OpenGL绘制（二）

上节学会了绘制三角形，我们开始绘制正方形

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1 顶点坐标，纹理坐标（图片来源于其他博友）

（1）顶点坐标系

 

（2）纹理坐标系

 

2．索引缓冲对象（Element Buffer Object，EBO）

索引缓冲对象EBO相当于OpenGL中的顶点索引数组，是为了解决同一个顶点多次重复调用的问题，可以减少内存空间浪费，提高执行效率。当需要使用重复的顶点时，通过顶点的位置索引来调用顶点，而不是对重复的顶点信息重复记录，重复调用。

3 画一个正方形= 画两个三角形拼在一起。顶点着色程序和片元着色程序无需改动

 定义应用程序的入口点。
//

#include "stdafx.h"

#include <glad/glad.h>
#include <GLFW/glfw3.h>
#include<string>
#include<fstream>
#include<sstream>
#include<iostream>

#include <stdio.h>

// settings
const unsigned int SCR_WIDTH = 800;
const unsigned int SCR_HEIGHT = 600;
unsigned int VBO = 0;
unsigned int VAO = 0;
unsigned int EBO = 0;
int shaderProgram = 0;

void render()
{
    glBindVertexArray(VAO);
    glUseProgram(shaderProgram);
    glDrawElements(GL_TRIANGLES,6,GL_UNSIGNED_INT,0);
    glUseProgram(0);
    glBindVertexArray(0);
}

void initmodule()
{
    //做个一模型;正方形四个顶点；顶点坐标系
    float vertexs[] = {

        0.0f,0.0f,0.0f,
        0.0f,0.5f,0.0f,
        0.5f,0.5f,0.0f,
        0.5f,0.0f,0.0f,
        
        
    };
    //一个正方形是由两个三角形得来的；记录顶点的索引顺序；0 1 2 3分别是四个顶点的排序，就可以画出两个三角形
    unsigned int indexs[] = {
        0,1,3,
        1,2,3,
    };
    
    //做VAO
    glGenVertexArrays(1,&VAO);
    glBindVertexArray(VAO);

    //做VBO

    glGenBuffers(1, &VBO);
    glBindBuffer(GL_ARRAY_BUFFER, VBO);
    //创建显存空间
    glBufferData(GL_ARRAY_BUFFER,sizeof(vertexs), vertexs, GL_STATIC_DRAW);
    
    //设置索引缓冲
    glGenBuffers(1,&EBO);
    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER,EBO);
    glBufferData(GL_ELEMENT_ARRAY_BUFFER,sizeof(indexs),indexs,GL_STATIC_DRAW);
    
    
    //设置第0个锚点,3个点，不需要归一化，跨度3个float可以读下一个点
    glVertexAttribPointer(0,3,GL_FLOAT,GL_FALSE,3*sizeof(float),(void*)0);
    //打开顶点
    glEnableVertexAttribArray(0);
    //解除绑定VBO
    glBindBuffer(GL_ARRAY_BUFFER,0);
    //解除绑定VEO？？？
    //glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, 0);会崩溃，目前不知道原因，后序补充

    //解绑VAO
    glBindVertexArray(0);

}

void initshader(const char* verpath,const char* fragpath)
{
    //编译shader，并记录shaderID
    std::string VerCode("");
    std::string fregCode("");
    //读文件
    std::ifstream  vShaderFile;
    std::ifstream  fShaderFile;

    vShaderFile.exceptions(std::ifstream::failbit | std::ifstream::badbit);
    fShaderFile.exceptions(std::ifstream::failbit | std::ifstream::badbit);
    
    try
    {
        vShaderFile.open(verpath);
        fShaderFile.open(fragpath);

        std::stringstream vsstream, fsstream;
        vsstream << vShaderFile.rdbuf();
        fsstream << fShaderFile.rdbuf();
        VerCode = vsstream.str();
        fregCode = fsstream.str();
        
