SKIP instead of ERROR if opentype/slant_direction's reference codepoint is not covered

CHANGELOG.md

@@ -4,6 +4,10 @@ A more detailed list of changes is available in the corresponding milestones for
 ##  Upcoming release: 0.13.1 (2025-Jan-??)
   - ...
 
+### Changes to existing checks
+### On the OpenType Profile
+  - **[opentype/slant_direction]:** SKIP instead of ERROR if a font does not contain 'H' (PR #4969)
+
 
 ##  0.13.0 (2025-Jan-10)
 ### Stable release notes

Lib/fontbakery/checks/opentype/slant_direction.py

@@ -1,6 +1,10 @@
-from fontbakery.prelude import check, condition, Message, FAIL, PASS
+from fontbakery.prelude import Message, check, condition
+from fontbakery.status import FAIL, PASS, SKIP
 from fontbakery.testable import Font
 
+# Reference codepoint to use to determine slant angle.
+REFERENCE = "H"
+
 
 @condition(Font)
 def uharfbuzz_blob(font):
@@ -33,10 +37,19 @@ def check_slant_direction(ttFont, uharfbuzz_blob):
         yield PASS, "Font has no slnt axis"
         return
 
+    if ord(REFERENCE) not in ttFont.getBestCmap():
+        yield SKIP, Message(
+            "no-reference-glyph",
+            f"This check uses '{REFERENCE}' as a reference codepoint to "
+            "determine slant direction, but it is not present in this font, "
+            "and so the slant direction cannot be checked.",
+        )
+        return
+
     hb_face = hb.Face(uharfbuzz_blob)
     hb_font = hb.Font(hb_face)
     buf = hb.Buffer()
-    buf.add_str("H")
+    buf.add_str(REFERENCE)
     features = {"kern": True, "liga": True}
     hb.shape(hb_font, buf, features)
 

tests/test_checks_opentype_varfont_family_axis_ranges.py

@@ -1,7 +1,8 @@
 from fontTools.ttLib import TTFont
 
 from conftest import check_id
-from fontbakery.status import FAIL
+from fontbakery.status import FAIL, SKIP
+from fontbakery.checks.opentype.slant_direction import REFERENCE
 from fontbakery.codetesting import (
     assert_PASS,
     assert_results_contain,
@@ -35,3 +36,19 @@ def test_check_slant_direction(check):
 
     font = TEST_FILE("slant_direction/Cairo_wrong_slnt_axis.ttf")
     assert_results_contain(check(font), FAIL, "positive-value-for-clockwise-lean")
+
+
+@check_id("opentype/slant_direction")
+def test_check_slant_direction_missing(check, tmp_path):
+    """Check that the slant direction check handles the case where its reference
+    codepoint is not present."""
+
+    missing_codepoint = tmp_path / "MissingCodepoint.ttf"
+
+    # Remove the reference codepoint from a TTF that would otherwise PASS.
+    ttf = TTFont(TEST_FILE("slant_direction/Cairo_correct_slnt_axis.ttf"), lazy=False)
+    for subtable in ttf["cmap"].tables:
+        subtable.cmap.pop(ord(REFERENCE), None)
+    ttf.save(missing_codepoint)
+
+    assert_results_contain(check(str(missing_codepoint)), SKIP, "no-reference-glyph")