在openGL里写一个Shader

1、总体代码

draw一个三角形，自写vertex着色器和fregment着色器，三角形染红色。

#include <GL/glew.h>
#include <GLFW/glfw3.h>
#include <iostream>

static unsigned int CompileShader( unsigned int type, const std::string& source)
{
    unsigned int id = glCreateShader(type);
    const char* src = source.c_str();
    glShaderSource(id, 1, &src,nullptr);
    glCompileShader(id);

    //TODO:Error handling
    int result;
    glGetShaderiv(id, GL_COMPILE_STATUS, &result);
    if (result == GL_FALSE)
    {
        int length;
        glGetShaderiv(id, GL_INFO_LOG_LENGTH, &length);
        char* message = (char*)alloca(length * sizeof(char));

        glGetShaderInfoLog(id, length, &length, message);
        std::cout << "Failed to compile " <<(type == GL_VERTEX_SHADER ? "vertex" : "fragment") << "shader!" 
            << std::endl;
        std::cout << message << std::endl;
        glDeleteShader(id);
    }

    return id;
}

static unsigned int CreateShader(const std::string& vectexShader, const std::string& fragmentShader)
{
   unsigned int program = glCreateProgram();
   unsigned int vs = CompileShader(GL_VERTEX_SHADER, vectexShader);
   unsigned int fs = CompileShader(GL_FRAGMENT_SHADER, fragmentShader);

   glAttachShader(program, vs);
   glAttachShader(program, fs);
   glLinkProgram(program);
   glValidateProgram(program);

   glDeleteShader(vs);
   glDeleteShader(fs);

   return program;
}

int main(void)
{
    GLFWwindow* window;

    /* Initialize the library */
    if (!glfwInit())
        return -1;
    
    /* Create a windowed mode window and its OpenGL context */
    window = glfwCreateWindow(640, 480, "Hello World", NULL, NULL);
    if (!window)
    {
        glfwTerminate();
        return -1;
    }

    /* Make the window's context current */
    glfwMakeContextCurrent(window);

    if (glewInit() != GLEW_OK)
        std::cout << "GLEW INIT ERR" << std::endl;

    std::cout << glGetString(GL_VERSION) << std::endl;

    float position[6] = {
        -0.5f, -0.5f,
         0.0f,  0.5f,
         0.5f, -0.5f,
    };

    unsigned int buffer;
    glGenBuffers(1, &buffer);
    glBindBuffer(GL_ARRAY_BUFFER, buffer);
    glBufferData(GL_ARRAY_BUFFER, 6 * sizeof(float), position, GL_STATIC_DRAW);

    glEnableVertexAttribArray(0);
    glVertexAttribPointer(0, 2, GL_FLOAT, GL_FALSE, sizeof(float) * 2, 0);

    std::string vertexShader =
        "#version 330 core\n"
        "\n"
        "layout(location = 0) in vec4 position;\n"
        "void main()\n"
        "{\n"
        "    gl_Position = position;\n"
        "}\n";

    std::string fragmentShader =
        "#version 330 core\n"
        "\n"
        "layout(location = 0) out vec4 color;\n"
        "void main()\n"
        "{\n"
        "    color = vec4(1.0, 0.0, 0.0, 1.0);\n" //RGB
        "}\n";

    unsigned int shader = CreateShader(vertexShader, fragmentShader);
    glUseProgram(shader);


    /* Loop until the user closes the window */
    while (!glfwWindowShouldClose(window))
    {
        /* Render here */
        glClear(GL_COLOR_BUFFER_BIT);

        glDrawArrays(GL_TRIANGLES, 0, 3);

        /* Swap front and back buffers */
        glfwSwapBuffers(window);

        /* Poll for and process events */
        glfwPollEvents();
    }

    glDeleteProgram(shader);

    glfwTerminate();
    return 0;
}

2、解析

具体的gl函数文档可以见这个网站： opengl文档.
一个十分好用的中文网站：learnOpenGL-CN

• glGenBuffers glGenBuffers：生成缓冲区对象名称

  • 声明：void glGenBuffers( GLsizei n,
    GLuint * buffers);
  • 参数：
    • n ：Specifies the number of buffer object names to be generated.指定要生成的缓冲区对象名称的数目。
    • buffers ：Specifies an array in which the generated buffer object names are stored. 指定存储生成的缓冲区对象名称的数组。
  • 说明：
    glGenBuffers returns n buffer object names in buffers. There is no guarantee that the names form a contiguous set of integers; however, it is guaranteed that none of the returned names was in use immediately before the call to glGenBuffers.

