基于C++的OpenGL 13 之Mesh

1. 引言

本文基于C++语言，描述OpenGL的Mesh

前置知识可参考：

• 基于C++的OpenGL 12 之多光源 - 当时明月在曾照彩云归 - 博客园 (cnblogs.com)

笔者这里不过多描述每个名词、函数和细节，更详细的文档可以参考：

• Assimp - LearnOpenGL CN (learnopengl-cn.github.io)
• 网格 - LearnOpenGL CN (learnopengl-cn.github.io)

2. 概述

3D建模工具(3D Modeling Tool)会在导出到模型文件的时候自动生成所有的顶点坐标、顶点法线以及纹理坐标

所以，解析这些导出的模型文件以及提取所有有用的信息，将它们储存为OpenGL能够理解的格式，就可以进行绘制

Assimp是一个非常流行的模型导入库，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），会将所有的模型数据加载至Assimp的通用数据结构中

Assimp的GitHub站点为：assimp/assimp: The official Open-Asset-Importer-Library Repository. Loads 40+ 3D-file-formats into one unified and clean data structure. (github.com)

Assimp的指导手册：The Asset-Importer-Lib Documentation — Asset-Importer-Lib December 2020 documentation (assimp-docs.readthedocs.io)

3. 构建Assimp

参考官方构建手册：assimp/Build.md at master · assimp/assimp (github.com)

笔者使用CMake在Linux上构建

拉取源码：

git clone https://github.com/assimp/assimp.git

CMake构建：

cd assimp
cmake CMakeLists.txt

编译：

make

安装：

sudo make install

4. 编写Mesh类

一个完整的模型往往是由多个组件构成，单个的组件称之为Mesh

多个Mesh组成Model，在这里，先定义Mesh类

定义顶点：

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

定义纹理：

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

定义Mesh类：

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();
};

构造函数：

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

    setupMesh();
}

初始化Mesh：

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);
}

渲染函数：

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);
}

Mesh类的调用见：基于C++的OpenGL 14 之模型加载 - 当时明月在曾照彩云归 - 博客园 (cnblogs.com)

5. 完整代码

主要文件mesh.hpp：

#ifndef MESH_HPP
#define MESH_HPP

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

#include "Shader.hpp"

#include <string>
#include <vector>

using namespace std;

struct Vertex {
    glm::vec3 Position;
    glm::vec3 Normal;
    glm::vec2 TexCoords;
};
struct Texture {
    unsigned int id;
    string type;
};
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)
        {
            this->vertices = vertices;
            this->indices = indices;
            this->textures = textures;

            setupMesh();
        }

        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);
        }

    private:
        /*  渲染数据  */
        unsigned int VAO, VBO, EBO;
        /*  函数  */
        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);
        }
};  

#endif

着色器shader.hpp：

#ifndef SHADER_HPP
#define SHADER_HPP

#include <glad/glad.h>
#include <glm/glm.hpp>

#include <string>
#include <fstream>
#include <sstream>
#include <iostream>

class Shader
{
public:
    unsigned int ID;
    // constructor generates the shader on the fly
    // ------------------------------------------------------------------------
    Shader(const char* vertexPath, const char* fragmentPath)
    {
        // 1. retrieve the vertex/fragment source code from filePath
        std::string vertexCode;
        std::string fragmentCode;
        std::ifstream vShaderFile;
        std::ifstream fShaderFile;
        // ensure ifstream objects can throw exceptions:
        vShaderFile.exceptions (std::ifstream::failbit | std::ifstream::badbit);
        fShaderFile.exceptions (std::ifstream::failbit | std::ifstream::badbit);
        try
        {
            // open files
            vShaderFile.open(vertexPath);
            fShaderFile.open(fragmentPath);
            std::stringstream vShaderStream, fShaderStream;
            // read file's buffer contents into streams
            vShaderStream << vShaderFile.rdbuf();
            fShaderStream << fShaderFile.rdbuf();
            // close file handlers
            vShaderFile.close();
            fShaderFile.close();
            // convert stream into string
            vertexCode = vShaderStream.str();
            fragmentCode = fShaderStream.str();
        }
        catch (std::ifstream::failure& e)
        {
            std::cout << "ERROR::SHADER::FILE_NOT_SUCCESFULLY_READ: " << e.what() << std::endl;
        }
        const char* vShaderCode = vertexCode.c_str();
        const char * fShaderCode = fragmentCode.c_str();
        // 2. compile shaders
        unsigned int vertex, fragment;
        // vertex shader
        vertex = glCreateShader(GL_VERTEX_SHADER);
        glShaderSource(vertex, 1, &vShaderCode, NULL);
        glCompileShader(vertex);
        checkCompileErrors(vertex, "VERTEX");
        // fragment Shader
        fragment = glCreateShader(GL_FRAGMENT_SHADER);
        glShaderSource(fragment, 1, &fShaderCode, NULL);
        glCompileShader(fragment);
        checkCompileErrors(fragment, "FRAGMENT");
        // shader Program
        ID = glCreateProgram();
        glAttachShader(ID, vertex);
        glAttachShader(ID, fragment);
        glLinkProgram(ID);
        checkCompileErrors(ID, "PROGRAM");
        // delete the shaders as they're linked into our program now and no longer necessery
        glDeleteShader(vertex);
        glDeleteShader(fragment);

