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Give OBJ Quad and Flipped Faces the Texture Coordinates of Their Own Corners (#15977)

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load_obj emits the second triangle of a quad from corners 0, 2 and 3,
but read its texture coordinates from corners 0, 1 and 2, so half of
every textured quad sampled the wrong part of the texture. The corner
indices are now passed down to where the coordinates are read.

A mesh with inward-facing triangles is flipped after loading, which
swaps corners 1 and 2 of every face. The texture coordinates were left
as they were. They are now swapped along with the corners.
This commit is contained in:
HanifKoh
2026-09-30 03:13:29 +08:00
committed by GitHub
parent 203bc63f35
commit 2769b12ce7
2 changed files with 66 additions and 17 deletions
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tests/libslic3r/test_obj.cpp
+54 -9
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@@ -19,9 +19,8 @@ struct LoadedObj
ObjInfo info;
};
// A tetrahedron with a material and two texture coordinates, (0.25, 0.5) and (0.75, 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, const std::string &vts = "vt 0.25 0.5\nvt 0.75 1\n")
// Loads an OBJ made of the given lines, with a material library that defines material "a".
LoadedObj load_textured_obj(const std::string &body)
{
ScopedTemporaryFile obj(".obj");
ScopedTemporaryFile mtl(".mtl");
@@ -31,12 +30,7 @@ LoadedObj load_textured_tetrahedron(const std::string &first_face, const std::st
}
{
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"
<< 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";
out << "mtllib " << mtl.path().filename().string() << "\n" << body;
}
LoadedObj loaded;
std::string message;
@@ -44,6 +38,21 @@ LoadedObj load_textured_tetrahedron(const std::string &first_face, const std::st
return loaded;
}
// A tetrahedron with a material and two texture coordinates, (0.25, 0.5) and (0.75, 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, const std::string &vts = "vt 0.25 0.5\nvt 0.75 1\n")
{
return load_textured_obj("v 0 0 0\nv 10 0 0\nv 0 10 0\nv 0 0 10\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");
}
// Texture coordinate n is (n / 10, n / 20), so a UV identifies the vt it came from.
void check_uv_is_vt(const Vec2f &uv, int vt)
{
CHECK_THAT(uv.x(), WithinAbs(vt / 10., 1e-6));
CHECK_THAT(uv.y(), WithinAbs(vt / 20., 1e-6));
}
} // namespace
TEST_CASE("An out-of-range texture index falls back to a zero UV and keeps the geometry", "[OBJ][Regression]")
@@ -120,3 +129,39 @@ TEST_CASE("A texture coordinate with w does not shift the indices of the ones af
CHECK_THAT(uv[2].x(), WithinAbs(0.75, 1e-6));
CHECK_THAT(uv[2].y(), WithinAbs(1., 1e-6));
}
TEST_CASE("Both triangles of a quad take the texture coordinates of their own corners", "[OBJ][Regression]")
{
const LoadedObj loaded = load_textured_obj("v 0 0 0\nv 10 0 0\nv 10 10 0\nv 0 10 0\n"
"vt 0.1 0.05\nvt 0.2 0.1\nvt 0.3 0.15\nvt 0.4 0.2\n"
"usemtl a\n"
"f 1/1 2/2 3/3 4/4\n");
REQUIRE(loaded.ok);
REQUIRE(loaded.mesh.facets_count() == 2);
REQUIRE(loaded.info.uvs.size() == 2);
check_uv_is_vt(loaded.info.uvs[0][0], 1);
check_uv_is_vt(loaded.info.uvs[0][1], 2);
check_uv_is_vt(loaded.info.uvs[0][2], 3);
check_uv_is_vt(loaded.info.uvs[1][0], 1);
check_uv_is_vt(loaded.info.uvs[1][1], 3);
check_uv_is_vt(loaded.info.uvs[1][2], 4);
}
TEST_CASE("Texture coordinates follow the corners of a mesh that is flipped on load", "[OBJ][Regression]")
{
// The faces wind inwards, so the loader flips them. Vertex n uses vt n.
const LoadedObj loaded = load_textured_obj("v 0 0 0\nv 10 0 0\nv 0 10 0\nv 0 0 10\n"
"vt 0.1 0.05\nvt 0.2 0.1\nvt 0.3 0.15\nvt 0.4 0.2\n"
"usemtl a\n"
"f 1/1 2/2 3/3\nf 1/1 4/4 2/2\nf 1/1 3/3 4/4\nf 2/2 4/4 3/3\n");
REQUIRE(loaded.ok);
const indexed_triangle_set &its = loaded.mesh.its;
CHECK(its_volume(its) > 0.f);
REQUIRE(its.indices.size() == 4);
REQUIRE(loaded.info.uvs.size() == 4);
for (size_t face = 0; face < its.indices.size(); ++face)
for (int corner = 0; corner < 3; ++corner)
check_uv_is_vt(loaded.info.uvs[face][corner], its.indices[face][corner] + 1);
}