    unsigned int objectId = 0;
    glGenObject(1, &objectId);//我们首先创建一个对象，然后用一个id保存它的引用（实际数据被储存在后台）
    

    返回n个缓冲对象名称,不能保证名字会组成一个连续的整数集，但是能保证在调用glGenBuffers前没有被立即使用过。

• glBindBuffer glBindBuffer
  ///查看文档即可，这里不做简单翻译了。

修改优化代码（去掉烦人的字符串）

cherno的方式，使用独立文件，#include ，注意和上述的区别
[1]头文件
[2]static ShaderProgramSource ParseShader(const std::string& filepath) 函数
这里main.cpp

#include <GL/glew.h>
#include <GLFW/glfw3.h>
#include <iostream>

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

struct ShaderProgramSource
{
    std::string VertexSource;
    std::string FragmentSource;
};

static ShaderProgramSource ParseShader(const std::string& filepath)
{
    std::ifstream stream(filepath);

    enum class ShaderType
    {
        NONE = -1,
        VERTEX = 0,
        FRAGMENT = 1
    };
    ShaderType type = ShaderType::NONE;

    std::string line;
    std::stringstream ss[2];
    while (getline(stream, line))
    {
        //找到一行不等于无效字符串位置，自定义语法令牌，（custom syntax token）
        if (line.find("#shader") != std::string::npos)
        {
            if (line.find("vertex") != std::string::npos)
                //set mode to  vertex
                type = ShaderType::VERTEX;
            else if (line.find("fragment") != std::string::npos)
                //set mode to fragment
                type = ShaderType::FRAGMENT;
        }
        else
        {
            ss[(int)type] << line << '\n';
        }
    }

    return { ss[0].str(),ss[1].str() };
}

static unsigned int CompileShader( unsigned int type, const std::string& source)
{
    unsigned int id = glCreateShader(type);
    const char* src = source.c_str();
    glShaderSource(id, 1, &src,nullptr);
    glCompileShader(id);

    //TODO:Error handling
    int result;
    glGetShaderiv(id, GL_COMPILE_STATUS, &result);
    if (result == GL_FALSE)
    {
        int length;
        glGetShaderiv(id, GL_INFO_LOG_LENGTH, &length);
        char* message = (char*)alloca(length * sizeof(char));

        glGetShaderInfoLog(id, length, &length, message);
        std::cout << "Failed to compile " <<(type == GL_VERTEX_SHADER ? "vertex" : "fragment") << "shader!" 
            << std::endl;
        std::cout << message << std::endl;
        glDeleteShader(id);
    }

    return id;
}


static unsigned int CreateShader(const std::string& vectexShader, const std::string& fragmentShader)
{
   unsigned int program = glCreateProgram();
   unsigned int vs = CompileShader(GL_VERTEX_SHADER, vectexShader);
   unsigned int fs = CompileShader(GL_FRAGMENT_SHADER, fragmentShader);

   glAttachShader(program, vs);
   glAttachShader(program, fs);
   glLinkProgram(program);
   glValidateProgram(program);

   glDeleteShader(vs);
   glDeleteShader(fs);

   return program;
}

int main(void)
{
    GLFWwindow* window;

    /* Initialize the library */
    if (!glfwInit())
        return -1;
    
    /* Create a windowed mode window and its OpenGL context */
    window = glfwCreateWindow(640, 480, "Hello World", NULL, NULL);
    if (!window)
    {
        glfwTerminate();
        return -1;
    }

    /* Make the window's context current */
    glfwMakeContextCurrent(window);

    if (glewInit() != GLEW_OK)
        std::cout << "GLEW INIT ERR" << std::endl;

    std::cout << glGetString(GL_VERSION) << std::endl;

    float position[6] = {
        -0.5f, -0.5f,
         0.0f,  0.5f,
         0.5f, -0.5f,
    };

    unsigned int buffer;
    glGenBuffers(1, &buffer);
	glBindBuffer(GL_ARRAY_BUFFER, buffer);
	glBufferData(GL_ARRAY_BUFFER, 6 * sizeof(float), position, GL_STATIC_DRAW);

    glEnableVertexAttribArray(0); //启用索引0
    glVertexAttribPointer(0, 2, GL_FLOAT, GL_FALSE, sizeof(float) * 2, 0);

