swap xy and yx fields in COLRv1 Affine2x3 struct

following the latest draft
See discussion at https://github.com/googlefonts/colr-gradients-spec/pull/85

Lib/fontTools/colorLib/builder.py

@@ -449,19 +449,21 @@ def buildPaintLinearGradient(
     return self
 
 
-def buildAffine2x3(
-    xx: _ScalarInput,
-    xy: _ScalarInput,
-    yx: _ScalarInput,
-    yy: _ScalarInput,
-    dx: _ScalarInput,
-    dy: _ScalarInput,
-) -> ot.Affine2x3:
+def buildAffine2x3(transform: _AffineTuple) -> ot.Affine2x3:
+    if len(transform) != 6:
+        raise ValueError(f"Expected 6-tuple of floats, found: {transform!}")
     self = ot.Affine2x3()
-    locs = locals()
-    for attr in ("xx", "xy", "yx", "yy", "dx", "dy"):
-        value = locs[attr]
-        setattr(self, attr, _to_variable_f16dot16_float(value))
+    # COLRv1 Affine2x3 uses the same column-major order to serialize a 2D
+    # Affine Transformation as the one used by fontTools.misc.transform.
+    # However, for historical reasons, the labels 'xy' and 'yx' are swapped.
+    # Their fundamental meaning is the same though.
+    # COLRv1 Affine2x3 follows the names found in FreeType and Cairo.
+    # In all case, the second element in the 6-tuple correspond to the
+    # y-part of the x basis vector, and the third to the x-part of the y
+    # basis vector.
+    # See https://github.com/googlefonts/colr-gradients-spec/pull/85
+    for i, attr in enumerate(("xx", "yx", "xy", "yy", "dx", "dy")):
+        setattr(self, attr, _to_variable_f16dot16_float(transform[i]))
     return self
 
 
@@ -517,7 +519,7 @@ def buildPaintTransform(transform: _AffineInput, paint: _PaintInput) -> ot.Paint
     self = ot.Paint()
     self.Format = int(ot.Paint.Format.PaintTransform)
     if not isinstance(transform, ot.Affine2x3):
-        transform = buildAffine2x3(*transform)
+        transform = buildAffine2x3(transform)
     self.Transform = transform
     self.Paint = buildPaint(paint)
     return self

Lib/fontTools/ttLib/tables/otData.py

@@ -1588,12 +1588,12 @@ otData = [
 	]),
 
 	('Affine2x3', [
-		('VarFixed', 'xx', None, None, ''),
-		('VarFixed', 'xy', None, None, ''),
-		('VarFixed', 'yx', None, None, ''),
-		('VarFixed', 'yy', None, None, ''),
-		('VarFixed', 'dx', None, None, ''),
-		('VarFixed', 'dy', None, None, ''),
+		('VarFixed', 'xx', None, None, 'x-part of x basis vector'),
+		('VarFixed', 'yx', None, None, 'y-part of x basis vector'),
+		('VarFixed', 'xy', None, None, 'x-part of y basis vector'),
+		('VarFixed', 'yy', None, None, 'y-part of y basis vector'),
+		('VarFixed', 'dx', None, None, 'Translation in x direction'),
+		('VarFixed', 'dy', None, None, 'Translation in y direction'),
 	]),
 
 	('ColorIndex', [

Tests/colorLib/builder_test.py

@@ -285,10 +285,10 @@ def test_buildColorLine():
 
 
 def test_buildAffine2x3():
-    matrix = builder.buildAffine2x3(1.5, 0, 0.5, 2.0, 1.0, -3.0)
+    matrix = builder.buildAffine2x3((1.5, 0, 0.5, 2.0, 1.0, -3.0))
     assert matrix.xx == builder.VariableFloat(1.5)
-    assert matrix.xy == builder.VariableFloat(0.0)
-    assert matrix.yx == builder.VariableFloat(0.5)
+    assert matrix.yx == builder.VariableFloat(0.0)
+    assert matrix.xy == builder.VariableFloat(0.5)
     assert matrix.yy == builder.VariableFloat(2.0)
     assert matrix.dx == builder.VariableFloat(1.0)
     assert matrix.dy == builder.VariableFloat(-3.0)
@@ -458,7 +458,7 @@ def test_buildPaintColrSlice():
 
 def test_buildPaintTransform():
     paint = builder.buildPaintTransform(
-        transform=builder.buildAffine2x3(1, 2, 3, 4, 5, 6),
+        transform=builder.buildAffine2x3((1, 2, 3, 4, 5, 6)),
         paint=builder.buildPaintGlyph(
             glyph="a",
             paint=builder.buildPaintSolid(paletteIndex=0, alpha=1.0),
@@ -467,8 +467,8 @@ def test_buildPaintTransform():
 
     assert paint.Format == ot.Paint.Format.PaintTransform
     assert paint.Transform.xx.value == 1.0
-    assert paint.Transform.xy.value == 2.0
-    assert paint.Transform.yx.value == 3.0
+    assert paint.Transform.yx.value == 2.0
+    assert paint.Transform.xy.value == 3.0
     assert paint.Transform.yy.value == 4.0
     assert paint.Transform.dx.value == 5.0
     assert paint.Transform.dy.value == 6.0
@@ -488,8 +488,8 @@ def test_buildPaintTransform():
 
     assert paint.Format == ot.Paint.Format.PaintTransform
     assert paint.Transform.xx.value == 1.0
-    assert paint.Transform.xy.value == 0.0
     assert paint.Transform.yx.value == 0.0
+    assert paint.Transform.xy.value == 0.0
     assert paint.Transform.yy.value == 0.3333
     assert paint.Transform.dx.value == 10
     assert paint.Transform.dy.value == 10

Tests/ttLib/tables/C_O_L_R_test.py

@@ -308,8 +308,8 @@ COLR_V1_XML = [
     "          </Paint>",
     "          <Transform>",
     '            <xx value="-13.0"/>',
-    '            <xy value="14.0"/>',
-    '            <yx value="15.0"/>',
+    '            <yx value="14.0"/>',
+    '            <xy value="15.0"/>',
     '            <yy value="-17.0"/>',
     '            <dx value="18.0"/>',
     '            <dy value="19.0"/>',
@@ -338,8 +338,8 @@ COLR_V1_XML = [
     "          </Paint>",
     "          <Transform>",
     '            <xx value="1.0"/>',
-    '            <xy value="0.0"/>',
     '            <yx value="0.0"/>',
+    '            <xy value="0.0"/>',
     '            <yy value="1.0"/>',
     '            <dx value="300.0"/>',
     '            <dy value="0.0"/>',