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模型加载

在实际项目中，我们很少手动定义顶点数据，而是从3D建模软件导出的模型文件中加载。本章介绍如何使用Assimp库加载复杂的3D模型。

模型文件格式​

常见的3D模型文件格式：

格式说明
OBJ简单、广泛支持，但不支持动画
FBXAutodesk格式，支持动画和骨骼
glTF现代格式，Web标准，支持PBR
COLLADAXML格式，功能完整
3DS3ds Max旧格式
BLENDBlender原生格式

Assimp库​

Assimp（Open Asset Import Library）是一个跨平台的3D模型加载库，支持多种格式。

安装Assimp​

# Windows (vcpkg)
vcpkg install assimp

# Linux
sudo apt-get install libassimp-dev

# macOS
brew install assimp

CMake配置​

find_package(assimp REQUIRED)
target_link_libraries(your_target assimp::assimp)

模型结构​

一个典型的模型结构：

Model
├── Mesh[]
│   ├── Vertex[]
│   │   ├── Position
│   │   ├── Normal
│   │   ├── TexCoords
│   │   └── Tangent/BiTangent
│   ├── Indices[]
│   └── Texture[]
│       ├── diffuse
│       ├── specular
│       └── normal
└── Node hierarchy

网格类（Mesh）​

// Mesh.h
#pragma once

#include <glad/glad.h>
#include <glm/glm.hpp>
#include <glm/gtc/matrix_transform.hpp>
#include <string>
#include <vector>

struct Vertex {
    glm::vec3 Position;
    glm::vec3 Normal;
    glm::vec2 TexCoords;
    glm::vec3 Tangent;
    glm::vec3 Bitangent;
};

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

class Mesh {
public:
    std::vector<Vertex> vertices;
    std::vector<unsigned int> indices;
    std::vector<Texture> textures;
    unsigned int VAO;
    
    Mesh(std::vector<Vertex> vertices, std::vector<unsigned int> indices, 
         std::vector<Texture> textures) {
        this->vertices = vertices;
        this->indices = indices;
        this->textures = textures;
        
        setupMesh();
    }
    
    void Draw(Shader& shader) {
        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);
            
            std::string number;
            std::string name = textures[i].type;
            if (name == "texture_diffuse")
                number = std::to_string(diffuseNr++);
            else if (name == "texture_specular")
                number = std::to_string(specularNr++);
            else if (name == "texture_normal")
                number = std::to_string(normalNr++);
            else if (name == "texture_height")
                number = std::to_string(heightNr++);
            
            glUniform1i(glGetUniformLocation(shader.ID, (name + number).c_str()), i);
            glBindTexture(GL_TEXTURE_2D, textures[i].id);
        }
        
        glBindVertexArray(VAO);
        glDrawElements(GL_TRIANGLES, indices.size(), GL_UNSIGNED_INT, 0);
        glBindVertexArray(0);
        
        glActiveTexture(GL_TEXTURE0);
    }
    
private:
    unsigned int 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);
        
        // 位置
        glEnableVertexAttribArray(0);
        glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, sizeof(Vertex), (void*)0);
        
        // 法线
        glEnableVertexAttribArray(1);
        glVertexAttribPointer(1, 3, GL_FLOAT, GL_FALSE, sizeof(Vertex), 
                              (void*)offsetof(Vertex, Normal));
        
        // 纹理坐标
        glEnableVertexAttribArray(2);
        glVertexAttribPointer(2, 2, GL_FLOAT, GL_FALSE, sizeof(Vertex), 
                              (void*)offsetof(Vertex, TexCoords));
        
        // 切线
        glEnableVertexAttribArray(3);
        glVertexAttribPointer(3, 3, GL_FLOAT, GL_FALSE, sizeof(Vertex), 
                              (void*)offsetof(Vertex, Tangent));
        
        // 副切线
        glEnableVertexAttribArray(4);
        glVertexAttribPointer(4, 3, GL_FLOAT, GL_FALSE, sizeof(Vertex), 
                              (void*)offsetof(Vertex, Bitangent));
        
        glBindVertexArray(0);
    }
};

模型类（Model）​

// Model.h
#pragma once

#include <glad/glad.h>
#include <glm/glm.hpp>
#include <glm/gtc/matrix_transform.hpp>
#include <assimp/Importer.hpp>
#include <assimp/scene.h>
#include <assimp/postprocess.h>
#include "Mesh.h"
#include "Shader.h"
#include <string>
#include <vector>
#include <map>

unsigned int TextureFromFile(const char* path, const std::string& directory);

class Model {
public:
    std::vector<Texture> textures_loaded;
    std::vector<Mesh> meshes;
    std::string directory;
    bool gammaCorrection;
    
    Model(std::string const& path, bool gamma = false) : gammaCorrection(gamma) {
        loadModel(path);
    }
    
    void Draw(Shader& shader) {
        for (unsigned int i = 0; i < meshes.size(); i++)
            meshes[i].Draw(shader);
    }
    
private:
    void loadModel(std::string const& path) {
        Assimp::Importer importer;
        const aiScene* scene = importer.ReadFile(path, 
            aiProcess_Triangulate | aiProcess_GenSmoothNormals | 
            aiProcess_FlipUVs | aiProcess_CalcTangentSpace);
        
        if (!scene || scene->mFlags & AI_SCENE_FLAGS_INCOMPLETE || !scene->mRootNode) {
            std::cout << "ERROR::ASSIMP:: " << importer.GetErrorString() << std::endl;
            return;
        }
        
        directory = path.substr(0, path.find_last_of('/'));
        
        processNode(scene->mRootNode, scene);
    }
    
    void processNode(aiNode* node, const aiScene* scene) {
        for (unsigned int i = 0; i < node->mNumMeshes; i++) {
            aiMesh* mesh = scene->mMeshes[node->mMeshes[i]];
            meshes.push_back(processMesh(mesh, scene));
        }
        
        for (unsigned int i = 0; i < node->mNumChildren; i++) {
            processNode(node->mChildren[i], scene);
        }
    }
    
    Mesh processMesh(aiMesh* mesh, const aiScene* scene) {
        std::vector<Vertex> vertices;
        std::vector<unsigned int> indices;
        std::vector<Texture> textures;
        
        for (unsigned int i = 0; i < mesh->mNumVertices; i++) {
            Vertex vertex;
            
            glm::vec3 vector;
            vector.x = mesh->mVertices[i].x;
            vector.y = mesh->mVertices[i].y;
            vector.z = mesh->mVertices[i].z;
            vertex.Position = vector;
            
            if (mesh->HasNormals()) {
                vector.x = mesh->mNormals[i].x;
                vector.y = mesh->mNormals[i].y;
                vector.z = mesh->mNormals[i].z;
                vertex.Normal = vector;
            }
            
            if (mesh->mTextureCoords[0]) {
                glm::vec2 vec;
                vec.x = mesh->mTextureCoords[0][i].x;
                vec.y = mesh->mTextureCoords[0][i].y;
                vertex.TexCoords = vec;
                
                vector.x = mesh->mTangents[i].x;
                vector.y = mesh->mTangents[i].y;
                vector.z = mesh->mTangents[i].z;
                vertex.Tangent = vector;
                
                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);
        }
        
        for (unsigned int i = 0; i < mesh->mNumFaces; i++) {
            aiFace face = mesh->mFaces[i];
            for (unsigned int j = 0; j < face.mNumIndices; j++)
                indices.push_back(face.mIndices[j]);
        }
        
        aiMaterial* material = scene->mMaterials[mesh->mMaterialIndex];
        
        std::vector<Texture> diffuseMaps = loadMaterialTextures(material, 
            aiTextureType_DIFFUSE, "texture_diffuse");
        textures.insert(textures.end(), diffuseMaps.begin(), diffuseMaps.end());
        
        std::vector<Texture> specularMaps = loadMaterialTextures(material, 
            aiTextureType_SPECULAR, "texture_specular");
        textures.insert(textures.end(), specularMaps.begin(), specularMaps.end());
        
        std::vector<Texture> normalMaps = loadMaterialTextures(material, 
            aiTextureType_HEIGHT, "texture_normal");
        textures.insert(textures.end(), normalMaps.begin(), normalMaps.end());
        
        std::vector<Texture> heightMaps = loadMaterialTextures(material, 
            aiTextureType_AMBIENT, "texture_height");
        textures.insert(textures.end(), heightMaps.begin(), heightMaps.end());
        
        return Mesh(vertices, indices, textures);
    }
    
    std::vector<Texture> loadMaterialTextures(aiMaterial* mat, aiTextureType type, 
                                               std::string typeName) {
        std::vector<Texture> textures;
        
        for (unsigned int i = 0; i < mat->GetTextureCount(type); i++) {
            aiString str;
            mat->GetTexture(type, i, &str);
            