    ShaderProgramSource source = ParseShader("res/shaders/Basic.shader");
    std::cout << "VERTEX" << std::endl;
    std::cout << source.VertexSource << std::endl;
    std::cout << "FRAGMENT" << std::endl;
    std::cout << source.FragmentSource << std::endl;
    
    unsigned int shader = CreateShader(source.VertexSource, source.FragmentSource);
    glUseProgram(shader);

    /* Loop until the user closes the window */
    while (!glfwWindowShouldClose(window))
    {
        /* Render here */
        glClear(GL_COLOR_BUFFER_BIT);

        glDrawArrays(GL_TRIANGLES, 0, 3);

        /* Swap front and back buffers */
        glfwSwapBuffers(window);

        /* Poll for and process events */
        glfwPollEvents();
    }

   // glDeleteProgram(shader);

    glfwTerminate();
    return 0;
}

下面是 res/shaders/Basic.shader的文件

#shader vertex
#version 330 core

layout(location = 0) in vec4 position;
void main()
{
    gl_Position = position;
};

#shader fragment
#version 330 core

layout(location = 0) out vec4 color;
void main()
{
    color = vec4(0.2, 0.3, 0.5, 1.0); //RGB
};

其实没什么区别

4、使用索引

当我们绘制一个正方形的时候，其实就是绘制俩个三角形，我们可以把上述的position增加为6组顶点坐标，进行相应改变，就可以绘制了，但是实际上，我们只需要四个顶点坐标就可以完成绘制（有重复节点，斜对角线）这个时候我们可以使用索引的方式
其他代码相同（shader什么的）

int main(void)
{
    GLFWwindow* window;

    /* Initialize the library */
    if (!glfwInit())
        return -1;
    
    /* Create a windowed mode window and its OpenGL context */
    window = glfwCreateWindow(640, 480, "Hello World", NULL, NULL);
    if (!window)
    {
        glfwTerminate();
        return -1;
    }

    /* Make the window's context current */
    glfwMakeContextCurrent(window);

    if (glewInit() != GLEW_OK)
        std::cout << "GLEW INIT ERR" << std::endl;

    std::cout << glGetString(GL_VERSION) << std::endl;

    float position[] = {
        -0.5f,  -0.5f, // 0
         0.5f,  -0.5f, // 1
         0.5f,   0.5f, // 2 
        -0.5f,   0.5f  // 3        
    };
    unsigned int indices[] =
    {
        0, 1, 2,
        2, 3, 0
    };

    unsigned int buffer;
    glGenBuffers(1, &buffer);
	glBindBuffer(GL_ARRAY_BUFFER, buffer);
	glBufferData(GL_ARRAY_BUFFER, 6 * 2 * sizeof(float), position, GL_STATIC_DRAW);

    glEnableVertexAttribArray(0); //启用索引0
    glVertexAttribPointer(0, 2, GL_FLOAT, GL_FALSE, sizeof(float) * 2, 0);


    unsigned int ibo;
    glGenBuffers(1, &ibo);
    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, ibo);
    glBufferData(GL_ELEMENT_ARRAY_BUFFER, 6 *  sizeof(unsigned int), indices, GL_STATIC_DRAW);

    ShaderProgramSource source = ParseShader("res/shaders/Basic.shader");
    //std::cout << "VERTEX" << std::endl;
    //std::cout << source.VertexSource << std::endl;
    //std::cout << "FRAGMENT" << std::endl;
    //std::cout << source.FragmentSource << std::endl;
    
    unsigned int shader = CreateShader(source.VertexSource, source.FragmentSource);
    glUseProgram(shader);

    /* Loop until the user closes the window */
    while (!glfwWindowShouldClose(window))
    {
        /* Render here */
        glClear(GL_COLOR_BUFFER_BIT);

       // glDrawArrays(GL_TRIANGLES, 0, 6);
        glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_INT,nullptr);

        /* Swap front and back buffers */
        glfwSwapBuffers(window);

        /* Poll for and process events */
        glfwPollEvents();
    }

   // glDeleteProgram(shader);

    glfwTerminate();
    return 0;
}