Keep OBJ Texture Coordinates That Carry a W Component

The vt parser stopped reading the optional third component when texture
coordinates were cut down to u and v, but the check that nothing is left
on the line stayed. A legal "vt u v w" line was therefore rejected and
silently dropped, shifting every later texture index. The w component is
parsed again and discarded.

The texture coordinate stride is now a named constant, OBJ_TEXCOORD_LENGTH,
used by the parser and the importer, so the relative-index rebase cannot
drift from the storage layout again.

src/libslic3r/Format/OBJ.cpp

@@ -168,11 +168,11 @@ bool load_obj(const char *path, TriangleMesh *meshptr, ObjInfo& obj_info, std::s
                         if (data.textureCoordinates.size() > 0) {
                             // A face vertex may omit vt or reference a missing one. Fall back to (0, 0) rather than
                             // skipping the face, so obj_info.uvs stays aligned with the face indices.
-                            const int uv_count = static_cast<int>(data.textureCoordinates.size() / 2);
+                            const int uv_count = static_cast<int>(data.textureCoordinates.size() / OBJ_TEXCOORD_LENGTH);
                             auto      uv_at    = [&data, uv_count](int idx) -> Vec2f {
                                 if (idx < 0 || idx >= uv_count)
                                     return Vec2f::Zero();
-                                return Vec2f(data.textureCoordinates[idx * 2], data.textureCoordinates[idx * 2 + 1]);
+                                return Vec2f(data.textureCoordinates[idx * OBJ_TEXCOORD_LENGTH], data.textureCoordinates[idx * OBJ_TEXCOORD_LENGTH + 1]);
                             };
                             std::array<Vec2f, 3> uv_array{uv_at(uvs[0]), uv_at(uvs[1]), uv_at(uvs[2])};
                             obj_info.uvs.emplace_back(uv_array);

src/libslic3r/Format/objparser.cpp

@@ -51,19 +51,18 @@ static bool obj_parseline(const char *line, ObjData &data)
 				line = endptr;
 				EATWS();
 			}
-			/*double w = 0;
+			// The optional w is accepted but not stored: only u and v are used.
 			if (*line != 0) {
-				w = strtod(line, &endptr);
+				strtod(line, &endptr);
 				if (endptr == 0 || (*endptr != ' ' && *endptr != '\t' && *endptr != 0))
 					return false;
 				line = endptr;
 				EATWS();
-			}*/
+			}
 			if (*line != 0)
 				return false;
 			data.textureCoordinates.push_back((float)u);
 			data.textureCoordinates.push_back((float)v);
-			//data.textureCoordinates.push_back((float)w);
 			break;
 		}
 		case 'n':
@@ -245,7 +244,7 @@ static bool obj_parseline(const char *line, ObjData &data)
             else
 				-- vertex.normalIdx;
 			if (vertex.textureCoordIdx < 0)
-                vertex.textureCoordIdx += (int)data.textureCoordinates.size() / 2;
+                vertex.textureCoordIdx += (int)data.textureCoordinates.size() / OBJ_TEXCOORD_LENGTH;
             else
 				-- vertex.textureCoordIdx;
 			data.vertices.push_back(vertex);

src/libslic3r/Format/objparser.hpp

@@ -92,6 +92,7 @@ inline bool operator==(const ObjSmoothingGroup &v1, const ObjSmoothingGroup &v2)
 }
 #define OBJ_VERTEX_COLOR_ALPHA 6
 #define OBJ_VERTEX_LENGTH   7  // x, y, z, color_x,color_y,color_z,color_w
+#define OBJ_TEXCOORD_LENGTH 2  // u, v
 #define ONE_FACE_SIZE 4//ONE_FACE format: f 8/4/6 7/3/6 6/2/6 -1/-1/-1
 struct ObjData {
 	// Version of the data structure for load / store in the private binary format.
@@ -100,7 +101,7 @@ struct ObjData {
 	// x, y, z, color_x,color_y,color_z,color_w
 	std::vector<float>				coordinates;
     bool                            has_vertex_color{false};
-	// u, v, w
+	// u, v
 	std::vector<float>				textureCoordinates;
 	// x, y, z
 	std::vector<float>				normals;

tests/libslic3r/test_obj.cpp

@@ -20,8 +20,8 @@ struct LoadedObj
 };
 
