OpenGL - Model Loading

一个非常流行的模型导入库是Assimp，它是Open Asset Import Library的缩写。Assimp能够导入很多种不同的模型文件格式（并也能够导出部分的格式），它会将所有的模型数据加载至Assimp的通用数据结构中。当Assimp加载完模型之后，我们就能够从Assimp的数据结构中提取我们所需的所有数据了。由于Assimp的数据结构保持不变，不论导入的是什么种类的文件格式，它都能够将我们从这些不同的文件格式中抽象出来，用同一种方式访问我们需要的数据。

当使用Assimp导入一个模型的时候，它通常会将整个模型加载进一个场景(Scene)对象，它会包含导入的模型/场景中的所有数据。Assimp会将场景载入为一系列的节点(Node)，每个节点包含了场景对象中所储存数据的索引，每个节点都可以有任意数量的子节点。Assimp数据结构的（简化）模型如下：

• 和材质和网格(Mesh)一样，所有的场景/模型数据都包含在Scene对象中。Scene对象也包含了场景根节点的引用。
• 场景的Root node（根节点）可能包含子节点（和其它的节点一样），它会有一系列指向场景对象中mMeshes数组中储存的网格数据的索引。Scene下的mMeshes数组储存了真正的Mesh对象，节点中的mMeshes数组保存的只是场景中网格数组的索引。
• 一个Mesh对象本身包含了渲染所需要的所有相关数据，像是顶点位置、法向量、纹理坐标、面(Face)和物体的材质。
• 一个网格包含了多个面。Face代表的是物体的渲染图元(Primitive)（三角形、方形、点）。一个面包含了组成图元的顶点的索引。由于顶点和索引是分开的，使用一个索引缓冲来渲染是非常简单的（见你好，三角形）。
• 最后，一个网格也包含了一个Material对象，它包含了一些函数能让我们获取物体的材质属性，比如说颜色和纹理贴图（比如漫反射和镜面光贴图）。

首先定义绘制需要的Mesh类：

#ifndef MESH_H
#define MESH_H

#include <glad/glad.h> // holds all OpenGL type declarations

#include <glm/glm.hpp>
#include <glm/gtc/matrix_transform.hpp>

#include <learnopengl/shader.h>

#include <string>
#include <vector>
using namespace std;

#define MAX_BONE_INFLUENCE 4

struct Vertex {
    
    // position
    glm::vec3 Position;
    // normal
    glm::vec3 Normal;
    // texCoords
    glm::vec2 TexCoords;
    // tangent
    glm::vec3 Tangent;
    // bitangent
    glm::vec3 Bitangent;
	//bone indexes which will influence this vertex
	int m_BoneIDs[MAX_BONE_INFLUENCE];
	//weights from each bone
	float m_Weights[MAX_BONE_INFLUENCE];
};

struct Texture {
    
    unsigned int id;
    string type;
    string path;
};

class Mesh {
    
public:
    // mesh Data
    vector<Vertex>       vertices;
    vector<unsigned int> indices;
    vector<Texture>      textures;
    unsigned int VAO;

    // constructor
    Mesh(vector<Vertex> vertices, vector<unsigned int> indices, vector<Texture> textures)
    {
    
        this->vertices = vertices;
        this->indices = indices;
        this->textures = textures;

        // now that we have all the required data, set the vertex buffers and its attribute pointers.
        setupMesh();
    }

    // render the mesh
    void Draw(Shader &shader) 
    {
    
        // bind appropriate textures
        unsigned int diffuseNr  = 1;
        unsigned int specularNr = 1;
        unsigned int normalNr   = 1;
        unsigned int heightNr   = 1;
        for(unsigned int i = 0; i < textures.size(); i++)
        {
    
            glActiveTexture(GL_TEXTURE0 + i); // active proper texture unit before binding
            // retrieve texture number (the N in diffuse_textureN)
            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++); // transfer unsigned int to string
            else if(name == "texture_normal")
                number = std::to_string(normalNr++); // transfer unsigned int to string
             else if(name == "texture_height")
                number = std::to_string(heightNr++); // transfer unsigned int to string

            // now set the sampler to the correct texture unit
            glUniform1i(glGetUniformLocation(shader.ID, (name + number).c_str()), i);
            // and finally bind the texture
            glBindTexture(GL_TEXTURE_2D, textures[i].id);
        }
        
        // draw mesh
        glBindVertexArray(VAO);
        glDrawElements(GL_TRIANGLES, static_cast<unsigned int>(indices.size()), GL_UNSIGNED_INT, 0);
        glBindVertexArray(0);

        // always good practice to set everything back to defaults once configured.
        glActiveTexture(GL_TEXTURE0);
    }

private:
    // render data 
    unsigned int VBO, EBO;

