Merge pull request #5313 from radarhere/alpha_composite

Allow alpha_composite destination to be negative

Tests/test_image.py

@@ -344,6 +344,12 @@ class TestImage:
         assert_image_equal(offset.crop((64, 64, 127, 127)), target.crop((0, 0, 63, 63)))
         assert offset.size == (128, 128)
 
+        # with negative offset
+        offset = src.copy()
+        offset.alpha_composite(over, (-64, -64))
+        assert_image_equal(offset.crop((0, 0, 63, 63)), target.crop((64, 64, 127, 127)))
+        assert offset.size == (128, 128)
+
         # offset and crop
         box = src.copy()
         box.alpha_composite(over, (64, 64), (0, 0, 32, 32))
@@ -367,8 +373,6 @@ class TestImage:
             source.alpha_composite(over, 0)
         with pytest.raises(ValueError):
             source.alpha_composite(over, (0, 0), 0)
-        with pytest.raises(ValueError):
-            source.alpha_composite(over, (0, -1))
         with pytest.raises(ValueError):
             source.alpha_composite(over, (0, 0), (0, -1))
 

docs/releasenotes/8.2.0.rst

@@ -13,6 +13,14 @@ when Tk/Tcl 8.5 will be the minimum supported.
 API Changes
 ===========
 
+Image.alpha_composite: dest
+^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+When calling :py:meth:`~PIL.Image.Image.alpha_composite`, the ``dest`` argument now
+accepts negative co-ordinates, like the upper left corner of the ``box`` argument of
+:py:meth:`~PIL.Image.Image.paste` can be negative. Naturally, this has effect of
+cropping the overlaid image.
+
 ImageDraw.rounded_rectangle
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^
 

src/PIL/Image.py

@@ -1544,8 +1544,6 @@ class Image:
             raise ValueError("Destination must be a 2-tuple")
         if min(source) < 0:
             raise ValueError("Source must be non-negative")
-        if min(dest) < 0:
-            raise ValueError("Destination must be non-negative")
 
         if len(source) == 2:
             source = source + im.size