new check: ensure glyph case mapping

Ensure that no glyph lacks its corresponding upper or lower counterpart (but only when unicode supports case-mapping).

com.google.fonts/check/case_mapping (EXPERIMENTAL)
Added to the Universal profile.

(issue fonttools#3230)

CHANGELOG.md

@@ -13,6 +13,7 @@ A more detailed list of changes is available in the corresponding milestones for
 #### Added to the Universal Profile
   - **EXPERIMENTAL - [com.google.fonts/check/varfont/family_axis_ranges]:** Check that family axis ranges are indentical. (issue #4445)
   - **EXPERIMENTAL - [com.google.fonts/check/tabular_kerning]:** Check that tabular numerals and symbols have no kerning. (issue #4440)
+  - **EXPERIMENTAL - [com.google.fonts/check/case_mapping]:** Ensure that no glyph lacks its corresponding upper or lower counterpart (but only when unicode supports case-mapping). (issue #3230)
 
 #### Added to the Google Fonts Profile
   - **EXPERIMENTAL - [com.google.fonts/check/metadata/has_tags]:** Check that the font family appears in the tags spreadsheet. (issue #4465)

Lib/fontbakery/profiles/universal.py

@@ -73,6 +73,7 @@
         "com.google.fonts/check/alt_caron",
         "com.google.fonts/check/arabic_spacing_symbols",
         "com.google.fonts/check/arabic_high_hamza",
+        "com.google.fonts/check/case_mapping",
     ]
 )
 
@@ -2585,5 +2586,83 @@ def com_google_fonts_check_alt_caron(ttFont):
         yield PASS, "Looks good!"
 
 
+@check(
+    id="com.google.fonts/check/case_mapping",
+    rationale="""
+        Ensure that no glyph lacks its corresponding upper or lower counterpart
+        (but only when unicode supports case-mapping).
+    """,
+    proposal="https://github.com/googlefonts/fontbakery/issues/3230",
+    experimental="Since 2024/Jan/19",
+    severity=10,  # if a font shows tofu in caps but not in lowercase
+    #               then it can be considered broken.
+)
+def com_google_fonts_check_case_mapping(ttFont):
+    """Ensure the font supports case swapping for all its glyphs."""
+    import unicodedata
+    from fontbakery.utils import markdown_table
+
+    # 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
+        0x0250,  # ɐ - Latin Small Letter Turned A
+        0x025C,  # ɜ - Latin Small Letter Reversed Open E
+        0x0252,  # ɒ - Latin Small Letter Turned Alpha
+        0x0271,  # ɱ - Latin Small Letter M with Hook
+        0x0282,  # ʂ - Latin Small Letter S with Hook
+        0x029E,  # ʞ - Latin Small Letter Turned K
+        0x0287,  # ʇ - Latin Small Letter Turned T
+        0x0127,  # ħ - Latin Small Letter H with Stroke
+        0x0140,  # ŀ - Latin Small Letter L with Middle Dot
+        0x023F,  # ȿ - Latin Small Letter S with Swash Tail
+        0x0240,  # ɀ - Latin Small Letter Z with Swash Tail
+        0x026B,  # ɫ - Latin Small Letter L with Middle Tilde
+    ]
+
+    missing_counterparts_table = []
+    cmap = ttFont["cmap"].getBestCmap()
+    for codepoint in cmap:
+        if codepoint in EXCEPTIONS:
+            continue
+
+        the_char = chr(codepoint)
+        swapped = the_char.swapcase()
+
+        # skip cases like 'ß' => 'SS'
+        if len(swapped) > 1:
+            continue
+
+        if the_char != swapped and ord(swapped) not in cmap:
+            name = unicodedata.name(the_char)
+            swapped_name = unicodedata.name(swapped)
+            row = {
+                "Glyph present in the font": f"U+{codepoint:04X}: {name}",
+                "Missing case-swapping counterpart": (
+                    f"U+{ord(swapped):04X}: {swapped_name}"
+                ),
+            }
+            missing_counterparts_table.append(row)
+
+    if missing_counterparts_table:
+        yield FAIL, Message(
+            "missing-case-counterparts",
+            f"The following glyphs lack their case-swapping counterparts:\n\n"
+            f"{markdown_table(missing_counterparts_table)}\n\n",
+        )
+    else:
+        yield PASS, "Looks good!"
+
+
 profile.auto_register(globals())
 profile.test_expected_checks(UNIVERSAL_PROFILE_CHECKS, exclusive=True)

tests/profiles/universal_test.py

@@ -1433,3 +1433,24 @@ def DISABLED_test_check_caps_vertically_centered():
 
     ttFont = TTFont(TEST_FILE("cairo/CairoPlay-Italic.leftslanted.ttf"))
     assert_results_contain(check(ttFont), WARN, "vertical-metrics-not-centered")
+
+
+def test_check_case_mapping():
+    """Ensure the font supports case swapping for all its glyphs."""
+    check = CheckTester(universal_profile, "com.google.fonts/check/case_mapping")
+
+    ttFont = TTFont(TEST_FILE("merriweather/Merriweather-Regular.ttf"))
+    # Glyph present in the font                  Missing case-swapping counterpart
+    # ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
+    # U+01D3: LATIN CAPITAL LETTER U WITH CARON  U+01D4: LATIN SMALL LETTER U WITH CARON
+    # U+01E6: LATIN CAPITAL LETTER G WITH CARON  U+01E7: LATIN SMALL LETTER G WITH CARON
+    # U+01F4: LATIN CAPITAL LETTER G WITH ACUTE  U+01F5: LATIN SMALL LETTER G WITH ACUTE
+    assert_results_contain(check(ttFont), FAIL, "missing-case-counterparts")
+
+    # While we'd expect designers to draw the missing counterparts,
+    # for testing purposes we can simply delete the glyphs that lack a counterpart
+    # to make the check PASS:
+    _remove_cmap_entry(ttFont, 0x01D3)
+    _remove_cmap_entry(ttFont, 0x01E6)
+    _remove_cmap_entry(ttFont, 0x01F4)
+    assert_PASS(check(ttFont))