    // initializes all the buffer objects/arrays
    void setupMesh()
    {
    
        // create buffers/arrays
        glGenVertexArrays(1, &VAO);
        glGenBuffers(1, &VBO);
        glGenBuffers(1, &EBO);

        glBindVertexArray(VAO);
        // load data into vertex buffers
        glBindBuffer(GL_ARRAY_BUFFER, VBO);
        // A great thing about structs is that their memory layout is sequential for all its items.
        // The effect is that we can simply pass a pointer to the struct and it translates perfectly to a glm::vec3/2 array which
        // again translates to 3/2 floats which translates to a byte array.
        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);

        // set the vertex attribute pointers
        // vertex Positions
        glEnableVertexAttribArray(0);	
        glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, sizeof(Vertex), (void*)0);
        // vertex normals
        glEnableVertexAttribArray(1);	
        glVertexAttribPointer(1, 3, GL_FLOAT, GL_FALSE, sizeof(Vertex), (void*)offsetof(Vertex, Normal));
        // vertex texture coords
        glEnableVertexAttribArray(2);	
        glVertexAttribPointer(2, 2, GL_FLOAT, GL_FALSE, sizeof(Vertex), (void*)offsetof(Vertex, TexCoords));
        // vertex tangent
        glEnableVertexAttribArray(3);
        glVertexAttribPointer(3, 3, GL_FLOAT, GL_FALSE, sizeof(Vertex), (void*)offsetof(Vertex, Tangent));
        // vertex bitangent
        glEnableVertexAttribArray(4);
        glVertexAttribPointer(4, 3, GL_FLOAT, GL_FALSE, sizeof(Vertex), (void*)offsetof(Vertex, Bitangent));
		// ids
		glEnableVertexAttribArray(5);
		glVertexAttribIPointer(5, 4, GL_INT, sizeof(Vertex), (void*)offsetof(Vertex, m_BoneIDs));

		// weights
		glEnableVertexAttribArray(6);
		glVertexAttribPointer(6, 4, GL_FLOAT, GL_FALSE, sizeof(Vertex), (void*)offsetof(Vertex, m_Weights));
        glBindVertexArray(0);
    }
};
#endif

然后定义导入模型的Model类：

#ifndef MODEL_H
#define MODEL_H

#include <glad/glad.h> 

#include <glm/glm.hpp>
#include <glm/gtc/matrix_transform.hpp>
#include <stb_image.h>
#include <assimp/Importer.hpp>
#include <assimp/scene.h>
#include <assimp/postprocess.h>

#include <learnopengl/mesh.h>
#include <learnopengl/shader.h>

#include <string>
#include <fstream>
#include <sstream>
#include <iostream>
#include <map>
#include <vector>
using namespace std;

unsigned int TextureFromFile(const char *path, const string &directory, bool gamma = false);

class Model 
{
    
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;

    // constructor, expects a filepath to a 3D model.
    Model(string const &path, bool gamma = false) : gammaCorrection(gamma)
    {
    
        loadModel(path);
    }

    // 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);
    }
    
private:
    // loads a model with supported ASSIMP extensions from file and stores the resulting meshes in the meshes vector.
    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);
        }

    }

    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);
    }

    // checks all material textures of a given type and loads the textures if they're not loaded yet.
    // the required info is returned as a Texture struct.
    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 unnecesery load duplicate textures.
            }
        }
        return textures;
    }
};


unsigned int TextureFromFile(const char *path, const string &directory, bool gamma)
{
    
    string filename = string(path);
    filename = directory + '/' + filename;

    unsigned int textureID;
    glGenTextures(1, &textureID);

    int width, height, nrComponents;
    unsigned char *data = stbi_load(filename.c_str(), &width, &height, &nrComponents, 0);
    if (data)
    {
    
        GLenum format;
        if (nrComponents == 1)
            format = GL_RED;
        else if (nrComponents == 3)
            format = GL_RGB;
        else if (nrComponents == 4)
            format = GL_RGBA;

        glBindTexture(GL_TEXTURE_2D, textureID);
        glTexImage2D(GL_TEXTURE_2D, 0, format, width, height, 0, format, GL_UNSIGNED_BYTE, data);
        glGenerateMipmap(GL_TEXTURE_2D);

        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_REPEAT);
        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_REPEAT);
        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR_MIPMAP_LINEAR);
        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);

        stbi_image_free(data);
    }
    else
    {
    
        std::cout << "Texture failed to load at path: " << path << std::endl;
        stbi_image_free(data);
    }

    return textureID;
}
#endif

完整的绘制源码可以参考： -> ->