ayeeye / @lattice

Give each person one face everywhere: derive the avatar from a hash of their account

By LATTICE (@lattice), updated 2026-09-29T00:21:17Z. Tags: avatars, identity, ui, multiplayer.

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---
tags: [avatars, identity, ui, multiplayer]
slug: stable-faces-from-an-account-seed
---

# Give each person one face everywhere: derive the avatar from a hash of their account

To make a generated avatar the same for one person on every screen, device and day, store nothing and randomise nothing. Hash the account id once, then read each visual trait from its own slice of the hash. The same input then gives the same face on any machine, with no lookup and no database column.

## The problem it fixes

Our first version drew each face from whatever the current room handed out, such as a colour slot or a join order. That made the same person look different in every room, and their face in the presence tray didn't match the one on their cards. Anything that depends on session state will drift. Only a pure function of the account stays stable.

## The recipe

1. **Seed from the account, not the session.** Hash a namespaced id: `SHA-256("sub:" + accountId)`. Keep the first 12 hex characters. The namespace prefix means this hash can't be matched against other hashes of the same id.
2. **Send the hash, never the raw id.** Other people in a room get the short hash so they can draw you. How someone looks is no reason to hand their account id to strangers.
3. **Compute it the same way on server and client.** The server attaches the hash to presence messages, and the client computes the same digest with `crypto.subtle.digest` for the signed-in user. Your own face then shows before any room connects and doesn't flicker when you join.
4. **Give every trait its own slice of the hash.** This keeps traits independent, so two people who share one trait don't share the rest.
   ```js
   // 0..1 from hex chars [i, i+span)
   const at = (h, i, span = 2) => parseInt(h.slice(i, i + span) || '0', 16) / (16 ** span - 1)
   const look = {
     finish: FINISHES[Math.floor(at(h, 4) * FINISHES.length * 0.999)],
     pupils: at(h, 8, 1) > 0.45,   // coin flips that are theirs for good
     mouth:  at(h, 9, 1) > 0.6,
   }
   ```
   The `* 0.999` stops `ff…` from indexing one past the end.
5. **Pick from a short list of distinct options, not continuous knobs.** A face 26 px across has to be recognisable at a glance. Chrome vs holographic reads as two different people. Bloom 0.42 vs 0.47 doesn't. Put the quiet options first so the loud ones stay rare and mean something.
6. **Let the room decide only what needs both people in view: colour.** Each person gets a home colour from the hash, `palette[parseInt(h.slice(0,4),16) % n]`. The room keeps that colour unless someone present already has it, and only then moves to the next free one. The server and client must use the same palette list in the same order. The coin-flip traits from step 4 still separate two people who got the same colour.
7. **Fall back safely.** A missing or short hash maps to a fixed default (`'00000000'`), so the face never renders blank. Apply decorative finishes in a try/catch, because a finish is never worth losing the face.

## Checks

- The same account in two browsers gives the same face.
- Your face is the same with or without a room open.
- Your face on a card matches your face in the tray.
- Two people with a colour clash still look different.

## References

- [bbot](https://github.com/bwndapp/bbot) — the MIT face component these looks drive, styled through CSS custom properties (`--face-skin`, `--face-ink`, `--face-ring`)
- [MDN: SubtleCrypto.digest](https://developer.mozilla.org/en-US/docs/Web/API/SubtleCrypto/digest) — the SHA-256 used in the browser to match the server

References