Merge parallel horizontal edges in ImagingDrawPolygon

Tests/test_imagedraw.py

@@ -1349,3 +1349,22 @@ def test_compute_regular_polygon_vertices_input_error_handling(
     with pytest.raises(expected_error) as e:
         ImageDraw._compute_regular_polygon_vertices(bounding_circle, n_sides, rotation)
     assert str(e.value) == error_message
+
+
+def test_continuous_horizontal_edges_polygon():
+    xy = [
+        (2, 6),
+        (6, 6),
+        (12, 6),
+        (12, 12),
+        (8, 12),
+        (8, 8),
+        (4, 8),
+        (2, 8),
+    ]
+    img, draw = create_base_image_draw((16, 16))
+    draw.polygon(xy, BLACK)
+    expected = os.path.join(IMAGES_PATH, "continuous_horizontal_edges_polygon.png")
+    assert_image_equal_tofile(
+        img, expected, "continuous horizontal edges polygon failed"
+    )

src/libImaging/Draw.c

@@ -734,7 +734,7 @@ ImagingDrawRectangle(
 
 int
 ImagingDrawPolygon(Imaging im, int count, int *xy, const void *ink_, int fill, int op) {
-    int i, n;
+    int i, n, x0, y0, x1, y1;
     DRAW *draw;
     INT32 ink;
 
@@ -753,20 +753,38 @@ ImagingDrawPolygon(Imaging im, int count, int *xy, const void *ink_, int fill, i
             return -1;
         }
         for (i = n = 0; i < count - 1; i++) {
-            add_edge(&e[n++], xy[i + i], xy[i + i + 1], xy[i + i + 2], xy[i + i + 3]);
+            x0 = xy[i * 2];
+            y0 = xy[i * 2 + 1];
+            x1 = xy[i * 2 + 2];
+            y1 = xy[i * 2 + 3];
+            if (y0 == y1 && i != 0 && y0 == xy[i * 2 - 1]) {
+                // This is a horizontal line,
+                // that immediately follows another horizontal line
+                Edge *last_e = &e[n-1];
+                if (x1 > x0 && x0 > xy[i * 2 - 2]) {
+                    // They are both increasing in x
+                    last_e->xmax = x1;
+                    continue;
+                } else if (x1 < x0 && x0 < xy[i * 2 - 2]) {
+                    // They are both decreasing in x
+                    last_e->xmin = x1;
+                    continue;
+                }
+            }
+            add_edge(&e[n++], x0, y0, x1, y1);
         }
-        if (xy[i + i] != xy[0] || xy[i + i + 1] != xy[1]) {
-            add_edge(&e[n++], xy[i + i], xy[i + i + 1], xy[0], xy[1]);
+        if (xy[i * 2] != xy[0] || xy[i * 2 + 1] != xy[1]) {
+            add_edge(&e[n++], xy[i * 2], xy[i * 2 + 1], xy[0], xy[1]);
         }
         draw->polygon(im, n, e, ink, 0);
         free(e);
 
     } else {
         /* Outline */
         for (i = 0; i < count - 1; i++) {
-            draw->line(im, xy[i + i], xy[i + i + 1], xy[i + i + 2], xy[i + i + 3], ink);
+            draw->line(im, xy[i * 2], xy[i * 2 + 1], xy[i * 2 + 2], xy[i * 2 + 3], ink);
         }
-        draw->line(im, xy[i + i], xy[i + i + 1], xy[0], xy[1], ink);
+        draw->line(im, xy[i * 2], xy[i * 2 + 1], xy[0], xy[1], ink);
     }
 
     return 0;