Sep 04 '16 at 11:47
so is there any solution to know that a particular glyph is missing? — binzhang, Jun 29 '17 at 08:59
Unfortunately i found no solution. I use a heuristic approach to detect invalid glyphs of a given range. — Szabolcs Dombi, Jun 29 '17 at 11:46

James K · Accepted Answer · 2017-06-28T12:16:03.017

There is the 'fontTools' package, that includes a python class for reading and querying TrueType fonts. Something like this is possible

from fontTools.ttLib import TTFont
f = TTFont('/path/to/font/arial.ttf')
print(f.getGlyphOrder())      # a list of the glyphs in the order 
                              # they appear
print(f.getReversedGlyphMap() # mapping from glyph names to Id
id = f.getGlyphID('Euro')     # The internal code for the Euro character, 
                              # Raises attribute error if the character
                              # isn't present.

The mapping from characters to glyphs is often complex, and is defined in the cmap table of the font. It's a binary section, but can be inspected with

f.getTableData('cmap')

A font can have multiple cmap tables. The freetype interface can also read ttf files. It is possible to use freetype to attempt to render a character, and see the result: this would be slow.

import freetype as ft
face = ft.Face('arial.ttf')
face.set_size(25*32)
face.load_char(‽)
bitmap = face.glyph.bitmap.buffer
if bitmap == [255, 255, 255, 255, 0, 255, 255, 0, 255, 255, 0, 255, 255, 0, 255, 255, 0, 255, 255, 255, 255]:
     print("No interobang in the font")

ImageFont detect missing glyph (Python Pillow)

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