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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 authored and GitHub committed 2026-09-30 03:13:29 +08:00
1 parent 203bc63f35
commit 2769b12ce7
2 files changed
+66 -17
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src/libslic3r/Format/OBJ.cpp
+12 -8
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@@ -141,7 +141,7 @@ bool load_obj(const char *path, TriangleMesh *meshptr, ObjInfo& obj_info, std::s
its.indices.emplace_back(indices[0], indices[1], indices[2]);
int face_index =its.indices.size() - 1;
RGBA face_color;
auto set_face_color = [&uvs, &data, &mtl_data, &obj_info, &face_color](int face_index, const std::string mtl_name) {
auto set_face_color = [&uvs, &data, &mtl_data, &obj_info, &face_color](int face_index, const std::string mtl_name, const std::array<int, 3> &corners) {
if (mtl_data.new_mtl_unmap.find(mtl_name) != mtl_data.new_mtl_unmap.end()) {
bool is_merge_ka_kd = true;
for (size_t n = 0; n < 3; n++) {
@@ -174,7 +174,7 @@ bool load_obj(const char *path, TriangleMesh *meshptr, ObjInfo& obj_info, std::s
return Vec2f::Zero();
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])};
std::array<Vec2f, 3> uv_array{uv_at(uvs[corners[0]]), uv_at(uvs[corners[1]]), uv_at(uvs[corners[2]])};
obj_info.uvs.emplace_back(uv_array);
}
obj_info.face_colors.emplace_back(face_color);
@@ -185,27 +185,27 @@ bool load_obj(const char *path, TriangleMesh *meshptr, ObjInfo& obj_info, std::s
}
}
};
auto set_face_color_by_mtl = [&data, &set_face_color](int face_index) {
auto set_face_color_by_mtl = [&data, &set_face_color](int face_index, const std::array<int, 3> &corners) {
if (data.usemtls.size() == 1) {
set_face_color(face_index, data.usemtls[0].name);
set_face_color(face_index, data.usemtls[0].name, corners);
} else {
for (size_t k = 0; k < data.usemtls.size(); k++) {
auto mtl = data.usemtls[k];
if (face_index >= mtl.face_start && face_index <= mtl.face_end) {
set_face_color(face_index, data.usemtls[k].name);
set_face_color(face_index, data.usemtls[k].name, corners);
break;
}
}
}
};
if (exist_mtl) {
set_face_color_by_mtl(face_index);
set_face_color_by_mtl(face_index, {0, 1, 2});
}
if (cnt == 4) {
its.indices.emplace_back(indices[0], indices[2], indices[3]);
int face_index = its.indices.size() - 1;
if (exist_mtl) {
set_face_color_by_mtl(face_index);
set_face_color_by_mtl(face_index, {0, 2, 3});
}
}
}
@@ -217,8 +217,12 @@ bool load_obj(const char *path, TriangleMesh *meshptr, ObjInfo& obj_info, std::s
message = _L("This OBJ file couldn't be read because it's empty.");
return false;
}
if (meshptr->volume() < 0)
if (meshptr->volume() < 0) {
meshptr->flip_triangles();
// Flipping swaps corners 1 and 2 of every face, so the UVs have to follow.
for (std::array<Vec2f, 3> &uv : obj_info.uvs)
std::swap(uv[1], uv[2]);
}
// Hand the parsed material table back so callers can build a TexturedMesh from it.
if (out_mtl)
*out_mtl = mtl_data;