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emoji-implementation.md

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Notes on Emoji font implementation

This document notes details on how common Emoji fonts implement sequence, modifier, varation-selection, text-presentation fallback, and other behavior, for the purposes of testing and debugging.

Emoji fonts are deployed by vendors using a variety of different image formats (including the SVG , COLRv0/CPAL, COLRv1/CPAL, glyf, and cff vector formats and the CBDT and sbix raster formats), which can make it difficult to characterize Emoji font behavior.

Similarly, Emoji font vendors have employed a variety of different OpenType features to implement support for standard sequences, modifier-based sequences, ZWJ-based sequences and permutations.

See the Emoji shaping document for more details on the sequences and definitions involved.

Format, features, and control-codepoint visiblity table

This table lists the image format, the GSUB feature(s) used for basic Emoji sequence support and ZWJ-based sequence support, and whether or not the font includes a visible glyph for the presentation selector codepoints (VS15, U+FE0E; VS16, U+FE0F) and modifier codepoints (U+1F3FB..U+1F3FF).

Font publisher image format sequence formation feature ZWJ sequence feature visible presentation selector visible modifier
Source Emoji Adobe cff ccmp ccmp, salt YES YES
Blobmoji C1710 CBDT ccmp ccmp no YES
Twemoji Twitter SVG liga liga no YES
Noto Color Emoji Google CBDT ccmp ccmp no YES
Noto Color Emoji Google COLRv1 ccmp ccmp no YES
EmojiTwo Android EmojiTwo CBDT ccmp ccmp no YES
EmojiTwo Apple EmojiTwo sbix morx morx no YES
EmojiTwo SVG EmojiTwo SVG ccmp ccmp no YES
Openmoji HfG Gmünd SVG liga liga no YES
FirefoxEmoji Mozilla COLRv0 rlig rlig no no
Noto Emoji Google glyf ccmp ccmp no YES
Old Noto B&W Emoji Google glyf ccmp ccmp no no
JoyPixels JoyPixels CBDT ccmp ccmp no YES
Apple Color Emoji Apple sbix morx morx no YES
Samsung Color Emoji Samsung CBDT ccmp ccmp no YES
Segoe UI Emoji Microsoft COLRv0 ccmp ccmp YES YES

Contributing additional data

Volunteers or implementers who wish to contribute data for additional Emoji fonts may need to collecting the information themselves by inspecting font binaries.

Options available include:

  1. FontTools / TTX

    • Users can run ttx -l somefontfilename.ttf (or .otf or .ttc or .otc) to get a short list of the tables. The presence of SVG , CBDT, sbix, or COLR indicates that whichever one of those exists is the image format. If none of the above are there but glyf or CFF or CFF2 is there, then whichever of those three exists is the image format (and means it's a black-and-white emoji font, which users would probably know beforehand anyway). If there's more than one of SVG , CBDT, sbix, or COLR present in the same font file, that would likely mean unknown behavior; comments on such cases are welcome.
    • Users can run the script layout-features.py somefontfilename.ttf script from FontTools/Snippets/ and it will print out an indented list of the GSUB and GPOS features used. All that matters for the table above is what the script reports on the Feature: line. For a typical emoji font there's probably only one feature -- but, if there are several, listing them is useful.

  2. allsorts / allsorts-tools

    • Users can use the dump tool from the allsorts-tools package to run allsorts dump somefilename.ttf and get a list of tables plus other metadata; the tables are the first output. Same interpretation as above.
    • At the moment it sounds like there isn't a single-command option in allsorts to list GSUB/GPOS features. Corrections are welcome.

  3. GUI font editors

    • Users can also open up the font file in a font editor and look at what it presents.
    • FontForge:
      • In FontForge, go to Element -> Font Info in the menu to open the font-info dialog box. It will show the GSUB/GPOS lookups in the "Lookups" tab (left-hand side).
      • FontForge does not just show a convenient list of all the tables. However, when users open the font file, the "Warnings" dialog box will report if it finds SVG , CBDT, sbix, or COLR tables. Unfortunately, it will only actually open the font for editing/inspection if it finds a glyf, CFF , or CFF2 table (which a COLR font would have) or an SVG table. So users can't use it to inspect the features of the other formats.

(Further instructions will be added to this list for other editors if volunteers can contribute them)

For determining if there's a printable glyph for the selectors/modifiers:

  1. GUI font editors
    • Users can open up the font in an editor and look at the slots for the Unicode codepoints for the presentation selectors (U+FE0E and U+FE0F) and the modifiers (U+1F3FB through U+1F3FF), if they exist (they might not).
  2. HarfBuzz
    • Users can run the hb-view utility to output glyph contents for specific Unicode codepoints, but one might have to try a couple of options, depending on the image format. Run hb-view --preserve-default-ignorables somefontfilename.ttf --unicodes=fe0e to start (for U+FE0E). Users may also try adding the --font-funcs=ot and/or --shapers=ot flags to that command if it gives trouble.