我正在使用 C++ 开发路径/光线追踪器。我现在正在加载 obj 文件。但是有些对象在加载后会翻转法线。我无法弄清楚这种行为来自何处或如何解决它。
请参阅图片以更好地理解该问题。
显示当前行为的图像：
首先，我认为这是表面后面的法线的问题。但是在基于表面法线渲染颜色之后。很明显，法线在某些情况下是翻转的。
这是我加载模型的非常基本的代码。
//OBJ Loader object.
bool OBJLoader::loadMesh (std::string filePath){
// If the file is not an .obj file return false
if (filePath.substr(filePath.size() - 4, 4) != ".obj"){
std::cout << "No .obj file found at given file location: "<<filePath << std::endl;
}
//Open file stream
std::ifstream file(filePath);
//check if file is open.
if (!file.is_open()){
std::cout << "File was not opened!" << std::endl;
return false;
}
//Do file loading.
std::cout << "Parsing obj-file: "<<filePath << std::endl;
//constuct mesh data.
bool smoothShading = false;
std::string obj_name;
std::vector<Vertex> vertices;
std::vector<Vect3> Positions;
std::vector<Vect3> Normals;
std::vector<Vect2> UVs;
std::vector<unsigned int> V_indices;
//the current line
std::string currentLine;
//loop over each line and parse the needed data.
while(std::getline(file, currentLine)){
//for now we just print the line
//std::cout << currentLine << std::endl;
if(algorithm::startsWith(currentLine, "s ")){
std::vector<std::string> line_split = algorithm::split(currentLine,' ');
if( line_split[1] == std::string("off")){
smoothShading = false;
}else if(line_split[1] == std::string("1")){
//enalbe smooth shading;
smoothShading = true;
}
}
//check if the line starts with v -> vertex.
if(algorithm::startsWith(currentLine, "o ")){
//construct new vertex position.
std::vector<std::string> line_split = algorithm::split(currentLine,' ');
obj_name = line_split[1];
}
//check if the line starts with v -> vertex.
if(algorithm::startsWith(currentLine, "v ")){
//construct new vertex position.
std::vector<std::string> line_split = algorithm::split(currentLine,' ');
float x = std::stof(line_split[1]);
float y = std::stof(line_split[2]);
float z = std::stof(line_split[3]);
Vect3 pos = Vect3(x,y,z);
Positions.push_back(pos);
}
//check if the line starts with vt -> vertex uv.
if(algorithm::startsWith(currentLine, "vt ")){
//construct new vertex uv.
std::vector<std::string> line_split = algorithm::split(currentLine,' ');
float u = std::stof(line_split[1]);
float v = std::stof(line_split[2]);
Vect2 uv = Vect2(u,v);
UVs.push_back(uv);
}
//check if the line starts with vn -> vertex normals.
if(algorithm::startsWith(currentLine, "vn ")){
//construct new vertex normal.
std::vector<std::string> line_split = algorithm::split(currentLine,' ');
float x = std::stof(line_split[1]);
float y = std::stof(line_split[2]);
float z = std::stof(line_split[3]);
Vect3 normal = Vect3(x,y,z);
Normals.push_back(normal);
}
//check if the line starts with f -> constuct faces.
if(algorithm::startsWith(currentLine, "f ")){
//construct new vertex position.
std::vector<std::string> line_split = algorithm::split(currentLine,' ');
//@NOTE: this only works when mesh is already triangulated.
//Parse all vertices.
std::vector<std::string> vertex1 = algorithm::split(line_split[1],'/');
std::vector<std::string> vertex2 = algorithm::split(line_split[2],'/');
std::vector<std::string> vertex3 = algorithm::split(line_split[3],'/');
if(vertex1.size() <= 1){
//VERTEX 1
Vect3 position = Positions[std::stoi(vertex1[0])-1];
Vertex v1(position);
vertices.push_back(v1);
//VERTEX 2
position = Positions[std::stoi(vertex2[0])-1];
Vertex v2(position);
vertices.push_back(v2);
//VERTEX 3
position = Positions[std::stoi(vertex3[0])-1];
Vertex v3(position);
vertices.push_back(v3);
//Add to Indices array.
//calculate the index number
//The 3 comes from 3 vertices per face.
unsigned int index = vertices.size() - 3;
V_indices.push_back(index);
V_indices.push_back(index+1);
V_indices.push_back(index+2);
}
//check if T exist.
else if(vertex1[1] == ""){
//NO Uv
//V -> index in the positions array.
