From 0b704d0971d4a8ee4ccda5f302321643a8687904 Mon Sep 17 00:00:00 2001
From: Jeremy Nimmer <jeremy.nimmer@tri.global>
Date: Wed, 7 Apr 2021 13:21:55 -0700
Subject: [PATCH] workspace: Upgrade tinyobjloader to latest commit (#14866)

Correct unit test based on tinyobj impl changes
---
 .../test/obj_to_surface_mesh_test.cc          | 52 +++++++++++++------
 tools/workspace/tinyobjloader/repository.bzl  |  4 +-
 2 files changed, 38 insertions(+), 18 deletions(-)

diff --git a/geometry/proximity/test/obj_to_surface_mesh_test.cc b/geometry/proximity/test/obj_to_surface_mesh_test.cc
index e3da1bea572f..742a8fd07a80 100644
--- a/geometry/proximity/test/obj_to_surface_mesh_test.cc
+++ b/geometry/proximity/test/obj_to_surface_mesh_test.cc
@@ -100,26 +100,46 @@ GTEST_TEST(ObjToSurfaceMeshTest, ReadObjToSurfaceMesh) {
     EXPECT_EQ(expect_vertices[i], surface.vertex(SurfaceVertexIndex(i)).r_MV());
   }
 
-  // This test relies on the specific content of the file quad_cube.obj. The
-  // last section of quad_cube.obj describes the six square faces of the cube.
-  // We expect that each square is subdivided into two triangles. Note that
-  // vertex indices in the file quad_cube.obj are from 1 to 8, but tinyobj
-  // has vertex indices from 0 to 7. We assume that tinyobj subdivides a
-  // polygon into a triangle fan around the first vertex; polygon ABCDE...
-  // becomes triangles ABC, ACD, ADE, etc.
+  // TODO(SeanCurtis-TRI) Devise a formulation of this that is less sensitive
+  //  to the details of the triangulation algorithm.
+  // This test is brittle. It depends on:
+  //   - the exact construction of quad faces in quad_cube.obj.
+  //   - the algorithm that tinyobj uses to triangulate n-gons into triangles.
+  // While we have 100% control over the former, we have no control over the
+  // latter. History has shown that the algorithm changes. In the past, it
+  // would create a fan around the *first* declared vertex, this current
+  // encoding creates a fan around the second. In other words, it's the
+  // difference between the following two triangulations
+  //
+  //    ┌───┐   ┌───┐
+  //    │╲  │   │  ╱│
+  //    │ ╲ │   │ ╱ │
+  //    │  ╲│   │╱  │
+  //    └───┘   └───┘
+  //
+  // This test may fail with subsequent updates to the triangulation algorithm.
   int expect_faces[12][3] {
-      {0, 1, 2}, {0, 2, 3},  // face 1 2 3 4 in quad_cube.obj
-      {4, 7, 6}, {4, 6, 5},  // face 5 8 7 6 in quad_cube.obj
-      {0, 4, 5}, {0, 5, 1},  // face 1 5 6 2 in quad_cube.obj
-      {1, 5, 6}, {1, 6, 2},  // face 2 6 7 3 in quad_cube.obj
-      {2, 6, 7}, {2, 7, 3},  // face 3 7 8 4 in quad_cube.obj
-      {4, 0, 3}, {4, 3, 7}   // face 5 1 4 8 in quad_cube.obj
+      {0, 1, 3}, {1, 2, 3},  // face 1 2 3 4 in quad_cube.obj
+      {4, 7, 5}, {7, 6, 5},  // face 5 8 7 6 in quad_cube.obj
+      {0, 4, 1}, {4, 5, 1},  // face 1 5 6 2 in quad_cube.obj
+      {1, 5, 2}, {5, 6, 2},  // face 2 6 7 3 in quad_cube.obj
+      {2, 6, 3}, {6, 7, 3},  // face 3 7 8 4 in quad_cube.obj
+      {4, 0, 7}, {0, 3, 7}   // face 5 1 4 8 in quad_cube.obj
   };
 
-  auto face_equal = [](const SurfaceFace& f, const SurfaceFace& g) -> bool {
-    return std::make_tuple(f.vertex(0), f.vertex(1), f.vertex(2)) ==
-           std::make_tuple(g.vertex(0), g.vertex(1), g.vertex(2));
+  auto face_equal = [](const SurfaceFace& f,
+                       const SurfaceFace& g) -> ::testing::AssertionResult {
+    const auto f_indices =
+        std::make_tuple(f.vertex(0), f.vertex(1), f.vertex(2));
+    const auto g_indices =
+        std::make_tuple(g.vertex(0), g.vertex(1), g.vertex(2));
+    if (f_indices == g_indices) return ::testing::AssertionSuccess();
+    return ::testing::AssertionFailure()
+           << "\n  Expected: " << f.vertex(0) << ", " << f.vertex(1) << ", "
+           << f.vertex(2) << "\n  Found: " << g.vertex(0) << ", " << g.vertex(1)
+           << ", " << g.vertex(2);
   };
+
   for (int i = 0; i < 12; ++i) {
     EXPECT_TRUE(face_equal(SurfaceFace(expect_faces[i]),
                            surface.element(SurfaceFaceIndex(i))));
diff --git a/tools/workspace/tinyobjloader/repository.bzl b/tools/workspace/tinyobjloader/repository.bzl
index 60f38d7b1404..4736d6e77866 100644
--- a/tools/workspace/tinyobjloader/repository.bzl
+++ b/tools/workspace/tinyobjloader/repository.bzl
@@ -8,8 +8,8 @@ def tinyobjloader_repository(
     github_archive(
         name = name,
         repository = "tinyobjloader/tinyobjloader",
-        commit = "9173980d1de273b17eba5e10eb189e8b4be89425",
-        sha256 = "fe06bf462bf386ea7f6bf34f94c78099c849df348d4a6df681707fba5b5b643c",  # noqa
+        commit = "15bc2685b51612748bcfdb820bb4d42087a7dce1",
+        sha256 = "a5a4b72db1799621567e156853e92215d07a21b3d84098990e0dd79645cce647",  # noqa
         build_file = "@drake//tools/workspace/tinyobjloader:package.BUILD.bazel",  # noqa
         mirrors = mirrors,
         patches = [