今天我们来学一些关于模型加载的内容：
之前的内容中，我们始终使用我们的两个图片作为我们的操作对象（笑脸和箱子），但现在让我们来考虑导入一些真正的cool的模型。
Assimp
要导入模型，就需要这个模型的加载库。
本质上来说ASSIMP库就是一个插件的集合，针对不同格式的模型我们使用不同的插件来进行解析。可以看到ASSIMP库的工作原理就是将导入的模型文件，划分为节点、网格和材质，我们的节点是树状结构，而节点中存在的网格值实际上是通过索引在网格中读取的，真正的网格值存储在mMeshes[]数组中。
现在我们需要去实现属于我们的Mesh类：
Mesh
我们可以回想网格中需要什么，一个网格中肯定需要有顶点，当然还需要纹理，我们将这两个要素写成两个结构体：
struct Vertex {
glm::vec3 Position;
glm::vec3 Normal;
glm::vec2 TexCoords;
};
struct Texture {
unsigned int id;
string type;
};顶点中包含位置向量、法线向量与纹理向量坐标，而纹理中包含一个ID与一个类型；
现在让我们完成基本的Mesh类的成员：
class Mesh {
public:
/* 网格数据 */
vector<Vertex> vertices;
vector<unsigned int> indices;
vector<Texture> textures;
/* 函数 */
Mesh(vector<Vertex> vertices, vector<unsigned int> indices, vector<Texture> textures);
void Draw(Shader &shader);
private:
/* 渲染数据 */
unsigned int VAO, VBO, EBO;
/* 函数 */
void setupMesh();
}; 顶点、纹理的数组与索引数组，构造函数与绘制函数（参数为我们之前写好的着色器），将VAO、VBO、EBO都作为private成员。
Mesh(vector<Vertex> vertices, vector<unsigned int> indices, vector<Texture> textures)
{
this->vertices = vertices;
this->indices = indices;
this->textures = textures;
setupMesh();
}这是构造函数的内容，我们调用了setupMesh函数来初始化网格。
既然要初始化网格，那当然得让网格类中的每个成员都初始化，回想我们的成员，显然顶点、纹理和索引是后续接收的输入，那么我们主要得初始化我们的VAO、VBO、EBO：
void setupMesh()
{
glGenVertexArrays(1, &VAO);
glGenBuffers(1, &VBO);
glGenBuffers(1, &EBO);
glBindVertexArray(VAO);
glBindBuffer(GL_ARRAY_BUFFER, VBO);
glBufferData(GL_ARRAY_BUFFER, vertices.size() * sizeof(Vertex), &vertices[0], GL_STATIC_DRAW);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, EBO);
glBufferData(GL_ELEMENT_ARRAY_BUFFER, indices.size() * sizeof(unsigned int),
&indices[0], GL_STATIC_DRAW);
// 顶点位置
glEnableVertexAttribArray(0);
glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, sizeof(Vertex), (void*)0);
// 顶点法线
glEnableVertexAttribArray(1);
glVertexAttribPointer(1, 3, GL_FLOAT, GL_FALSE, sizeof(Vertex), (void*)offsetof(Vertex, Normal));
// 顶点纹理坐标
glEnableVertexAttribArray(2);
glVertexAttribPointer(2, 2, GL_FLOAT, GL_FALSE, sizeof(Vertex), (void*)offsetof(Vertex, TexCoords));
glBindVertexArray(0);
} 可以看到和之前我们写的代码并没有太大区别。
然后是定义我们的Draw函数：
void Draw(Shader shader)
{
unsigned int diffuseNr = 1;
unsigned int specularNr = 1;
for(unsigned int i = 0; i < textures.size(); i++)
{
glActiveTexture(GL_TEXTURE0 + i); // 在绑定之前激活相应的纹理单元
// 获取纹理序号（diffuse_textureN 中的 N）
string number;
string name = textures[i].type;
if(name == "texture_diffuse")
number = std::to_string(diffuseNr++);
else if(name == "texture_specular")
number = std::to_string(specularNr++);
shader.setInt(("material." + name + number).c_str(), i);
glBindTexture(GL_TEXTURE_2D, textures[i].id);
}
glActiveTexture(GL_TEXTURE0);
// 绘制网格
glBindVertexArray(VAO);
glDrawElements(GL_TRIANGLES, indices.size(), GL_UNSIGNED_INT, 0);
glBindVertexArray(0);
}在这段代码中，我们主要做的事就是将纹理数组中的纹理激活并获取其类型（漫反射/镜面反射），然后绑定并整合后绑定在VAO上并使用glDrawElements来绘制。
创建好网格类之后，让我们来创建最终的Model类：
Model
我们先来看public的部分：
public:
// model data
vector<Texture> textures_loaded; // stores all the textures loaded so far, optimization to make sure textures aren't loaded more than once.
