Limit radius to half width or height of rounded rectangle

Tests/test_imagedraw.py

@@ -945,6 +945,21 @@ def test_rounded_rectangle_zero_radius(bbox: Coords) -> None:
     assert_image_equal_tofile(im, "Tests/images/imagedraw_rectangle_width_fill.png")
 
 
+@pytest.mark.parametrize("w, h", ((200, 100), (100, 200)))
+def test_rounded_rectangle_large_radius(w: int, h: int) -> None:
+    im = Image.new("RGB", (w, h))
+    draw = ImageDraw.Draw(im)
+    draw.rounded_rectangle(
+        (0, 0, w, h), 100, "red", corners=(True, False, False, False)
+    )
+
+    expected = Image.new("RGB", (w, h))
+    draw = ImageDraw.Draw(expected)
+    draw.rounded_rectangle((0, 0, w, h), 50, "red", corners=(True, False, False, False))
+
+    assert_image_equal(im, expected)
+
+
 @pytest.mark.parametrize(
     "xy, suffix",
     [

docs/reference/ImageDraw.rst

@@ -382,7 +382,8 @@ Methods
     :param xy: Two points to define the bounding box. Sequence of either
             ``[(x0, y0), (x1, y1)]`` or ``[x0, y0, x1, y1]``, where ``x1 >= x0`` and
             ``y1 >= y0``. The bounding box is inclusive of both endpoints.
-    :param radius: Radius of the corners.
+    :param radius: Radius of the corners, limited to half of the smallest dimension of
+            the bounding box.
     :param fill: Color to use for the fill.
     :param outline: Color to use for the outline.
     :param width: The line width, in pixels.

src/PIL/ImageDraw.py

@@ -419,7 +419,7 @@ class ImageDraw:
         if corners is None:
             corners = (True, True, True, True)
 
-        d = radius * 2
+        d = min(x1 - x0, y1 - y0, radius * 2)
 
         x0 = round(x0)
         y0 = round(y0)