//N -> index in the normals array.
//VERTEX 1
Vect3 position = Positions[std::stoi(vertex1[0])-1];
Vect3 normal = Normals[std::stoi(vertex1[2])-1];
Vertex v1(position,normal);
vertices.push_back(v1);
//VERTEX 2
position = Positions[std::stoi(vertex2[0])-1];
normal = Normals[std::stoi(vertex2[2])-1];
Vertex v2(position,normal);
vertices.push_back(v2);
//VERTEX 3
position = Positions[std::stoi(vertex3[0])-1];
normal = Normals[std::stoi(vertex3[2])-1];
Vertex v3(position,normal);
vertices.push_back(v3);
//Add to Indices array.
//calculate the index number
//The 3 comes from 3 vertices per face.
unsigned int index = vertices.size() - 3;
V_indices.push_back(index);
V_indices.push_back(index+1);
V_indices.push_back(index+2);
}else if (vertex1[1] != ""){
//We have UV
//V -> index in the positions array.
//T -> index of UV
//N -> index in the normals array.
//VERTEX 1
Vect3 position = Positions[std::stoi(vertex1[0])-1];
Vect2 uv = UVs[std::stoi(vertex1[1])-1];
Vect3 normal = Normals[std::stoi(vertex1[2])-1];
Vertex v1(position,normal,uv);
vertices.push_back(v1);
//VERTEX 2
position = Positions[std::stoi(vertex2[0])-1];
uv = UVs[std::stoi(vertex2[1])-1];
normal = Normals[std::stoi(vertex2[2])-1];
Vertex v2(position,normal,uv);
vertices.push_back(v2);
//VERTEX 3
position = Positions[std::stoi(vertex3[0])-1];
uv = UVs[std::stoi(vertex3[1])-1];
normal = Normals[std::stoi(vertex3[2])-1];
Vertex v3(position,normal,uv);
vertices.push_back(v3);
//Add to Indices array.
//calculate the index number
//The 3 comes from 3 vertices per face.
unsigned int index = vertices.size() - 3;
V_indices.push_back(index);
V_indices.push_back(index+1);
V_indices.push_back(index+2);
}
//We can check here in which format. V/T/N, V//N, V//, ...
//For now we ignore this and use V//N.
}
}
//close stream
file.close();
Positions.clear();
Normals.clear();
UVs.clear();
//reorder the arrays so the coresponding index match the position,uv and normal.
for (Vertex v: vertices) {
Positions.push_back(v.getPosition());
Normals.push_back(v.getNormal());
UVs.push_back(v.getUV());
}
//Load mesh data.
_mesh = Mesh(smoothShading,obj_name, Positions, Normals, UVs, V_indices);
//return true, succes.
return true;
在此之后，模型被插入到网格结构中以进行更快的交叉测试。
for(int i= 0;i<mesh._indices.size();i=i+3){
Triangle* tri;
if(mesh.smoothShading){
tri = new SmoothTriangle(Point3(mesh._positions[mesh._indices[i]]),
Point3(mesh._positions[mesh._indices[i+1]]),
Point3(mesh._positions[mesh._indices[i+2]]),
Normal(mesh._normals[mesh._indices[i]]),
Normal(mesh._normals[mesh._indices[i+1]]),
Normal(mesh._normals[mesh._indices[i+2]]),material);
}else{
tri = new Triangle(Point3(mesh._positions[mesh._indices[i]]),
Point3(mesh._positions[mesh._indices[i+1]]),Point3(mesh._positions[mesh._indices[i+2]]),Normal(mesh._normals[mesh._indices[i]]),material);
}
add_object(tri);
}
constructCells();
可能需要添加的是用于插入法线的代码
Normal SmoothTriangle::calculate_normal(double gamma, double beta){
return (Normal((1 - beta - gamma) * n0 + beta * n1 + gamma * n2)).normalize();
}
固定的
我修复了这个问题。它不在我的 OBJ 加载器中。该模型是从搅拌机中导出的，并且在导出时应用了所有修改器，但是在导出到 .obj 文件后，Solidify 导致一些背面与正面剪辑。删除此修饰符后，一切都恢复了“正常”（只是一个有趣的双关语来完成这个答案）