Support more affine expression forms in Image.point

In modes I and F, Image.point only supported affine expressions of the
forms (lambda x:) x * a, x + a, and x * a + b. Expressions like 1 - x
had to be written x * -1 + 1.

This rewrite, though still limited to affine transformations, supports
far more expression forms, including 1 - x, (2 * x + 1) / 3, etc.

Tests/test_image_point.py

@@ -18,10 +18,18 @@ def test_sanity():
     im.point(lambda x: x * 1)
     im.point(lambda x: x + 1)
     im.point(lambda x: x * 1 + 1)
+    im.point(lambda x: 0.1 + 0.2 * x)
+    im.point(lambda x: -x)
+    im.point(lambda x: x - 0.5)
+    im.point(lambda x: 1 - x / 2)
+    im.point(lambda x: (2 + x) / 3)
+    im.point(lambda x: 0.5)
     with pytest.raises(TypeError):
-        im.point(lambda x: x - 1)
+        im.point(lambda x: x * x)
     with pytest.raises(TypeError):
-        im.point(lambda x: x / 1)
+        im.point(lambda x: 1 / x)
+    with pytest.raises(TypeError):
+        im.point(lambda x: x // 2)
 
 
 def test_16bit_lut():

src/PIL/Image.py

@@ -431,45 +431,44 @@ def _getencoder(mode, encoder_name, args, extra=()):
 # Simple expression analyzer
 
 
-def coerce_e(value):
-    return value if isinstance(value, _E) else _E(value)
+# _Affine(m, b) represents the polynomial m x + b
+class _Affine:
+    def __init__(self, m, b):
+        self.m = m
+        self.b = b
 
-
-class _E:
-    def __init__(self, data):
-        self.data = data
+    def __neg__(self):
+        return _Affine(-self.m, -self.b)
 
     def __add__(self, other):
-        return _E((self.data, "__add__", coerce_e(other).data))
+        if isinstance(other, _Affine):
+            return _Affine(self.m + other.m, self.b + other.b)
+        return _Affine(self.m, self.b + other)
+
+    __radd__ = __add__
+
+    def __sub__(self, other):
+        return self + -other
+
+    def __rsub__(self, other):
+        return other + -self
 
     def __mul__(self, other):
-        return _E((self.data, "__mul__", coerce_e(other).data))
+        if isinstance(other, _Affine):
+            return NotImplemented
+        return _Affine(self.m * other, self.b * other)
+
+    __rmul__ = __mul__
+
+    def __truediv__(self, other):
+        if isinstance(other, _Affine):
+            return NotImplemented
+        return _Affine(self.m / other, self.b / other)
 
 
 def _getscaleoffset(expr):
-    stub = ["stub"]
-    data = expr(_E(stub)).data
-    try:
-        (a, b, c) = data  # simplified syntax
-        if a is stub and b == "__mul__" and isinstance(c, numbers.Number):
-            return c, 0.0
-        if a is stub and b == "__add__" and isinstance(c, numbers.Number):
-            return 1.0, c
-    except TypeError:
-        pass
-    try:
-        ((a, b, c), d, e) = data  # full syntax
-        if (
-            a is stub
-            and b == "__mul__"
-            and isinstance(c, numbers.Number)
-            and d == "__add__"
-            and isinstance(e, numbers.Number)
-        ):
-            return c, e
-    except TypeError:
-        pass
-    raise ValueError("illegal expression")
+    a = expr(_Affine(1.0, 0.0))
+    return (a.m, a.b) if isinstance(a, _Affine) else (0.0, a)
 
 
 # --------------------------------------------------------------------