For the past few weeks, I’ve been implementing emoji support for Skija. I thought it might be fun sharing a few nitty-gritty details of how this “biggest innovation in human communication since the invention of the letter 🅰️” works under the hood.
Warning: some emoji might not display as expected on your device. In that case, use this text version: ¯\(ツ)/¯
As you might know, all text inside computers is encoded with numbers. One letter—one number. The most popular encoding we use is called Unicode, with the two most popular variations called UTF-8 and UTF-16.
Unicode allocates 2²¹ (~2 mil) characters called codepoints. Sorry, programmers, 21 is not a multiple of 8 🤷. Out of those 2 million, actually defined are ~150k characters.
150k defined characters cover all the scripts used on 🌍, many dead languages, a lot of weird stuff like 𝔣𝔲𝔫𝔫𝔶 𝕝𝕖𝕥𝕥𝕖𝕣𝕤, sɹǝʇʇǝl uʍop-ǝpᴉsdn, GHz as one glyph: ㎓, “rightwards two-headed arrow with tail with double vertical stroke”: ⤘, seven-eyed monster: ꙮ, and a duck:
As a side note, definitely check out the Egyptian Hieroglyphs block (U+13000–U+1342F). They have some really weird stuff.
So, emoji. At their simplest, they are just that: another symbol in a Unicode table. Most of them are grouped in U+1F300–1F6FF and U+1F900–1FAFF.
That’s why emoji behave like any other letter: they can be typed in a text field, copied, pasted, rendered in a plain text document, embedded in a tweet, etc. When you type “A”, the computer sees U+0041. When you type “🌵”, the computer sees U+1F335. Not much difference.
Why are emoji rendered as images then? Well, bitmap fonts. Apparently, you can create a font that has pngs for glyphs instead of boring black-and-white vector shapes.
Every OS comes with a single pre-installed font for emoji. On macOS/iOS, that’s Apple Color Emoji. Windows has Segoe UI Emoji, Android has Noto Color Emoji.
As far as I can tell, Apple is a bitmap font with 160×160 raster glyphs, Noto uses 128×128 bitmaps, and Segoe is a vector color font 🆒.
That’s why emoji look different on different devices—just like fonts look different! On top of that, many apps bundle their own emoji fonts, too: WhatsApp, Twitter, Facebook.
