Script to derive somewhat more correct COLRv1 font from a good CBDT one

colrv1/README.md

@@ -1,9 +1,7 @@
 # COLRv1 Build
 
-Assumptions:
-
-* bitmap font already exists.
-* a font that contains complete emojicompat metadata exists
+We assume the bitmap version with equivalent coverage exists and
+contains emojicompat metadata.
 
 ## Build Steps
 
@@ -14,4 +12,10 @@ Assumptions:
    ```
 
 1. Post-process COLRv1 font for Android
-   * At time of writing only the noflags version is for Android
+   * At time of writing only the noflags version is for Android
+
+   ```shell
+   # Assumed to be in a python3 environment with requirements.txt fulfilled
+   python postproc.py build/NotoColorEmoji-noflags.ttf \
+   	PATH_TO/NotoColorEmojiCompat.ttf
+   ```

colrv1/postproc.py

@@ -0,0 +1,70 @@
+"""
+Post-nanoemoji processing of the Noto COLRv1 Emoji file.
+
+For now substantially based on copying from a correct bitmap build.
+"""
+from absl import app
+from fontTools import ttLib
+
+
+def _is_colrv1(font):
+  return (
+    "COLR" in font
+    and font["COLR"].version == 1
+  )
+
+
+def _is_cbdt(font):
+  return "CBDT" in font
+
+
+def _is_compat_font(font):
+  return (
+    "meta" in font
+    and "Emji" in font["meta"].data
+  )
+
+
+def _copy_emojicompat_data(colr_font, cbdt_font):
+    colr_font["meta"] = cbdt_font["meta"]
+
+
+def _set_name(name_table, nameID):
+  name_table.getName(value, nameID, 3, 1, 0x409)
+
+
+def _set_name(name_table, nameID, value):
+  name_table.setName(value, nameID, 3, 1, 0x409)
+
+
+def _copy_names(colr_font, cbdt_font):
+  colr_font["name"] = cbdt_font["name"]
+  name_table = colr_font["name"]
+  assert all((n.platformID, n.platEncID, n.langID) == (3, 1, 0x409)
+             for n in name_table.names), "Should only have names Android uses"
+
+  # Amendments
+  _set_name(name_table, 10, "Color emoji font using COLRv1.")
+  _set_name(name_table, 11, "https://github.com/googlefonts/noto-emoji")
+
+
+def main(argv):
+    if len(argv) != 3:
+      raise ValueError("Must have two args, a COLRv1 font and a CBDT emojicompat font")
+
+    colr_font = ttLib.TTFont(argv[1])
+    if not _is_colrv1(colr_font):
+      raise ValueError("First arg must be a COLRv1 font")
+
+    cbdt_font = ttLib.TTFont(argv[2])
+    if not _is_cbdt(cbdt_font) or not _is_compat_font(cbdt_font):
+      raise ValueError("Second arg must be a CBDT emojicompat font")
+
+    _copy_emojicompat_data(colr_font, cbdt_font)
+    _copy_names(colr_font, cbdt_font)
+
+    colr_font.save('../fonts/Noto-COLRv1-noflags.ttf')
+
+
+if __name__ == "__main__":
+    app.run(main)