我一直在尝试实现一个所有面法线都指向外部的网格。为了实现这一点，我从 *.ctm 文件中加载了一个网格，然后遍历所有三角形以使用叉积确定法线，如果法线指向负 z 方向，我翻转 v1 和 v2（因此正常方向）。完成后，我将结果保存到 *.ctm 文件并使用 Meshlab 进行查看。
Meshlab 中的结果仍然显示法线指向正和负 z 方向（可以从黑色三角形中看出）。同样，在 Meshlab 中查看法线时，它们确实是向后指向的。
谁能给我一些关于如何解决这个问题的建议？
规范化部分的源代码是：
pcl::PointCloud<pcl::PointXYZRGBA>::Ptr cloud1 (new pcl::PointCloud<pcl::PointXYZRGBA> ());
pcl::fromROSMsg (meshFixed.cloud,*cloud1);for(std::vector<pcl::Vertices>::iterator it = meshFixed.polygons.begin(); it != meshFixed.polygons.end(); ++it)
{
alglib::real_2d_array v0;
double _v0[] = {cloud1->points[it->vertices[0]].x,cloud1->points[it->vertices[0]].y,cloud1->points[it->vertices[0]].z};
v0.setcontent(3,1,_v0); //3 rows, 1col
alglib::real_2d_array v1;
double _v1[] = {cloud1->points[it->vertices[1]].x,cloud1->points[it->vertices[1]].y,cloud1->points[it->vertices[1]].z};
v1.setcontent(3,1,_v1); //3 rows, 1col
alglib::real_2d_array v2;
double _v2[] = {cloud1->points[it->vertices[2]].x,cloud1->points[it->vertices[2]].y,cloud1->points[it->vertices[2]].z};
v2.setcontent(1,3,_v2); //3 rows, 1col
alglib::real_2d_array normal;
normal = cross(v1-v0,v2-v0);
//if z<0 change indices order v1->v2 and v2->v1
alglib::real_2d_array normalizedNormal;
if(normal[2][0]<0)
{
int index1,index2;
index1 = it->vertices[1];
index2 = it->vertices[2];
it->vertices[1] = index2;
it->vertices[2] = index1;
//make normal of length 1
double normalScaling = 1.0/sqrt(dot(normal,normal));
normal[0][0] = -1*normal[0][0];
normal[1][0] = -1*normal[1][0];
normal[2][0] = -1*normal[2][0];
normalizedNormal = normalScaling * normal;
}
else
{
//make normal of length 1
double normalScaling = 1.0/sqrt(dot(normal,normal));
normalizedNormal = normalScaling * normal;
}
//add to normal cloud
pcl::Normal pclNormalizedNormal;
pclNormalizedNormal.normal_x = normalizedNormal[0][0];
pclNormalizedNormal.normal_y = normalizedNormal[1][0];
pclNormalizedNormal.normal_z = normalizedNormal[2][0];
normalsFixed.push_back(pclNormalizedNormal);
}
这段代码的结果是：
我在 VCG 库中找到了一些代码来定位面和顶点法线。使用此功能后，大部分网格具有正确的面法线，但不是全部。
新代码：
// VCG library implementation
MyMesh m;
// Convert pcl::PolygonMesh to VCG MyMesh
m.Clear();
// Create temporary cloud in to have handy struct object
pcl::PointCloud<pcl::PointXYZRGBA>::Ptr cloud1 (new pcl::PointCloud<pcl::PointXYZRGBA> ());
pcl::fromROSMsg (meshFixed.cloud,*cloud1);
// Now convert the vertices to VCG MyMesh
int vertCount = cloud1->width*cloud1->height;
vcg::tri::Allocator<MyMesh>::AddVertices(m, vertCount);
for(unsigned int i=0;i<vertCount;++i)
m.vert[i].P()=vcg::Point3f(cloud1->points[i].x,cloud1->points[i].y,cloud1->points[i].z);
// Now convert the polygon indices to VCG MyMesh => make VCG faces..
int triCount = meshFixed.polygons.size();
if(triCount==1)
{
if(meshFixed.polygons[0].vertices[0]==0 && meshFixed.polygons[0].vertices[1]==0 && meshFixed.polygons[0].vertices[2]==0)
triCount=0;
}
Allocator<MyMesh>::AddFaces(m, triCount);
for(unsigned int i=0;i<triCount;++i)
{
m.face[i].V(0)=&m.vert[meshFixed.polygons[i].vertices[0]];
m.face[i].V(1)=&m.vert[meshFixed.polygons[i].vertices[1]];
m.face[i].V(2)=&m.vert[meshFixed.polygons[i].vertices[2]];
}
vcg::tri::UpdateBounding<MyMesh>::Box(m);
vcg::tri::UpdateNormal<MyMesh>::PerFace(m);
vcg::tri::UpdateNormal<MyMesh>::PerVertexNormalizedPerFace(m);
printf("Input mesh vn:%i fn:%i\n",m.VN(),m.FN());
// Start to flip all normals to outside
vcg::face::FFAdj<MyMesh>::FFAdj();
vcg::tri::UpdateTopology<MyMesh>::FaceFace(m);
bool oriented, orientable;
if ( vcg::tri::Clean<MyMesh>::CountNonManifoldEdgeFF(m)>0 ) {
std::cout << "Mesh has some not 2-manifold faces, Orientability requires manifoldness" << std::endl; // text
return; // can't continue, mesh can't be processed
}
vcg::tri::Clean<MyMesh>::OrientCoherentlyMesh(m, oriented,orientable);
vcg::tri::Clean<MyMesh>::FlipNormalOutside(m);
vcg::tri::Clean<MyMesh>::FlipMesh(m);
//vcg::tri::UpdateTopology<MyMesh>::FaceFace(m);
//vcg::tri::UpdateTopology<MyMesh>::TestFaceFace(m);
vcg::tri::UpdateNormal<MyMesh>::PerVertexNormalizedPerFace(m);
vcg::tri::UpdateNormal<MyMesh>::PerVertexFromCurrentFaceNormal(m);
// now convert VCG back to pcl::PolygonMesh
pcl::PointCloud<pcl::PointXYZRGBA>::Ptr cloud (new pcl::PointCloud<pcl::PointXYZRGBA>);
cloud->is_dense = false;
cloud->width = vertCount;
cloud->height = 1;
cloud->points.resize (vertCount);
// Now fill the pointcloud of the mesh
for(int i=0; i<vertCount; i++)
{
cloud->points[i].x = m.vert[i].P()[0];
cloud->points[i].y = m.vert[i].P()[1];
cloud->points[i].z = m.vert[i].P()[2];
}
pcl::toROSMsg(*cloud,meshFixed.cloud);
std::vector<pcl::Vertices> polygons;
// Now fill the indices of the triangles/faces of the mesh
for(int i=0; i<triCount; i++)
{
pcl::Vertices vertices;
vertices.vertices.push_back(m.face[i].V(0)-&*m.vert.begin());
vertices.vertices.push_back(m.face[i].V(1)-&*m.vert.begin());
vertices.vertices.push_back(m.face[i].V(2)-&*m.vert.begin());
polygons.push_back(vertices);
}
meshFixed.polygons = polygons;
这导致：（Meshlab 仍然显示法线面向两侧）