    }
    catch (const std::exception&)
    {
        std::cout << "read file error" << std::endl;
    }

    const char* vshader = VerCode.c_str();
    const char* fshader = fregCode.c_str();

    //shader 编译连接
    unsigned int vertexID = 0, fragID = 0;
    char infoLog[512];//存储错误信息
    int  successflag = 0;
    vertexID = glCreateShader(GL_VERTEX_SHADER);
    glShaderSource(vertexID,1,&vshader,NULL );
    glCompileShader(vertexID);
    //获取编译是否成功
    glGetShaderiv(vertexID,GL_COMPILE_STATUS,&successflag);
    if (!successflag)
    {
        glGetShaderInfoLog(vertexID,512,NULL,infoLog);
        std::string errstr(infoLog);
        std::cout << "v shader err"<<infoLog;
    }
    //frag
    fragID = glCreateShader(GL_FRAGMENT_SHADER);
    glShaderSource(fragID, 1, &fshader, NULL);
    glCompileShader(fragID);
    //获取编译是否成功
    glGetShaderiv(fragID, GL_COMPILE_STATUS, &successflag);
    if (!successflag)
    {
        glGetShaderInfoLog(fragID, 512, NULL, infoLog);
        std::string errstr(infoLog);
        std::cout << "f shader err"<<infoLog;
    }
    //链接
    shaderProgram = glCreateProgram();
    glAttachShader(shaderProgram,vertexID);
    glAttachShader(shaderProgram,fragID);
    glLinkProgram(shaderProgram);
    glGetProgramiv(shaderProgram,GL_LINK_STATUS,&successflag);
    if (!successflag)
    {
        glGetShaderInfoLog(shaderProgram, 512, NULL, infoLog);
        std::string errstr(infoLog);
        std::cout << "link error";
    }

    //编译完成后，可以把中间的步骤程序删除
    glDeleteShader(vertexID);
    glDeleteShader(fragID);
}
void processInput(GLFWwindow *window)
{
    if (glfwGetKey(window, GLFW_KEY_ESCAPE) == GLFW_PRESS)
    {
        //将窗口设置为关闭，跳出循环
        glfwSetWindowShouldClose(window, true);
    }
}

void framebuffer_size_callback(GLFWwindow* window, int width, int height)
{
    glViewport(0, 0, width, height);
}

int main()
{
    //glfw初始化
    glfwInit();
    glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 3);
    glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 3);
    glfwWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE);

    //glfw创建窗口
    GLFWwindow* window = glfwCreateWindow(SCR_WIDTH, SCR_HEIGHT, "LearnOpenGL", NULL, NULL);
    if (window == NULL)
    {
        printf("创建窗口失败");
        //终止
        glfwTerminate();
        return -1;
    }
    //显示窗口
    glfwMakeContextCurrent(window);

    //设置回调，当窗口大小调整后将调用该回调函数
    glfwSetFramebufferSizeCallback(window, framebuffer_size_callback);

    // glad初始化
    if (!gladLoadGLLoader((GLADloadproc)glfwGetProcAddress))
    {
        printf("加载失败");
        return -1;
    }
    initmodule();
    initshader("vertexShader.glsl","fragmentShader.glsl");

    // 使用循环达到循环渲染效果
    while (!glfwWindowShouldClose(window))
    {
        //自定义输入事件
        processInput(window);

        glClearColor(0.5f,0.5f,0.3f,1.0f);
        glClear(GL_COLOR_BUFFER_BIT);
        render();
        //交互缓冲区，否则显示空白
        glfwSwapBuffers(window);
        //输入输出事件,否则无法对窗口进行交互
        glfwPollEvents();
    }

    //终止渲染 关闭并清理glfw本地资源
    glfwTerminate();
    return 0;
}

#version 330 core
layout(location = 0) in vec3 aPos;
void main()
{
   gl_Position = vec4(aPos.x,aPos.y,aPos.z,1.0);
};

#version 330 core
out vec4 FragColor;
void main()
{
    FragColor = vec4(0.5f,0.9f,0.5f,1.0f);
};