fixed banding bug with draw.circle (#2032)

* fixed banding bug with draw.circle when center of circle is to the left of the screen

* formatting

* expanded the test to test all 4 directions

* cleaned up test some more

* added test for regression for upstream bug

* removed redundant variable

* changed comment to be a message in the asserts

Author: oddbookworm

src_c/draw.c

@@ -1992,11 +1992,10 @@ draw_circle_filled(SDL_Surface *surf, int x0, int y0, int radius, Uint32 color,
     int ddF_y = -2 * radius;
     int x = 0;
     int y = radius;
-    int xmin = INT_MAX;
     int xmax = INT_MIN;
 
     if (x0 < 0) {
-        xmin = x0 + INT_MAX + 1;
+        xmax = x0 + INT_MAX + 1;
     }
     else {
         xmax = INT_MAX - x0;
@@ -2015,15 +2014,14 @@ draw_circle_filled(SDL_Surface *surf, int x0, int y0, int radius, Uint32 color,
         /* optimisation to avoid overdrawing and repeated return rect checks:
            only draw a line if y-step is about to be decreased. */
         if (f >= 0) {
-            drawhorzlineclipbounding(surf, color, x0 - MIN(x, xmin),
-                                     y0 + y - 1, x0 + MIN(x - 1, xmax),
-                                     drawn_area);
-            drawhorzlineclipbounding(surf, color, x0 - MIN(x, xmin), y0 - y,
+            drawhorzlineclipbounding(surf, color, x0 - x, y0 + y - 1,
+                                     x0 + MIN(x - 1, xmax), drawn_area);
+            drawhorzlineclipbounding(surf, color, x0 - x, y0 - y,
                                      x0 + MIN(x - 1, xmax), drawn_area);
         }
-        drawhorzlineclipbounding(surf, color, x0 - MIN(y, xmin), y0 + x - 1,
+        drawhorzlineclipbounding(surf, color, x0 - y, y0 + x - 1,
                                  x0 + MIN(y - 1, xmax), drawn_area);
-        drawhorzlineclipbounding(surf, color, x0 - MIN(y, xmin), y0 - x,
+        drawhorzlineclipbounding(surf, color, x0 - y, y0 - x,
                                  x0 + MIN(y - 1, xmax), drawn_area);
     }
 }

test/draw_test.py

@@ -5673,6 +5673,63 @@ def test_circle__diameter(self):
             self.assertEqual(bounding_rect.width, radius * 2)
             self.assertEqual(bounding_rect.height, radius * 2)
 
+    def test_circle__offscreen(self):
+        """Ensures that drawing a circle completely off-screen doesn't create
+        a solid band across the screen
+        see https://github.com/pygame/pygame/issues/3143
+        """
+        width = 200
+        height = 200
+        surf = pygame.Surface((width, height))
+        surf_center = surf.get_rect().center
+        radius = 10
+
+        # way way outside the screen
+        test_centers = [
+            (-1e30, height / 2),
+            (1e30, height / 2),
+            (width / 2, -1e30),
+            (width / 2, 1e30),
+        ]
+        for center in test_centers:
+            surf.fill("black")
+            self.draw_circle(surf, "white", center, radius)
+
+            self.assertEqual(
+                surf.get_at(surf_center)[0:3],
+                (0, 0, 0),
+                msg="pixel at center should be black, not white",
+            )
+
+    def test_circle__no_band(self):
+        """Ensures that drawing a circle with the center off-screen doesn't create a
+        solid band across the screen
+        This tests a bug introduced in
+        https://github.com/pygame-community/pygame-ce/commit/c88a1d8f7b31099cec84dc5cb7aeebdada783e83
+        """
+        width = 200
+        height = 200
+        surf = pygame.Surface((width, height))
+        surf_center = surf.get_rect().center
+        radius = 10
+
+        # one pixel outside the screen on the center of each edge
+        test_centers = [
+            (-1, height / 2),
+            (width + 1, height / 2),
+            (width / 2, -1),
+            (width / 2, height + 1),
+        ]
+        for center in test_centers:
+            surf.fill("black")
+            self.draw_circle(surf, "white", center, radius)
+
+            self.assertEqual(
+                surf.get_at(surf_center)[0:3],
+                (0, 0, 0),
+                msg="pixel at center should be black, not white",
+            )
+
 
 class DrawCircleTest(DrawCircleMixin, DrawTestCase):
     """Test draw module function circle.