            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;
                    break;
                }
            }
            
            if (!skip) {
                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);
            }
        }
        
        return textures;
    }
};

unsigned int TextureFromFile(const char* path, const std::string& directory) {
    std::string filename = std::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;
}

使用模型类​

#include "Model.h"
#include "Shader.h"
#include "Camera.h"

int main() {
    // 初始化...
    
    Shader ourShader("model.vs", "model.fs");
    Model ourModel("resources/objects/backpack/backpack.obj");
    
    while (!glfwWindowShouldClose(window)) {
        processInput(window);
        
        glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
        
        ourShader.use();
        
        glm::mat4 projection = glm::perspective(glm::radians(camera.Zoom), 
                                                 800.0f / 600.0f, 0.1f, 100.0f);
        glm::mat4 view = camera.GetViewMatrix();
        glm::mat4 model = glm::mat4(1.0f);
        model = glm::translate(model, glm::vec3(0.0f, 0.0f, 0.0f));
        model = glm::scale(model, glm::vec3(1.0f, 1.0f, 1.0f));
        
        ourShader.setMat4("projection", projection);
        ourShader.setMat4("view", view);
        ourShader.setMat4("model", model);
        
        ourModel.Draw(ourShader);
        
        glfwSwapBuffers(window);
        glfwPollEvents();
    }
    
    glfwTerminate();
    return 0;
}

模型着色器​

// model.vs
#version 330 core
layout (location = 0) in vec3 aPos;
layout (location = 1) in vec3 aNormal;
layout (location = 2) in vec2 aTexCoords;

out vec2 TexCoords;

uniform mat4 model;
uniform mat4 view;
uniform mat4 projection;

void main() {
    TexCoords = aTexCoords;
    gl_Position = projection * view * model * vec4(aPos, 1.0);
}
// model.fs
#version 330 core
out vec4 FragColor;

in vec2 TexCoords;

uniform sampler2D texture_diffuse1;

void main() {
    FragColor = texture(texture_diffuse1, TexCoords);
}

Assimp后处理标志​

加载模型时可以指定后处理标志：

标志说明
aiProcess_Triangulate将所有面转换为三角形
aiProcess_GenNormals为没有法线的面生成法线
aiProcess_GenSmoothNormals生成平滑法线
aiProcess_FlipUVs翻转纹理坐标Y轴
aiProcess_CalcTangentSpace计算切线和副切线
aiProcess_JoinIdenticalVertices合并相同顶点
aiProcess_ImproveCacheLocality优化顶点缓存
aiProcess_RemoveRedundantMaterials移除冗余材质

优化建议​

1. 纹理缓存​

避免重复加载相同的纹理：

std::map<std::string, unsigned int> textureCache;

unsigned int loadTextureCached(const std::string& path) {
    if (textureCache.find(path) != textureCache.end()) {
        return textureCache[path];
    }
    unsigned int id = loadTexture(path);
    textureCache[path] = id;
    return id;
}

2. 实例化渲染​

对于相同模型的多个实例，使用实例化渲染：

glDrawElementsInstanced(GL_TRIANGLES, indexCount, GL_UNSIGNED_INT, 0, instanceCount);

3. LOD（细节层次）​

根据距离使用不同精度的模型：

Model* getModelByDistance(float distance) {
    if (distance < 10.0f) return highDetailModel;
    else if (distance < 50.0f) return mediumDetailModel;
    else return lowDetailModel;
}

小结​

模型加载是3D应用开发的重要环节：

  • Assimp库支持多种模型格式
  • 模型由网格层次结构组成
  • 每个网格包含顶点、索引和纹理
  • 正确处理后处理标志可以获得更好的模型数据

下一章我们将学习 高级OpenGL，包括帧缓冲、立方体贴图等高级技术。

模型资源

可以从以下网站获取免费3D模型：