vector<Mesh> meshes;
string directory;
bool gammaCorrection;//是否开启伽马矫正
// constructor, expects a filepath to a 3D model.
Model(string const& path, bool gamma = false) : gammaCorrection(gamma)//默认不开启伽马矫正
{
loadModel(path);//加载模型
}
// draws the model, and thus all its meshes
void Draw(Shader& shader)
{
for (unsigned int i = 0; i < meshes.size(); i++)
meshes[i].Draw(shader);//使用Mesh类中的Draw方法
}我们主要关注的是纹理和网格的部分，因为我们知道网格中已经定义好了顶点和纹理了，而我们这里还需要一个纹理数组来获得导入模型的纹理后再传入我们的网格。
现在让我们来看看具体的loadModel函数的实现：
void loadModel(string const& path)
{
// read file via ASSIMP
Assimp::Importer importer;
const aiScene* scene = importer.ReadFile(path, aiProcess_Triangulate | aiProcess_GenSmoothNormals | aiProcess_FlipUVs | aiProcess_CalcTangentSpace);
// check for errors
if (!scene || scene->mFlags & AI_SCENE_FLAGS_INCOMPLETE || !scene->mRootNode) // if is Not Zero
{
cout << "ERROR::ASSIMP:: " << importer.GetErrorString() << endl;
return;
}
// retrieve the directory path of the filepath
directory = path.substr(0, path.find_last_of('/'));
// process ASSIMP's root node recursively
processNode(scene->mRootNode, scene);
}
// processes a node in a recursive fashion. Processes each individual mesh located at the node and repeats this process on its children nodes (if any).
void processNode(aiNode* node, const aiScene* scene)
{
// process each mesh located at the current node
for (unsigned int i = 0; i < node->mNumMeshes; i++)
{
// the node object only contains indices to index the actual objects in the scene.
// the scene contains all the data, node is just to keep stuff organized (like relations between nodes).
aiMesh* mesh = scene->mMeshes[node->mMeshes[i]];
meshes.push_back(processMesh(mesh, scene));
}
// after we've processed all of the meshes (if any) we then recursively process each of the children nodes
for (unsigned int i = 0; i < node->mNumChildren; i++)
{
processNode(node->mChildren[i], scene);
}
}实例化一个Assimp库中的Importer类的对象，去读取文件路径下的。。。，如果发现异常就报错；更新路径并执行我们的processNode函数,具体内容包括将每个节点下的网格传输到我们之前的meshes数组中，注意这里使用递归，以形成树状结构。
类似的，我们的processMesh的代码也是这样：
Mesh processMesh(aiMesh* mesh, const aiScene* scene)
{
// data to fill
vector<Vertex> vertices;
vector<unsigned int> indices;
vector<Texture> textures;
// walk through each of the mesh's vertices
for (unsigned int i = 0; i < mesh->mNumVertices; i++)
{
Vertex vertex;
glm::vec3 vector; // we declare a placeholder vector since assimp uses its own vector class that doesn't directly convert to glm's vec3 class so we transfer the data to this placeholder glm::vec3 first.
// positions
vector.x = mesh->mVertices[i].x;
vector.y = mesh->mVertices[i].y;
vector.z = mesh->mVertices[i].z;
vertex.Position = vector;
// normals
if (mesh->HasNormals())
{
vector.x = mesh->mNormals[i].x;
vector.y = mesh->mNormals[i].y;
vector.z = mesh->mNormals[i].z;
vertex.Normal = vector;
}
// texture coordinates
if (mesh->mTextureCoords[0]) // does the mesh contain texture coordinates?
