Only draw each pixel once

Tests/test_imagedraw.py

@@ -734,6 +734,30 @@ def test_rounded_rectangle_zero_radius():
     assert_image_equal_tofile(im, "Tests/images/imagedraw_rectangle_width_fill.png")
 
 
+@pytest.mark.parametrize(
+    "xy, suffix",
+    [
+        ((20, 10, 80, 90), "x"),
+        ((10, 20, 90, 80), "y"),
+        ((20, 20, 80, 80), "both"),
+    ],
+)
+def test_rounded_rectangle_translucent(xy, suffix):
+    # Arrange
+    im = Image.new("RGB", (W, H))
+    draw = ImageDraw.Draw(im, "RGBA")
+
+    # Act
+    draw.rounded_rectangle(
+        xy, 30, fill=(255, 0, 0, 127), outline=(0, 255, 0, 127), width=5
+    )
+
+    # Assert
+    assert_image_equal_tofile(
+        im, "Tests/images/imagedraw_rounded_rectangle_" + suffix + ".png"
+    )
+
+
 def test_floodfill():
     red = ImageColor.getrgb("red")
 

src/PIL/ImageDraw.py

@@ -264,19 +264,65 @@ class ImageDraw:
         else:
             x0, y0, x1, y1 = xy
 
-        # Do not allow the diameter to be greater than the width or height
+        d = radius * 2
-        d = min(radius * 2, x1 - x0, y1 - y0)
+
+        full_x = d >= x1 - x0
+        if full_x:
+            # The two left and two right corners are joined
+            d = x1 - x0
+        full_y = d >= y1 - y0
+        if full_y:
+            # The two top and two bottom corners are joined
+            d = y1 - y0
+        if full_x and full_y:
+            # If all corners are joined, that is a circle
+            return self.ellipse(xy, fill, outline, width)
+
         if d == 0:
+            # If the corners have no curve, that is a rectangle
             return self.rectangle(xy, fill, outline, width)
 
         ink, fill = self._getink(outline, fill)
-        if fill is not None:
-            self.draw.draw_pieslice((x1 - d, y0, x1, y0 + d), 270, 360, fill, 1)
-            self.draw.draw_pieslice((x1 - d, y1 - d, x1, y1), 0, 90, fill, 1)
-            self.draw.draw_pieslice((x0, y1 - d, x0 + d, y1), 90, 180, fill, 1)
-            self.draw.draw_pieslice((x0, y0, x0 + d, y0 + d), 180, 270, fill, 1)
 
-            self.draw.draw_rectangle((x0 + d / 2 + 1, y0, x1 - d / 2 - 1, y1), fill, 1)
+        def draw_corners(pieslice):
+            if full_x:
+                # Draw top and bottom halves
+                parts = (
+                    ((x0, y0, x0 + d, y0 + d), 180, 360),
+                    ((x0, y1 - d, x0 + d, y1), 0, 180),
+                )
+            elif full_y:
+                # Draw left and right halves
+                parts = (
+                    ((x0, y0, x0 + d, y0 + d), 90, 270),
+                    ((x1 - d, y0, x1, y0 + d), 270, 90),
+                )
+            else:
+                # Draw four separate corners
+                parts = (
+                    ((x1 - d, y0, x1, y0 + d), 270, 360),
+                    ((x1 - d, y1 - d, x1, y1), 0, 90),
+                    ((x0, y1 - d, x0 + d, y1), 90, 180),
+                    ((x0, y0, x0 + d, y0 + d), 180, 270),
+                )
+            for part in parts:
+                if pieslice:
+                    self.draw.draw_pieslice(*(part + (fill, 1)))
+                else:
+                    self.draw.draw_arc(*(part + (ink, width)))
+
+        if fill is not None:
+            draw_corners(True)
+
+            if full_x:
+                self.draw.draw_rectangle(
+                    (x0, y0 + d / 2 + 1, x1, y1 - d / 2 - 1), fill, 1
+                )
+            else:
+                self.draw.draw_rectangle(
+                    (x0 + d / 2 + 1, y0, x1 - d / 2 - 1, y1), fill, 1
+                )
+            if not full_x and not full_y:
                 self.draw.draw_rectangle(
                     (x0, y0 + d / 2 + 1, x0 + d / 2, y1 - d / 2 - 1), fill, 1
                 )
@@ -284,22 +330,21 @@ class ImageDraw:
                     (x1 - d / 2, y0 + d / 2 + 1, x1, y1 - d / 2 - 1), fill, 1
                 )
         if ink is not None and ink != fill and width != 0:
-            self.draw.draw_arc((x1 - d, y0, x1, y0 + d), 270, 360, ink, width)
+            draw_corners(False)
-            self.draw.draw_arc((x1 - d, y1 - d, x1, y1), 0, 90, ink, width)
-            self.draw.draw_arc((x0, y1 - d, x0 + d, y1), 90, 180, ink, width)
-            self.draw.draw_arc((x0, y0, x0 + d, y0 + d), 180, 270, ink, width)
 
+            if not full_x:
                 self.draw.draw_rectangle(
-                (x1 - width + 1, y0 + d / 2 + 1, x1, y1 - d / 2 - 1), ink, 1
+                    (x0 + d / 2 + 1, y0, x1 - d / 2 - 1, y0 + width - 1), ink, 1
                 )
                 self.draw.draw_rectangle(
                     (x0 + d / 2 + 1, y1 - width + 1, x1 - d / 2 - 1, y1), ink, 1
                 )
+            if not full_y:
                 self.draw.draw_rectangle(
                     (x0, y0 + d / 2 + 1, x0 + width - 1, y1 - d / 2 - 1), ink, 1
                 )
                 self.draw.draw_rectangle(
-                (x0 + d / 2 + 1, y0, x1 - d / 2 - 1, y0 + width - 1), ink, 1
+                    (x1 - width + 1, y0 + d / 2 + 1, x1, y1 - d / 2 - 1), ink, 1
                 )
 
     def _multiline_check(self, text):