support proposed PaintSweepGradient's angles with +1.0 (180°) bias

In the upcoming draft of COLR spec, PaintSweepGradient's startAngle/endAngle are encoded with a +1.0 bias to allow for representation of a full +360° positive angle. Normal F2Dot14-fraction-of-half-circle angles can only represent angles between -360 <= angle < +360

This is a breaking change and will need to be coordinated with rendering implementations (at least FreeType/Skia).

Lib/fontTools/ttLib/tables/otConverters.py

@@ -452,13 +452,14 @@ class F2Dot14(BaseFixedValue):
 class Angle(F2Dot14):
 	# angles are specified in degrees, and encoded as F2Dot14 fractions of half
 	# circle: e.g. 1.0 => 180, -0.5 => -90, -2.0 => -360, etc.
+	bias = 0.0
 	factor = 1.0/(1<<14) * 180  # 0.010986328125
 	@classmethod
 	def fromInt(cls, value):
-		return super().fromInt(value) * 180
+		return (super().fromInt(value) + cls.bias) * 180
 	@classmethod
 	def toInt(cls, value):
-		return super().toInt(value / 180)
+		return super().toInt((value / 180) - cls.bias)
 	@classmethod
 	def fromString(cls, value):
 		# quantize to nearest multiples of minimum fixed-precision angle
@@ -467,6 +468,11 @@ class Angle(F2Dot14):
 	def toString(cls, value):
 		return nearestMultipleShortestRepr(value, cls.factor)
 
+class BiasedAngle(Angle):
+	# A bias of 1.0 is used in the representation of start and end angles
+	# of COLRv1 PaintSweepGradients to allow for encoding +360deg
+	bias = 1.0
+
 class Version(SimpleValue):
 	staticSize = 4
 	def read(self, reader, font, tableDict):
@@ -1773,6 +1779,7 @@ converterMapping = {
 	"Fixed":	Fixed,
 	"F2Dot14":	F2Dot14,
 	"Angle":	Angle,
+	"BiasedAngle":	BiasedAngle,
 	"struct":	Struct,
 	"Offset":	Table,
 	"LOffset":	LTable,

Lib/fontTools/ttLib/tables/otData.py

@@ -1755,8 +1755,8 @@ otData = [
 		('Offset24', 'ColorLine', None, None, 'Offset (from beginning of PaintSweepGradient table) to ColorLine subtable.'),
 		('int16', 'centerX', None, None, 'Center x coordinate.'),
 		('int16', 'centerY', None, None, 'Center y coordinate.'),
-		('Angle', 'startAngle', None, None, 'Start of the angular range of the gradient.'),
-		('Angle', 'endAngle', None, None, 'End of the angular range of the gradient.'),
+		('BiasedAngle', 'startAngle', None, None, 'Start of the angular range of the gradient.'),
+		('BiasedAngle', 'endAngle', None, None, 'End of the angular range of the gradient.'),
 	]),
 	# PaintVarSweepGradient
 	('PaintFormat9', [
@@ -1764,8 +1764,8 @@ otData = [
 		('LOffset24To(VarColorLine)', 'ColorLine', None, None, 'Offset (from beginning of PaintVarSweepGradient table) to VarColorLine subtable.'),
 		('int16', 'centerX', None, None, 'Center x coordinate. VarIndexBase + 0.'),
 		('int16', 'centerY', None, None, 'Center y coordinate. VarIndexBase + 1.'),
-		('Angle', 'startAngle', None, None, 'Start of the angular range of the gradient. VarIndexBase + 2.'),
-		('Angle', 'endAngle', None, None, 'End of the angular range of the gradient. VarIndexBase + 3.'),
+		('BiasedAngle', 'startAngle', None, None, 'Start of the angular range of the gradient. VarIndexBase + 2.'),
+		('BiasedAngle', 'endAngle', None, None, 'End of the angular range of the gradient. VarIndexBase + 3.'),
 		('VarIndex', 'VarIndexBase', None, None, 'Base index into DeltaSetIndexMap.'),
 	]),
 

Tests/ttLib/tables/C_O_L_R_test.py

@@ -400,8 +400,8 @@ COLR_V1_XML = [
     "        </ColorLine>",
     '        <centerX value="259"/>',
     '        <centerY value="300"/>',
-    '        <startAngle value="45.0"/>',
-    '        <endAngle value="135.0"/>',
+    '        <startAngle value="225.0"/>',
+    '        <endAngle value="315.0"/>',
     "      </Paint>",
     '      <Glyph value="glyph00011"/>',
     "    </Paint>",

Tests/varLib/merger_test.py

@@ -619,8 +619,8 @@ class COLRVariationMergerTest:
                             },
                             "centerX": 0,
                             "centerY": 0,
-                            "startAngle": -360,
-                            "endAngle": 0,
+                            "startAngle": 0,
+                            "endAngle": 360,
                         },
                         "Transform": (1.0, 0, 0, 1.0, 0, 0),
                     },
@@ -641,8 +641,8 @@ class COLRVariationMergerTest:
                             },
                             "centerX": 256,
                             "centerY": 0,
-                            "startAngle": -360,
-                            "endAngle": 0,
+                            "startAngle": 0,
+                            "endAngle": 360,
                         },
                         # Transform.xx below produces the same VarStore delta as the
                         # above PaintSweepGradient's centerX because, when Fixed16.16
@@ -675,8 +675,8 @@ class COLRVariationMergerTest:
                     "    </ColorLine>",
                     '    <centerX value="0"/>',
                     '    <centerY value="0"/>',
-                    '    <startAngle value="-360.0"/>',
-                    '    <endAngle value="0.0"/>',
+                    '    <startAngle value="0.0"/>',
+                    '    <endAngle value="360.0"/>',
                     '    <VarIndexBase value="0"/>',
                     "  </Paint>",
                     "  <Transform>",