使用了2个着色器。

• glUseProgram可任意切换，后面的绘制操作都用此着色器
• vertices的顶点扩展到6个，VBO调用glBufferData塞数据的时候，是需要传入数组的大小；VAO的调用参数需要修改
• VHO能省一些重复的顶点数据；使用时，glDrawArrays要改为glDrawElements

#include <iostream>
#ifdef __APPLE__
#include <OpenGL/gl3.h>
#include <OpenGL/glext.h>
#else
// GLEW
#define GLEW_STATIC
#include <GL/glew.h>
#endif

// GLFW
#include "glfw3.h"


// Function prototypes
void key_callback(GLFWwindow* window, int key, int scancode, int action, int mode);

// Window dimensions
const GLuint WIDTH = 800, HEIGHT = 600;

// Shaders
const GLchar* vertexShaderSource = "#version 330 core\n"
"layout (location = 0) in vec3 position;\n"
"void main()\n"
"{\n"
"gl_Position = vec4(position.x, position.y, position.z, 1.0);\n"
"}\0";
const GLchar* fragmentShaderSource = "#version 330 core\n"
"out vec4 color;\n"
"void main()\n"
"{\n"
"color = vec4(1.0f, 0.5f, 0.2f, 1.0f);\n"
"}\n\0";

const GLchar* yellowfragmentShaderSource = "#version 330 core\n"
"out vec4 color;\n"
"void main()\n"
"{\n"
"color = vec4(1.0f, 1.0f, 0.0f, 1.0f);\n"
"}\n\0";

static void error_callback(int error, const char* description)
{
    fprintf(stderr, "Error: %s\n", description);
}

// The MAIN function, from here we start the application and run the game loop
int main()
{
    glfwSetErrorCallback(error_callback);
    
    // Init GLFW
    glfwInit();
    // Set all the required options for GLFW
    glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 3);
    glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 3);
    glfwWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE);
    glfwWindowHint(GLFW_RESIZABLE, GL_FALSE);
#ifdef __APPLE__
    glfwWindowHint(GLFW_OPENGL_FORWARD_COMPAT, GL_TRUE);
#endif
    
    // Create a GLFWwindow object that we can use for GLFW's functions
    GLFWwindow* window = glfwCreateWindow(WIDTH, HEIGHT, "LearnOpenGL", nullptr, nullptr);
    glfwMakeContextCurrent(window);
    
    // Set the required callback functions
    glfwSetKeyCallback(window, key_callback);
    
    // Set this to true so GLEW knows to use a modern approach to retrieving function pointers and extensions
//    glewExperimental = GL_TRUE;
    // Initialize GLEW to setup the OpenGL Function pointers
//    glewInit();
    
    // Define the viewport dimensions
    int width, height;
    glfwGetFramebufferSize(window, &width, &height);
    glViewport(0, 0, width, height);
    
    
    // Build and compile our shader program
    // Vertex shader
    GLuint vertexShader = glCreateShader(GL_VERTEX_SHADER);
    glShaderSource(vertexShader, 1, &vertexShaderSource, NULL);
    glCompileShader(vertexShader);
    // Check for compile time errors
    GLint success;
    GLchar infoLog[512];
    glGetShaderiv(vertexShader, GL_COMPILE_STATUS, &success);
    if (!success)
    {
        glGetShaderInfoLog(vertexShader, 512, NULL, infoLog);
        std::cout << "ERROR::SHADER::VERTEX::COMPILATION_FAILED\n" << infoLog << std::endl;
    }
    // Fragment shader
    GLuint fragmentShader = glCreateShader(GL_FRAGMENT_SHADER);
    glShaderSource(fragmentShader, 1, &fragmentShaderSource, NULL);
    glCompileShader(fragmentShader);
    // Check for compile time errors
    glGetShaderiv(fragmentShader, GL_COMPILE_STATUS, &success);
    if (!success)
    {
        glGetShaderInfoLog(fragmentShader, 512, NULL, infoLog);
        std::cout << "ERROR::SHADER::FRAGMENT::COMPILATION_FAILED\n" << infoLog << std::endl;
    }
    // Link shaders
    GLuint shaderProgram = glCreateProgram();
    glAttachShader(shaderProgram, vertexShader);
    glAttachShader(shaderProgram, fragmentShader);
    glLinkProgram(shaderProgram);
    // Check for linking errors
    glGetProgramiv(shaderProgram, GL_LINK_STATUS, &success);
    if (!success) {
        glGetProgramInfoLog(shaderProgram, 512, NULL, infoLog);
        std::cout << "ERROR::SHADER::PROGRAM::LINKING_FAILED\n" << infoLog << std::endl;
    }
    glDeleteShader(vertexShader);
    glDeleteShader(fragmentShader);
    
