case_mapping: Dynamically exclude incomplete Greek glyphs.

'case_mapping' check on Universal profile.

(PR #4721)

CHANGELOG.md

@@ -4,6 +4,7 @@ A more detailed list of changes is available in the corresponding milestones for
 ##  Upcoming release: 0.13.0 (2024-Sep-??)
 ### Changes to existing checks
 #### On the Universal profile
+  - **[case_mapping]:** Dynamically exclude incomplete Greek glyphs (PR #4721)
   - **[missing_small_caps_glyphs]:** Rewrote it from scratch, marked it as **experimental** (issue #4713)
   - **[name/family_and_style_max_length"]:** Use nameID 16 (Typographic family name) to determine name length if it exists. (PR #4811)
 

Lib/fontbakery/checks/glyphset.py

@@ -23,19 +23,15 @@
 )
 def check_case_mapping(ttFont):
     """Ensure the font supports case swapping for all its glyphs."""
-    from fontbakery.utils import markdown_table
+    from fontbakery.utils import markdown_table, characters_per_script
 
     # These are a selection of codepoints for which the corresponding case-swap
     # glyphs are missing way too often on the Google Fonts library,
     # so we'll ignore for now:
     EXCEPTIONS = [
         0x0192,  # ƒ - Latin Small Letter F with Hook
         0x00B5,  # µ - Micro Sign
-        0x03C0,  # π - Greek Small Letter Pi
         0x2126,  # Ω - Ohm Sign
-        0x03BC,  # μ - Greek Small Letter Mu
-        0x03A9,  # Ω - Greek Capital Letter Omega
-        0x0394,  # Δ - Greek Capital Letter Delta
         0x0251,  # ɑ - Latin Small Letter Alpha
         0x0261,  # ɡ - Latin Small Letter Script G
         0x00FF,  # ÿ - Latin Small Letter Y with Diaeresis
@@ -53,6 +49,13 @@ def check_case_mapping(ttFont):
         0x026B,  # ɫ - Latin Small Letter L with Middle Tilde
     ]
 
+    # Font has incomplete legacy Greek coverage, so ignore Greek dynamically
+    # (minimal Greek coverage is 2x24=48 characters, so we assume incomplete
+    # if coverage is less than half of 48)
+    greek = characters_per_script(ttFont, "Greek")
+    if 0 < len(greek) < 24:
+        EXCEPTIONS.extend(greek)
+
     missing_counterparts_table = []
     cmap = ttFont["cmap"].getBestCmap()
     for codepoint in cmap: