TTF Fonts
I've a niggling mini-project to write code to insert new emoji into my mac's system emoji font file. For no good reason than to say that I can, I'm wondering whether I can (for example) show ๐ + the zero-width-joiner + ๐ฒ as a newly inserted glyph of Deliveroo's logo. Naturally this wouldn't be useful to anyone except myself (as no-one else would have this glyph), but I'm using it as a way to learn how TrueType Fonts are put together.
I've found the docs on the OpenType spec at Microsoft, as well as the TrueType reference at Apple, but there is a lot. I think I need to start with making a trivial font with a tool like Glyphs (which I used to make Caspian) or FontForge, and seeing what's created. There is definitely bytecode in the Apple Emoji TTF fileโฆ
Zero Width Joiner
The ZWJ is a non-visible unicode character that can be used to link codepoints together into single glyphs or more accurately, as wikipedia tells me, indicates that the following grapheme is positioned relative to the previous one.
The classic example for me is the various family emoji, which look like this: ๐จโ๐ฉโ๐ฆโ๐ฆ
That particular one is ๐จ + ZWJ + ๐ฉโ๐ฆฐ + ZWJ + ๐ฆ + ZWJ + ๐ฆ โ so a full 7 multi-byte characters for a single glyph! Then of course you can add skin colour indicators for each of the members of the family, and you can rapidly get to enormous single "characters".
I saw a post recently that called out the Welsh flag emoji ๐ด๓ ง๓ ข๓ ท๓ ฌ๓ ณ๓ ฟ as the longest flag, being ๐ด + the letters gbwls + โ. Designing emoji fonts must be a multi-year effort at this point!