    //---------------------- YELLOW -----------------------
    GLuint vertexShader2 = glCreateShader(GL_VERTEX_SHADER);
    glShaderSource(vertexShader2, 1, &vertexShaderSource, NULL);
    glCompileShader(vertexShader2);
    // Check for compile time errors
    glGetShaderiv(vertexShader2, GL_COMPILE_STATUS, &success);
    if (!success)
    {
        glGetShaderInfoLog(vertexShader2, 512, NULL, infoLog);
        std::cout << "ERROR::SHADER::VERTEX::COMPILATION_FAILED\n" << infoLog << std::endl;
    }
    // Fragment shader
    GLuint fragmentShader2 = glCreateShader(GL_FRAGMENT_SHADER);
    glShaderSource(fragmentShader2, 1, &yellowfragmentShaderSource, NULL);
    glCompileShader(fragmentShader2);
    // Check for compile time errors
    glGetShaderiv(fragmentShader2, GL_COMPILE_STATUS, &success);
    if (!success)
    {
        glGetShaderInfoLog(fragmentShader2, 512, NULL, infoLog);
        std::cout << "ERROR::SHADER::FRAGMENT::COMPILATION_FAILED\n" << infoLog << std::endl;
    }
    // Link shaders
    GLuint shaderProgram2 = glCreateProgram();
    glAttachShader(shaderProgram2, vertexShader2);
    glAttachShader(shaderProgram2, fragmentShader2);
    glLinkProgram(shaderProgram2);
    // Check for linking errors
    glGetProgramiv(shaderProgram2, GL_LINK_STATUS, &success);
    if (!success) {
        glGetProgramInfoLog(shaderProgram2, 512, NULL, infoLog);
        std::cout << "ERROR::SHADER::PROGRAM::LINKING_FAILED\n" << infoLog << std::endl;
    }
    glDeleteShader(vertexShader2);
    glDeleteShader(fragmentShader2);
    
    //---------------------- YELLOW -----------------------
    
    // Set up vertex data (and buffer(s)) and attribute pointers
    GLfloat vertices[] = {
        0.5, -0.5, 0,
        -0.5, -0.5, 0,
        -0.5, 0.5, 0,
        -0.5, 0.5, 0,
        0.5, 0.5, 0,
        0.5, -0.5, 0,
    };
    GLuint VBO, VAO;
    glGenVertexArrays(1, &VAO);
    glGenBuffers(1, &VBO);
    // Bind the Vertex Array Object first, then bind and set vertex buffer(s) and attribute pointer(s).
    glBindVertexArray(VAO);
    
    glBindBuffer(GL_ARRAY_BUFFER, VBO);
    glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW);
    
    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 3 * sizeof(GLfloat), (GLvoid*)0);
    glEnableVertexAttribArray(0);
    
    glBindBuffer(GL_ARRAY_BUFFER, 0); // Note that this is allowed, the call to glVertexAttribPointer registered VBO as the currently bound vertex buffer object so afterwards we can safely unbind
    
    glBindVertexArray(0); // Unbind VAO (it's always a good thing to unbind any buffer/array to prevent strange bugs)
    
//    glPolygonMode(GL_FRONT_AND_BACK, GL_LINE);
    
    // Game loop
    while (!glfwWindowShouldClose(window))
    {
        // Check if any events have been activiated (key pressed, mouse moved etc.) and call corresponding response functions
        glfwPollEvents();
        
        // Render
        // Clear the colorbuffer
        glClearColor(0.2f, 0.3f, 0.3f, 1.0f);
        glClear(GL_COLOR_BUFFER_BIT);
        
        // Draw our first triangle
        glUseProgram(shaderProgram);
        glBindVertexArray(VAO);
        glDrawArrays(GL_TRIANGLES, 0, 3);
        
        glUseProgram(shaderProgram2);
        glDrawArrays(GL_TRIANGLES, 3, 3);
        
        glBindVertexArray(0);
        
        // Swap the screen buffers
        glfwSwapBuffers(window);
    }
    // Properly de-allocate all resources once they've outlived their purpose
    glDeleteVertexArrays(1, &VAO);
    glDeleteBuffers(1, &VBO);
    // Terminate GLFW, clearing any resources allocated by GLFW.
    glfwTerminate();
    return 0;
}

// Is called whenever a key is pressed/released via GLFW
void key_callback(GLFWwindow* window, int key, int scancode, int action, int mode)
{
    if (key == GLFW_KEY_ESCAPE && action == GLFW_PRESS)
        glfwSetWindowShouldClose(window, GL_TRUE);
}