 // A tetrahedron with a material and two texture coordinates, (0.25, 0.5) and (0.75, 1).
-// Only the first face is varied; the other three reference vt 1.
+// Only the first face and the vt lines are varied; the other three faces reference vt 1.
-LoadedObj load_textured_tetrahedron(const std::string &first_face)
+LoadedObj load_textured_tetrahedron(const std::string &first_face, const std::string &vts = "vt 0.25 0.5\nvt 0.75 1\n")
 {
     ScopedTemporaryFile obj(".obj");
     ScopedTemporaryFile mtl(".mtl");
@@ -33,7 +33,7 @@ LoadedObj load_textured_tetrahedron(const std::string &first_face)
         boost::nowide::ofstream out(obj.string());
         out << "mtllib " << mtl.path().filename().string() << "\n"
             << "v 0 0 0\nv 10 0 0\nv 0 10 0\nv 0 0 10\n"
-            << "vt 0.25 0.5\nvt 0.75 1\n"
+            << vts
             << "usemtl a\n"
             << first_face << "\n"
             << "f 1/1 2/1 4/1\nf 1/1 4/1 3/1\nf 2/1 3/1 4/1\n";
@@ -90,3 +90,33 @@ TEST_CASE("A negative texture index counts back from the last texture coordinate
     CHECK_THAT(uv[1].x(), WithinAbs(0.75, 1e-6));
     CHECK_THAT(uv[1].y(), WithinAbs(1., 1e-6));
 }
+
+TEST_CASE("Texture coordinates with a w component are kept", "[OBJ][Regression]")
+{
+    const LoadedObj loaded = load_textured_tetrahedron("f 1/1 3/2 2/2", "vt 0.25 0.5 0\nvt 0.75 1 0\n");
+
+    REQUIRE(loaded.ok);
+    CHECK(loaded.mesh.facets_count() == 4);
+    REQUIRE(loaded.info.uvs.size() == 4);
+    const std::array<Vec2f, 3> &uv = loaded.info.uvs.front();
+    CHECK_THAT(uv[0].x(), WithinAbs(0.25, 1e-6));
+    CHECK_THAT(uv[0].y(), WithinAbs(0.5, 1e-6));
+    CHECK_THAT(uv[1].x(), WithinAbs(0.75, 1e-6));
+    CHECK_THAT(uv[1].y(), WithinAbs(1., 1e-6));
+}
+
+TEST_CASE("A texture coordinate with w does not shift the indices of the ones after it", "[OBJ][Regression]")
+{
+    // The w on the first vt used to drop that line, so vt 2 resolved to the third coordinate.
+    const LoadedObj loaded = load_textured_tetrahedron("f 1/2 3/3 2/-1", "vt 0.1 0.2 0\nvt 0.25 0.5\nvt 0.75 1\n");
+
+    REQUIRE(loaded.ok);
+    REQUIRE(loaded.info.uvs.size() == 4);
+    const std::array<Vec2f, 3> &uv = loaded.info.uvs.front();
+    CHECK_THAT(uv[0].x(), WithinAbs(0.25, 1e-6));
+    CHECK_THAT(uv[0].y(), WithinAbs(0.5, 1e-6));
+    CHECK_THAT(uv[1].x(), WithinAbs(0.75, 1e-6));
+    CHECK_THAT(uv[1].y(), WithinAbs(1., 1e-6));
+    CHECK_THAT(uv[2].x(), WithinAbs(0.75, 1e-6));
+    CHECK_THAT(uv[2].y(), WithinAbs(1., 1e-6));
+}