{
glm::vec2 vec;
// a vertex can contain up to 8 different texture coordinates. We thus make the assumption that we won't
// use models where a vertex can have multiple texture coordinates so we always take the first set (0).
vec.x = mesh->mTextureCoords[0][i].x;
vec.y = mesh->mTextureCoords[0][i].y;
vertex.TexCoords = vec;
// tangent
vector.x = mesh->mTangents[i].x;
vector.y = mesh->mTangents[i].y;
vector.z = mesh->mTangents[i].z;
vertex.Tangent = vector;
// bitangent
vector.x = mesh->mBitangents[i].x;
vector.y = mesh->mBitangents[i].y;
vector.z = mesh->mBitangents[i].z;
vertex.Bitangent = vector;
}
else
vertex.TexCoords = glm::vec2(0.0f, 0.0f);
vertices.push_back(vertex);
}
// now wak through each of the mesh's faces (a face is a mesh its triangle) and retrieve the corresponding vertex indices.
for (unsigned int i = 0; i < mesh->mNumFaces; i++)
{
aiFace face = mesh->mFaces[i];
// retrieve all indices of the face and store them in the indices vector
for (unsigned int j = 0; j < face.mNumIndices; j++)
indices.push_back(face.mIndices[j]);
}
// process materials
aiMaterial* material = scene->mMaterials[mesh->mMaterialIndex];
// we assume a convention for sampler names in the shaders. Each diffuse texture should be named
// as 'texture_diffuseN' where N is a sequential number ranging from 1 to MAX_SAMPLER_NUMBER.
// Same applies to other texture as the following list summarizes:
// diffuse: texture_diffuseN
// specular: texture_specularN
// normal: texture_normalN
// 1. diffuse maps
vector<Texture> diffuseMaps = loadMaterialTextures(material, aiTextureType_DIFFUSE, "texture_diffuse");
textures.insert(textures.end(), diffuseMaps.begin(), diffuseMaps.end());
// 2. specular maps
vector<Texture> specularMaps = loadMaterialTextures(material, aiTextureType_SPECULAR, "texture_specular");
textures.insert(textures.end(), specularMaps.begin(), specularMaps.end());
// 3. normal maps
std::vector<Texture> normalMaps = loadMaterialTextures(material, aiTextureType_HEIGHT, "texture_normal");
textures.insert(textures.end(), normalMaps.begin(), normalMaps.end());
// 4. height maps
std::vector<Texture> heightMaps = loadMaterialTextures(material, aiTextureType_AMBIENT, "texture_height");
textures.insert(textures.end(), heightMaps.begin(), heightMaps.end());
// return a mesh object created from the extracted mesh data
return Mesh(vertices, indices, textures);
}我们将Mesh按照之前的设想拆分成：顶点属性、索引数据以及材质纹理，顶点属性中包括顶点坐标、法线坐标以及纹理坐标，材质纹理分为漫反射、镜面反射、法线贴图和高度贴图。
vector<Texture> loadMaterialTextures(aiMaterial* mat, aiTextureType type, string typeName)
{
vector<Texture> textures;
for (unsigned int i = 0; i < mat->GetTextureCount(type); i++)
{
aiString str;
mat->GetTexture(type, i, &str);
// check if texture was loaded before and if so, continue to next iteration: skip loading a new texture
bool skip = false;
for (unsigned int j = 0; j < textures_loaded.size(); j++)
{
if (std::strcmp(textures_loaded[j].path.data(), str.C_Str()) == 0)
{
textures.push_back(textures_loaded[j]);
skip = true; // a texture with the same filepath has already been loaded, continue to next one. (optimization)
break;
}
}
if (!skip)
{ // if texture hasn't been loaded already, load it
Texture texture;
texture.id = TextureFromFile(str.C_Str(), this->directory);
texture.type = typeName;
texture.path = str.C_Str();
textures.push_back(texture);
textures_loaded.push_back(texture); // store it as texture loaded for entire model, to ensure we won't unnecessary load duplicate textures.
}
}
return textures;
}同样的，加载我们的材质纹理，不难发现我们的model类内部的方法做的事就是从一开始导入的模型开始逐层地拆解，先得到节点，然后得到网格，最后得到材质。我们所有所做的事无非就是读取到模型后进行拆解并将拆解后的信息总结到我们自己定义的顶点、网格数组中。
通过我们的Model类之后，我们就可以读取到导入的模型的信息了，现在让我们在主程序中将导入的模型放入场景中：
Model ourModel("D:/Downloads/backpack/backpack.obj");
就可以得到如下效果：
