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zulip/docs/subsystems/emoji.md

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Emoji

Emoji seem like a simple idea, but there's actually a ton of complexity that goes into an effective emoji implementation. This document discusses a number of these issues.

Currently, Zulip uses the Noto (Android) emoji set, but we are close to being able to support the user choosing which emoji set they want to use.

Emoji codes

The Unicode standard has various ranges of characters set aside for emoji. So you can put emoji in your terminal using actual unicode characters like 😀 and 👍. If you paste those into Zulip, Zulip will render them as the corresponding emoji image.

However, the Unicode committee did not standardize on a set of human-readable names for emoji. So, for example, when using the popular : based style for entering emoji from the keyboard, we have to decide whether to use :angry: or :angry_face: to represent an angry face. Different products use different approaches, but for purposes like emoji pickers or autocomplete, you definitely want to pick exactly one of these names, since otherwise users will always be seeing duplicates of a given emoji next to each other.

Picking which emoji name to use is surprisingly complicated! Zulip has a nice library, tools/setup/emoji/emoji_setup_utils.py, which we use to make these decisions systematically, with a relatively small list of hand-coded exceptions.

Custom emoji

Zulip supports custom user-uploaded emoji. We manage those by having the name of the emoji be its "emoji code", and using an emoji_type field to keep track of it. We are in the progress of migrating Zulip to refer to these emoji only by ID, which is a requirement for being able to support deprecating old realm emoji in a sensible way.

Tooling

We use the iamcal emoji data package to provide sprite sheets and individual images for our emoji, as well as a data set of emoji categories, code points, names, etc. The sprite sheets are used by the Zulip webapp to display emoji in messages, emoji reactions, etc. However, we can't use the sprite sheets in some contexts, such as missed-message and digestemails, that need to have self-contained assets. For those, we use individual emoji files under static/generated/emoji. The structure of that repository contains both files named after the unicode representation of emoji (as actual image files) as well as symlinks pointing to those emoji.

We need to maintain those both for the names used in the iamcal emoji data set as well as our old emoji data set (emoji_map.json). Zulip has a tool, tools/setup/emoji/build_emoji, that combines the emoji.json file from iamcal with the old emoji-map.json data set to construct the various symlink farms and output files described below that support our emoji experience.

The build_emoji tool generates the set of files under static/generated/emoji (or really, it generates the /srv/zulip-emoji-cache/<sha1>/emoji tree, and static/generated/emoji is a symlink to that tree; we do this in order to cache old versions to make provisioning and production deployments super fast in the common case that we haven't changed the emoji tooling). See our dependencies document for more details on this strategy.

The emoji tree generated by this process contains several import elements:

  • emoji_codes.js: A set of mappings used by the Zulip frontend to understand what unicode emoji exist and what their shortnames are, used for autocomplete, emoji pickers, etc. This has been deduplicated using the logic in tools/setup/emoji/emoji_setup_utils.py to generally only have :angry: and not also :angry_face:, since having both is ugly and pointless for purposes like autocomplete and emoji pickers.
  • images/emoji/unicode/*.png: A farm of emoji
  • images/emoji/*.png: A farm of symlinks from emoji names to the images/emoji/unicode/ tree. This is used to serve individual emoji images, as well as for the backend markdown processor to know which emoji names exist and what unicode emoji / images they map to. In this tree, we currently include all of the emoji in emoji-map.json; this means that if you send :angry_face:, it won't autocomplete, but will still work (but not in previews).
  • Some CSS and PNGs for the emoji spritesheets, used in Zulip for emoji pickers where we would otherwise need to download over 1000 of individual emoji images (which would cause a browser performance problem). We have multiple spritesheets: one for each emoji provider that we support (Google, Twitter, EmojiOne, etc.).