varLib: add set_default_weight_width_slant

When building a variable font, varLib.build must make sure that
the OS/2.usWeightClass is equal to the wght axis default location,
that the OS/2.usWidthClass is set to the equivalent value (1-9) of
the wdth axis default location, and finally that post.italicAngle
is set to the same default value as slnt axis.

Sometimes the base master doesn't have these values correctly
set leading to discrepancies between OS/2 and post, on the one
hand, and the fvar axes' default values.

Lib/fontTools/varLib/__init__.py

@@ -857,6 +857,45 @@ def load_designspace(designspace):
 	)
 
 
+# https://docs.microsoft.com/en-us/typography/opentype/spec/os2#uswidthclass
+WDTH_VALUE_TO_OS2_WIDTH_CLASS = {
+	50: 1,
+	62.5: 2,
+	75: 3,
+	87.5: 4,
+	100: 5,
+	112.5: 6,
+	125: 7,
+	150: 8,
+	200: 9,
+}
+
+
+def set_default_weight_width_slant(font, location):
+	if "OS/2" in font:
+		if "wght" in location:
+			weight_class = otRound(max(1, min(location["wght"], 1000)))
+			if font["OS/2"].usWeightClass != weight_class:
+				log.info("Setting OS/2.usWidthClass = %s", weight_class)
+				font["OS/2"].usWeightClass = weight_class
+
+		if "wdth" in location:
+			# map 'wdth' axis (50..200) to OS/2.usWidthClass (1..9), rounding to closest
+			widthValue = min(max(location["wdth"], 50), 200)
+			widthClass = otRound(
+				models.piecewiseLinearMap(widthValue, WDTH_VALUE_TO_OS2_WIDTH_CLASS)
+			)
+			if font["OS/2"].usWidthClass != widthClass:
+				log.info("Setting OS/2.usWidthClass = %s", widthClass)
+				font["OS/2"].usWidthClass = widthClass
+
+	if "slnt" in location and "post" in font:
+		italicAngle = max(-90, min(location["slnt"], 90))
+		if font["post"].italicAngle != italicAngle:
+			log.info("Setting post.italicAngle = %s", italicAngle)
+			font["post"].italicAngle = italicAngle
+
+
 def build(designspace, master_finder=lambda s:s, exclude=[], optimize=True):
 	"""
 	Build variation font from a designspace file.
@@ -930,6 +969,10 @@ def build(designspace, master_finder=lambda s:s, exclude=[], optimize=True):
 				post.extraNames = []
 				post.mapping = {}
 
+	set_default_weight_width_slant(
+		vf, location={axis.axisTag: axis.defaultValue for axis in vf["fvar"].axes}
+	)
+
 	for tag in exclude:
 		if tag in vf:
 			del vf[tag]

Tests/varLib/data/test_results/BuildMain.ttx

@@ -55,7 +55,7 @@
          will be recalculated by the compiler -->
     <version value="4"/>
     <xAvgCharWidth value="506"/>
-    <usWeightClass value="400"/>
+    <usWeightClass value="368"/>
     <usWidthClass value="5"/>
     <fsType value="00000000 00000100"/>
     <ySubscriptXSize value="650"/>

Tests/varLib/data/test_results/SparseMasters.ttx

@@ -55,7 +55,7 @@
          will be recalculated by the compiler -->
     <version value="4"/>
     <xAvgCharWidth value="580"/>
-    <usWeightClass value="400"/>
+    <usWeightClass value="350"/>
     <usWidthClass value="5"/>
     <fsType value="00000000 00000100"/>
     <ySubscriptXSize value="650"/>

Tests/varLib/varLib_test.py

@@ -4,6 +4,7 @@ from fontTools.ttLib import TTFont, newTable
 from fontTools.varLib import build
 from fontTools.varLib.mutator import instantiateVariableFont
 from fontTools.varLib import main as varLib_main, load_masters
+from fontTools.varLib import set_default_weight_width_slant
 from fontTools.designspaceLib import (
     DesignSpaceDocumentError, DesignSpaceDocument, SourceDescriptor,
 )
@@ -661,5 +662,81 @@ def _extract_flat_kerning(font, pairpos_table):
     return extracted_kerning
 
 
+@pytest.fixture
+def ttFont():
+    f = TTFont()
+    f["OS/2"] = newTable("OS/2")
+    f["OS/2"].usWeightClass = 400
+    f["OS/2"].usWidthClass = 100
+    f["post"] = newTable("post")
+    f["post"].italicAngle = 0
+    return f
+
+
+class SetDefaultWeightWidthSlantTest(object):
+    @pytest.mark.parametrize(
+        "location, expected",
+        [
+            ({"wght": 0}, 1),
+            ({"wght": 1}, 1),
+            ({"wght": 100}, 100),
+            ({"wght": 1000}, 1000),
+            ({"wght": 1001}, 1000),
+        ],
+    )
+    def test_wght(self, ttFont, location, expected):
+        set_default_weight_width_slant(ttFont, location)
+
+        assert ttFont["OS/2"].usWeightClass == expected
+
+    @pytest.mark.parametrize(
+        "location, expected",
+        [
+            ({"wdth": 0}, 1),
+            ({"wdth": 56}, 1),
+            ({"wdth": 57}, 2),
+            ({"wdth": 62.5}, 2),
+            ({"wdth": 75}, 3),
+            ({"wdth": 87.5}, 4),
+            ({"wdth": 100}, 5),
+            ({"wdth": 112.5}, 6),
+            ({"wdth": 125}, 7),
+            ({"wdth": 150}, 8),
+            ({"wdth": 200}, 9),
+            ({"wdth": 201}, 9),
+            ({"wdth": 1000}, 9),
+        ],
+    )
+    def test_wdth(self, ttFont, location, expected):
+        set_default_weight_width_slant(ttFont, location)
+
+        assert ttFont["OS/2"].usWidthClass == expected
+
+    @pytest.mark.parametrize(
+        "location, expected",
+        [
+            ({"slnt": -91}, -90),
+            ({"slnt": -90}, -90),
+            ({"slnt": 0}, 0),
+            ({"slnt": 11.5}, 11.5),
+            ({"slnt": 90}, 90),
+            ({"slnt": 91}, 90),
+        ],
+    )
+    def test_slnt(self, ttFont, location, expected):
+        set_default_weight_width_slant(ttFont, location)
+
+        assert ttFont["post"].italicAngle == expected
+
+    def test_all(self, ttFont):
+        set_default_weight_width_slant(
+            ttFont, {"wght": 500, "wdth": 150, "slnt": -12.0}
+        )
+
+        assert ttFont["OS/2"].usWeightClass == 500
+        assert ttFont["OS/2"].usWidthClass == 8
+        assert ttFont["post"].italicAngle == -12.0
+
+
 if __name__ == "__main__":
     sys.exit(unittest.main())