new check for STAT table in static fonts

Added to the Universal Profile
com.google.fonts/check/STAT_in_statics
Static fonts with more than a single entry per design axis cause trouble on Windows
(issue #4149)

CHANGELOG.md

@@ -1,8 +1,11 @@
 Below are the most important changes from each release.
 A more detailed list of changes is available in the corresponding milestones for each release in the Github issue tracker (https://github.com/googlefonts/fontbakery/milestones?state=closed).
 
-## Upcoming release: 0.8.12 (2023-Apr-??)
+## Upcoming release: 0.8.12 (2023-May-??)
 ### New Checks
+#### Added to the Universal Profile
+  - **[com.google.fonts/check/STAT_in_statics]:** Static fonts with more than a single entry per design axis cause trouble on Windows (issue #4149)
+
 #### Added to the Google Fonts Profile
   - **[com.google.fonts/check/metadata/unreachable_subsetting]:** Implemented checks to ensure that all encoded glyphs in the font are covered by a subset declared in the METADATA.pb (issue #4097)
 

Lib/fontbakery/profiles/universal.py

@@ -76,6 +76,7 @@
         'com.google.fonts/check/interpolation_issues',
         'com.google.fonts/check/math_signs_width',
         'com.google.fonts/check/linegaps',
+        'com.google.fonts/check/STAT_in_statics',
     ]
 
 
@@ -2207,5 +2208,69 @@ def com_google_fonts_check_linegaps(ttFont):
         yield PASS, "OS/2 sTypoLineGap and hhea lineGap are both 0."
 
 
+@check(
+    id = 'com.google.fonts/check/STAT_in_statics',
+    conditions = ['not is_variable_font'],
+    rationale = """
+        Adobe feature syntax allows for the definition of a STAT table. Fonts built
+        with a hand-coded STAT table in feature syntax may be built either as static
+        or variable, but will end up with the same STAT table.
+        
+        This is a problem, because a STAT table which works on variable fonts
+        will not be appropriate for static instances. The examples in the OpenType spec
+        of non-variable fonts with a STAT table show that the table entries must be
+        restricted to those entries which refer to the static font's position in
+        the designspace. i.e. a Regular weight static should only have the following
+        entry for the weight axis:
+
+        <AxisIndex value="0"/>
+        <Flags value="2"/>  <!-- ElidableAxisValueName -->
+        <ValueNameID value="265"/>  <!-- Regular -->
+        <Value value="400.0"/>
+
+        However, if the STAT table intended for a variable font is compiled into a
+        static, it will have many entries for this axis. In this case, Windows will
+        read the first entry only, causing all instances to report themselves
+        as "Thin Condensed".
+    """,
+    proposal = 'https://github.com/googlefonts/fontbakery/issues/4149'
+)
+def com_google_fonts_check_STAT_in_statics(ttFont):
+    """Checking STAT table entries in static fonts."""
+
+    entries = {}
+    def count_entries(tag_name):
+        if tag_name in entries:
+            entries[tag_name] += 1
+        else:
+            entries[tag_name] = 1
+
+    passed = True
+    if "STAT" in ttFont:
+        stat = ttFont["STAT"].table
+        designAxes = stat.DesignAxisRecord.Axis
+        for axisValueTable in stat.AxisValueArray.AxisValue:
+            axisValueFormat = axisValueTable.Format
+            if axisValueFormat in (1, 2, 3):
+                axisTag = designAxes[axisValueTable.AxisIndex].AxisTag
+                count_entries(axisTag)
+            elif axisValueFormat == 4:
+                for rec in axisValueTable.AxisValueRecord:
+                    axisTag = designAxes[rec.AxisIndex].AxisTag
+                    count_entries(axisTag)
+
+        for tag_name in entries:
+            if entries[tag_name] > 1:
+                passed = False
+                yield FAIL,\
+                      Message("multiple-STAT-entries",
+                              f"The STAT table has more than a single entry for the"
+                              f" '{tag_name}' axis ({entries[tag_name]}) on this"
+                              f" static font which will causes problems on Windows.")
+
+    if passed:
+        yield PASS, "Looks good!"
+
+
 profile.auto_register(globals())
 profile.test_expected_checks(UNIVERSAL_PROFILE_CHECKS, exclusive=True)

tests/profiles/universal_test.py

@@ -1275,3 +1275,33 @@ def test_check_linegaps():
     # Confirm the check yields FAIL if the font doesn't have a required table
     del ttFont['OS/2']
     assert_results_contain(check(ttFont), FAIL, "lacks-table")
+
+
+def test_check_STAT_in_statics():
+    """ Checking STAT table on static fonts. """
+    check = CheckTester(universal_profile,
+                        "com.google.fonts/check/STAT_in_statics")
+
+    ttFont = TTFont(TEST_FILE("varfont/RobotoSerif[GRAD,opsz,wdth,wght].ttf"))
+    assert_SKIP(check(ttFont),
+                'with a variable font...')
+
+    # fake it: Remove fvar table to make Font Bakery think it is dealing with a static font
+    del ttFont["fvar"]
+
+    # We know that our reference RobotoSerif varfont (which the check is induced
+    # here to think it is a static font) has multiple records per design axis in its
+    # STAT table:
+    assert_results_contain(check(ttFont),
+                           FAIL, 'multiple-STAT-entries',
+                           'with a static font with a bad STAT table...')
+
+    # Remove all entries except the very first one:
+    stat = ttFont["STAT"].table
+    stat.AxisValueArray.AxisCount = 1
+    stat.AxisValueArray.AxisValue = [stat.AxisValueArray.AxisValue[0]]
+
+    # And finally, completely remove STAT and it should PASS:
+    del ttFont["STAT"]
+    assert_PASS(check(ttFont))
+