From b894c8c73df04cb9ee2dc7b2e858278c59ece1be Mon Sep 17 00:00:00 2001
From: Andrew Murray <radarhere@users.noreply.github.com>
Date: Fri, 21 Jan 2022 11:55:59 +1100
Subject: [PATCH] Connected discontiguous polygon corners

---
 .../discontiguous_corners_polygon.png         | Bin 0 -> 486 bytes
 Tests/test_imagedraw.py                       |  12 +++++
 src/libImaging/Draw.c                         |  47 +++++++++++++++---
 3 files changed, 52 insertions(+), 7 deletions(-)
 create mode 100644 Tests/images/imagedraw/discontiguous_corners_polygon.png

diff --git a/Tests/images/imagedraw/discontiguous_corners_polygon.png b/Tests/images/imagedraw/discontiguous_corners_polygon.png
new file mode 100644
index 0000000000000000000000000000000000000000..509c42b26e0cbc5e01853915b083d367c1587579
GIT binary patch
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diff --git a/Tests/test_imagedraw.py b/Tests/test_imagedraw.py
index b661494c733..3df4e7ca1db 100644
--- a/Tests/test_imagedraw.py
+++ b/Tests/test_imagedraw.py
@@ -1440,3 +1440,15 @@ def test_continuous_horizontal_edges_polygon():
     assert_image_equal_tofile(
         img, expected, "continuous horizontal edges polygon failed"
     )
+
+
+def test_discontiguous_corners_polygon():
+    img, draw = create_base_image_draw((84, 68))
+    draw.polygon(((1, 21), (34, 4), (71, 1), (38, 18)), BLACK)
+    draw.polygon(((71, 44), (38, 27), (1, 24)), BLACK)
+    draw.polygon(
+        ((38, 66), (5, 49), (77, 49), (47, 66), (82, 63), (82, 47), (1, 47), (1, 63)),
+        BLACK,
+    )
+    expected = os.path.join(IMAGES_PATH, "discontiguous_corners_polygon.png")
+    assert_image_similar_tofile(img, expected, 1)
diff --git a/src/libImaging/Draw.c b/src/libImaging/Draw.c
index 0e4899b5b49..86cd6c3a058 100644
--- a/src/libImaging/Draw.c
+++ b/src/libImaging/Draw.c
@@ -450,7 +450,8 @@ polygon_generic(Imaging im, int n, Edge *e, int ink, int eofill, hline_handler h
     int edge_count = 0;
     int ymin = im->ysize - 1;
     int ymax = 0;
-    int i;
+    int i, j, k;
+    float adjacent_line_x, adjacent_line_x_other_edge;
 
     if (n <= 0) {
         return 0;
@@ -493,16 +494,48 @@ polygon_generic(Imaging im, int n, Edge *e, int ink, int eofill, hline_handler h
         return -1;
     }
     for (; ymin <= ymax; ymin++) {
-        int j = 0;
+        j = 0;
         for (i = 0; i < edge_count; i++) {
             Edge *current = edge_table[i];
             if (ymin >= current->ymin && ymin <= current->ymax) {
                 xx[j++] = (ymin - current->y0) * current->dx + current->x0;
-            }
-            /* Needed to draw consistent polygons */
-            if (ymin == current->ymax && ymin < ymax) {
-                xx[j] = xx[j - 1];
-                j++;
+
+                if (ymin == current->ymax && ymin < ymax) {
+                    // Needed to draw consistent polygons
+                    xx[j] = xx[j - 1];
+                    j++;
+                } else if (current->dx != 0 && roundf(xx[j-1]) == xx[j-1]) {
+                    // Connect discontiguous corners
+                    for (k = 0; k < i; k++) {
+                        Edge *other_edge = edge_table[k];
+                        if ((current->dx > 0 && other_edge->dx <= 0) ||
+                            (current->dx < 0 && other_edge->dx >= 0)) {
+                            continue;
+                        }
+                        // Check if the two edges join to make a corner
+                        if (xx[j-1] == (ymin - other_edge->y0) * other_edge->dx + other_edge->x0) {
+                            // Determine points from the edges on the next row
+                            // Or if this is the last row, check the previous row
+                            int offset = ymin == ymax ? -1 : 1;
+                            adjacent_line_x = (ymin + offset - current->y0) * current->dx + current->x0;
+                            adjacent_line_x_other_edge = (ymin + offset - other_edge->y0) * other_edge->dx + other_edge->x0;
+                            if (ymin == current->ymax) {
+                                if (current->dx > 0) {
+                                    xx[k] = fmax(adjacent_line_x, adjacent_line_x_other_edge) + 1;
+                                } else {
+                                    xx[k] = fmin(adjacent_line_x, adjacent_line_x_other_edge) - 1;
+                                }
+                            } else {
+                                if (current->dx > 0) {
+                                    xx[k] = fmin(adjacent_line_x, adjacent_line_x_other_edge);
+                                } else {
+                                    xx[k] = fmax(adjacent_line_x, adjacent_line_x_other_edge) + 1;
+                                }
+                            }
+                            break;
+                        }
+                    }
+                }
             }
         }
         qsort(xx, j, sizeof(float), x_cmp);