    }
    // activate the shader
    // ------------------------------------------------------------------------
    void use() const
    {
        glUseProgram(ID);
    }
    // utility uniform functions
    // ------------------------------------------------------------------------
    void setBool(const std::string &name, bool value) const
    {
        glUniform1i(glGetUniformLocation(ID, name.c_str()), (int)value);
    }
    // ------------------------------------------------------------------------
    void setInt(const std::string &name, int value) const
    {
        glUniform1i(glGetUniformLocation(ID, name.c_str()), value);
    }
    // ------------------------------------------------------------------------
    void setFloat(const std::string &name, float value) const
    {
        glUniform1f(glGetUniformLocation(ID, name.c_str()), value);
    }
    // ------------------------------------------------------------------------
    void setVec2(const std::string &name, const glm::vec2 &value) const
    {
        glUniform2fv(glGetUniformLocation(ID, name.c_str()), 1, &value[0]);
    }
    void setVec2(const std::string &name, float x, float y) const
    {
        glUniform2f(glGetUniformLocation(ID, name.c_str()), x, y);
    }
    // ------------------------------------------------------------------------
    void setVec3(const std::string &name, const glm::vec3 &value) const
    {
        glUniform3fv(glGetUniformLocation(ID, name.c_str()), 1, &value[0]);
    }
    void setVec3(const std::string &name, float x, float y, float z) const
    {
        glUniform3f(glGetUniformLocation(ID, name.c_str()), x, y, z);
    }
    // ------------------------------------------------------------------------
    void setVec4(const std::string &name, const glm::vec4 &value) const
    {
        glUniform4fv(glGetUniformLocation(ID, name.c_str()), 1, &value[0]);
    }
    void setVec4(const std::string &name, float x, float y, float z, float w) const
    {
        glUniform4f(glGetUniformLocation(ID, name.c_str()), x, y, z, w);
    }
    // ------------------------------------------------------------------------
    void setMat2(const std::string &name, const glm::mat2 &mat) const
    {
        glUniformMatrix2fv(glGetUniformLocation(ID, name.c_str()), 1, GL_FALSE, &mat[0][0]);
    }
    // ------------------------------------------------------------------------
    void setMat3(const std::string &name, const glm::mat3 &mat) const
    {
        glUniformMatrix3fv(glGetUniformLocation(ID, name.c_str()), 1, GL_FALSE, &mat[0][0]);
    }
    // ------------------------------------------------------------------------
    void setMat4(const std::string &name, const glm::mat4 &mat) const
    {
        glUniformMatrix4fv(glGetUniformLocation(ID, name.c_str()), 1, GL_FALSE, &mat[0][0]);
    }

private:
    // utility function for checking shader compilation/linking errors.
    // ------------------------------------------------------------------------
    void checkCompileErrors(GLuint shader, std::string type)
    {
        GLint success;
        GLchar infoLog[1024];
        if (type != "PROGRAM")
        {
            glGetShaderiv(shader, GL_COMPILE_STATUS, &success);
            if (!success)
            {
                glGetShaderInfoLog(shader, 1024, NULL, infoLog);
                std::cout << "ERROR::SHADER_COMPILATION_ERROR of type: " << type << "\n" << infoLog << "\n -- --------------------------------------------------- -- " << std::endl;
            }
        }
        else
        {
            glGetProgramiv(shader, GL_LINK_STATUS, &success);
            if (!success)
            {
                glGetProgramInfoLog(shader, 1024, NULL, infoLog);
                std::cout << "ERROR::PROGRAM_LINKING_ERROR of type: " << type << "\n" << infoLog << "\n -- --------------------------------------------------- -- " << std::endl;
            }
        }
    }
};
#endif

6. 参考资料

[1]Assimp - LearnOpenGL CN (learnopengl-cn.github.io)

[2]网格 - LearnOpenGL CN (learnopengl-cn.github.io)

[3]assimp/Build.md at master · assimp/assimp (github.com)