New check: overlapping_path_segments

"Check there are no overlapping path segments"

'overlapping_path_segments'
Added to the Universal profile.

Some rasterizers encounter difficulties when rendering glyphs with
overlapping path segments.

A path segment is a section of a path defined by two on-curve points.
When two segments share the same coordinates, they are considered
overlapping.

(PR #4853)

CHANGELOG.md

@@ -5,6 +5,10 @@ A more detailed list of changes is available in the corresponding milestones for
 ### Noteworthy code-changes
   - **[FontBakeryCondition:remote_styles]:** Fetching static fonts no longer raises an error. (PR # #4863)
 
+### New checks
+#### Added to the Universal profile
+  - **[overlapping_path_segments]:** Check there are no overlapping path segments. (PR #4853)
+
 ### Migration of checks
 #### Moved from OpenType to Universal profile
   - **[name/no_copyright_on_description]**: Adding copyright information to the description name entry is bad practice but is not a breach of specification. (issue #4857)

Lib/fontbakery/checks/outline.py

@@ -5,7 +5,7 @@
 
 from fontbakery.callable import condition, check
 from fontbakery.testable import Font
-from fontbakery.status import PASS, WARN
+from fontbakery.status import PASS, WARN, FAIL
 from fontbakery.message import Message
 from fontbakery.utils import bullet_list
 
@@ -382,3 +382,40 @@ def bounds_contains(bb1, bb2):
             f"The following glyphs have a counter-clockwise outer contour:\n\n"
             f"{formatted_list}",
         )
+
+
+@check(
+    id="overlapping_path_segments",
+    rationale="""
+        Some rasterizers encounter difficulties when rendering glyphs with
+        overlapping path segments.
+
+        A path segment is a section of a path defined by two on-curve points.
+        When two segments share the same coordinates, they are considered
+        overlapping.
+    """,
+    conditions=["outlines_dict", "is_ttf"],
+    proposal="https://github.com/google/fonts/issues/7594#issuecomment-2401909084",
+)
+def check_overlapping_path_segments(ttFont, outlines_dict, config):
+    """Check there are no overlapping path segments"""
+    failed = []
+    for glyph, outlines in outlines_dict.items():
+        seen = set()
+        for p in outlines:
+            for seg in p.asSegments():
+                normal = ((seg.start.x, seg.start.y), (seg.end.x, seg.end.y))
+                flipped = ((seg.end.x, seg.end.y), (seg.start.x, seg.start.y))
+                if normal in seen or flipped in seen:
+                    failed.append(
+                        f"{glyph[1]}: {seg} has the same coordinates as a previous segment."
+                    )
+                seen.add(normal)
+    if failed:
+        yield FAIL, Message(
+            "overlapping-path-segments",
+            f"The following glyphs have overlapping path segments:\n\n"
+            f"{bullet_list(config, failed, bullet='*')}",
+        )
+    else:
+        yield PASS, "No overlapping path segments found."

Lib/fontbakery/profiles/adobefonts.py

@@ -26,6 +26,7 @@
     "pending_review": [
         "notofonts/cmap/alien_codepoints",  # Note: These two checks had not been previously marked as permanently excluded,
         "notofonts/unicode_range_bits",  # so maybe there's still some change they may be considered useful here?
+        #
         "opentype/caret_slope",
         "opentype/dsig",
         "opentype/fsselection",
@@ -41,6 +42,7 @@
         "opentype/varfont/family_axis_ranges",
         "opentype/varfont/ital_range",
         "opentype/vendor_id",
+        #
         "alt_caron",
         "arabic_high_hamza",
         "arabic_spacing_symbols",
@@ -70,6 +72,7 @@
         "name/italic_names",
         "no_debugging_tables",
         "no_mac_entries",
+        "overlapping_path_segments",
         "render_own_name",
         "smart_dropout",
         "soft_hyphen",

Lib/fontbakery/profiles/fontbureau.py

@@ -25,6 +25,7 @@
         "name/family_and_style_max_length",
         "name/ascii_only_entries",
         "no_mac_entries",
+        "overlapping_path_segments",
         "render_own_name",
         "smart_dropout",
         "stylisticset_description",

Lib/fontbakery/profiles/fontwerk.py

@@ -18,6 +18,7 @@
     ],
     "pending_review": [
         "googlefonts/axes_match",
+        "overlapping_path_segments",
         "typographic_family_name",
         "vtt_volt_data",  # very similar to vttclean, may be a good idea to merge them.
         #

Lib/fontbakery/profiles/microsoft.py

@@ -10,6 +10,7 @@
         "opentype/cff_ascii_strings",
         "opentype/slant_direction",
         "opentype/weight_class_fvar",
+        #
         "cjk_not_enough_glyphs",
         "cmap/format_12",
         "color_cpal_brightness",
@@ -38,6 +39,7 @@
         "name/family_and_style_max_length",
         "no_debugging_tables",
         "no_mac_entries",
+        "overlapping_path_segments",
         "render_own_name",
         "smart_dropout",
         "stylisticset_description",

Lib/fontbakery/profiles/typenetwork.py

@@ -54,6 +54,7 @@
         "legacy_accents",
         "name/ascii_only_entries",
         "no_debugging_tables",
+        "overlapping_path_segments",
         "render_own_name",
         "tabular_kerning",
         "typoascender_exceeds_Agrave",

Lib/fontbakery/profiles/universal.py

@@ -67,6 +67,7 @@
             "no_mac_entries",
             "os2_metrics_match_hhea",
             "ots",
+            "overlapping_path_segments",
             "required_tables",
             "render_own_name",
             "rupee",

tests/test_checks_outline.py

@@ -1,5 +1,5 @@
 from fontTools.ttLib import TTFont
-from fontbakery.status import WARN, SKIP
+from fontbakery.status import WARN, SKIP, FAIL
 from fontbakery.codetesting import (
     assert_results_contain,
     CheckTester,
@@ -154,3 +154,17 @@ def test_check_outline_direction():
 
     font = TEST_FILE("wonky_paths/OutlineTest.ttf")
     assert_PASS(check(font))
+
+
+def test_check_overlapping_path_segments():
+    check = CheckTester("overlapping_path_segments")
+
+    # Check a font that contains overlapping path segments
+    filename = TEST_FILE("overlapping_path_segments/Figtree[wght].ttf")
+    results = check(filename)
+    assert_results_contain(results, FAIL, "overlapping-path-segments")
+
+    # check a font that doesn't contain overlapping path segments
+    filename = TEST_FILE("merriweather/Merriweather-Regular.ttf")
+    results = check(filename)
+    assert_PASS(results)