From 3afe846570c365bb218b8ae6e4edc0f21d15515d Mon Sep 17 00:00:00 2001
From: Emmett Lalish <elalish@gmail.com>
Date: Sat, 8 Jun 2024 17:31:33 -0700
Subject: [PATCH] Fixed backward zebra polygon (#834)

fixed two more bugs
---
 src/polygon/src/polygon.cpp | 19 ++++++++++---------
 test/polygon_test.cpp       | 12 +++++-------
 test/zebra.cpp              |  2 +-
 3 files changed, 16 insertions(+), 17 deletions(-)

diff --git a/src/polygon/src/polygon.cpp b/src/polygon/src/polygon.cpp
index 7c3d8ed19..e122893bb 100644
--- a/src/polygon/src/polygon.cpp
+++ b/src/polygon/src/polygon.cpp
@@ -354,6 +354,13 @@ class EarClip {
       return left->InsideEdge(left->right, precision, true);
     }
 
+    // Subtly different from !IsConvex because IsConvex will return true for
+    // colinear non-folded verts, while IsReflex will always check until actual
+    // certainty is determined.
+    bool IsReflex(float precision) const {
+      return !left->InsideEdge(left->right, precision, true);
+    }
+
     // This function is the core of finding a proper place to keyhole. It runs
     // on this Vert, which represents the edge from this to right. It returns
     // an iterator to the vert to connect to (either this or right) and a bool
@@ -368,7 +375,8 @@ class EarClip {
     std::pair<VertItr, bool> InterpY2X(glm::vec2 start, int onTop,
                                        float precision) const {
       const auto none = std::make_pair(left, false);
-      if (pos.y < start.y && right->pos.y >= start.y) {
+      if (pos.y < start.y && right->pos.y >= start.y &&
+          (pos.x > start.x - precision || right->pos.x > start.x - precision)) {
         return std::make_pair(left->right, true);
       } else if (onTop != 0 && pos.x > start.x - precision &&
                  pos.y > start.y - precision && pos.y < start.y + precision &&
@@ -532,13 +540,6 @@ class EarClip {
       // triangulations of polygons with holes, due to vert duplication.
       triangles_.push_back(
           {ear->left->mesh_idx, ear->mesh_idx, ear->right->mesh_idx});
-#ifdef MANIFOLD_DEBUG
-      if (params.verbose) {
-        std::cout << "output tri: " << ear->mesh_idx << ", "
-                  << ear->right->mesh_idx << ", " << ear->left->mesh_idx
-                  << std::endl;
-      }
-#endif
     } else {
       PRINT("Topological degenerate!");
     }
@@ -704,7 +705,7 @@ class EarClip {
           vert->pos.y * above > start->pos.y * above - precision_ &&
           (inside > 0 || (inside == 0 && vert->pos.x < best->pos.x)) &&
           vert->InsideEdge(edge, precision_, true) &&
-          !vert->IsConvex(precision_)) {
+          vert->IsReflex(precision_)) {
         if (vert->pos.y > start->pos.y - precision_ &&
             vert->pos.y < start->pos.y + precision_) {
           if (onTop > 0 && vert->left->pos.x < vert->pos.x &&
diff --git a/test/polygon_test.cpp b/test/polygon_test.cpp
index f90454bc4..3fd4f6602 100644
--- a/test/polygon_test.cpp
+++ b/test/polygon_test.cpp
@@ -56,20 +56,18 @@ Polygons Duplicate(Polygons polys) {
 }
 
 void TestPoly(const Polygons &polys, int expectedNumTri,
-              float precision = -1.0f, bool onlyBasic = false) {
+              float precision = -1.0f) {
   PolygonParams().verbose = options.params.verbose;
 
   std::vector<glm::ivec3> triangles;
   EXPECT_NO_THROW(triangles = Triangulate(polys, precision));
   EXPECT_EQ(triangles.size(), expectedNumTri) << "Basic";
 
-  if (!onlyBasic) {
-    EXPECT_NO_THROW(triangles = Triangulate(Turn180(polys), precision));
-    EXPECT_EQ(triangles.size(), expectedNumTri) << "Turn 180";
+  EXPECT_NO_THROW(triangles = Triangulate(Turn180(polys), precision));
+  EXPECT_EQ(triangles.size(), expectedNumTri) << "Turn 180";
 
-    EXPECT_NO_THROW(triangles = Triangulate(Duplicate(polys), precision));
-    EXPECT_EQ(triangles.size(), 2 * expectedNumTri) << "Duplicate";
-  }
+  EXPECT_NO_THROW(triangles = Triangulate(Duplicate(polys), precision));
+  EXPECT_EQ(triangles.size(), 2 * expectedNumTri) << "Duplicate";
 
   PolygonParams().verbose = false;
 }
diff --git a/test/zebra.cpp b/test/zebra.cpp
index 312af6c34..877c27702 100644
--- a/test/zebra.cpp
+++ b/test/zebra.cpp
@@ -62639,7 +62639,7 @@ TEST(Polygon, Zebra) {
       {5.59076977, 14.00315},    //
       {5.59097004, 14.00315},    //
   });
-  TestPoly(polys, 62597, -1, true);
+  TestPoly(polys, 62597);
 }
 
 TEST(Polygon, Zebra1) {