A project similar to the Schriftenkartei unfolded in East Germany (GDR) around the same time, producing a set of cards for typefaces from VEB Typoart in Latin (79 typefaces) and Cyrillic.

but even ᴡindows® Notepad tries to substitute unknown characters from glyphs in other fonts instead of using the replacement character directly. Sometimes it even substitute characters whereas they exist in the font

No, there is no “glyph not found” character. Different programs use different graphic presentations. An empty narrow rectangle is a common rendering, but not the only one. It could also be a rectangle with a question mark in it or with the code number of the character, in hexadecimal, in it.

The glyph-not-found character is specified by the font engine and by the font; there is no fixed character for it.

Determine if a glyph exists in a given font

I'm looking to see if there is a way, in PHP, to determine if a glyph exists for a given character in a font.

I know this is older but I'm surprised by the "Is redrawing 110K glyphs (with metrics and kerning and combining attributes and hinting) too hard?" I used to do typography. A plain, unoriginal typeface with 255 straightforward latin-# oriented letters is at least a couple days of work; probably a couple weeks; couple months for truly good work. 110K is the equivalent of 400+ faces with much harder metrics and such. 15,000 hours of work or drastically more; so at least 7 or so years. So, kinda hard.

Pango is one of the most common font rendering libraries on Linux. It's used by GTK/GNOME and a lot of standalone apps, like Rofi, Polybar, and a lot of terminals.

cp src/glyphs/Symbols-2048-em\ Nerd\ Font\ Complete.ttf ~/.local/share/fonts fc-cache -fv pango-view -t "Playing some  for you right now"