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[DecomposedTransform] Document and implement always skewY == 0

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Spotted by Cosimo. I convinced myself he is right, since a and b
are zero in that branch.
This commit is contained in:
Behdad Esfahbod 2024-11-21 17:00:49 -07:00
parent b8635fee62
commit 3fe083e8d1
1 changed files with 15 additions and 4 deletions
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19
Lib/fontTools/misc/transform.py
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@ -437,8 +437,20 @@ class DecomposedTransform:
@classmethod
def fromTransform(self, transform):
"""Return a DecomposedTransform() equivalent of this transformation.
The returned solution always has skewY = 0, and angle in the (-180, 180].
:Example:
>>> DecomposedTransform.fromTransform(Transform(3, 0, 0, 2, 0, 0))
DecomposedTransform(translateX=0, translateY=0, rotation=0.0, scaleX=3.0, scaleY=2.0, skewX=0.0, skewY=0.0, tCenterX=0, tCenterY=0)
>>> DecomposedTransform.fromTransform(Transform(0, 0, 0, 1, 0, 0))
DecomposedTransform(translateX=0, translateY=0, rotation=0.0, scaleX=0.0, scaleY=1.0, skewX=0.0, skewY=0.0, tCenterX=0, tCenterY=0)
>>> DecomposedTransform.fromTransform(Transform(0, 0, 1, 1, 0, 0))
DecomposedTransform(translateX=0, translateY=0, rotation=-45.0, scaleX=0.0, scaleY=1.4142135623730951, skewX=0.0, skewY=0.0, tCenterX=0, tCenterY=0)
"""
# Adapted from an answer on
# https://math.stackexchange.com/questions/13150/extracting-rotation-scale-values-from-2d-transformation-matrix
a, b, c, d, x, y = transform
sx = math.copysign(1, a)
@ -450,21 +462,20 @@ class DecomposedTransform:
rotation = 0
scaleX = scaleY = 0
skewX = skewY = 0
skewX = 0
# Apply the QR-like decomposition.
if a != 0 or b != 0:
r = math.sqrt(a * a + b * b)
rotation = math.acos(a / r) if b >= 0 else -math.acos(a / r)
scaleX, scaleY = (r, delta / r)
skewX, skewY = (math.atan((a * c + b * d) / (r * r)), 0)
skewX = math.atan((a * c + b * d) / (r * r))
elif c != 0 or d != 0:
s = math.sqrt(c * c + d * d)
rotation = math.pi / 2 - (
math.acos(-c / s) if d >= 0 else -math.acos(c / s)
)
scaleX, scaleY = (delta / s, s)
skewX, skewY = (0, math.atan((a * c + b * d) / (s * s)))
else:
# a = b = c = d = 0
pass
@ -476,7 +487,7 @@ class DecomposedTransform:
scaleX * sx,
scaleY,
math.degrees(skewX) * sx,
math.degrees(skewY),
0.0,
0,
0,
)