text: #include <glad/glad.h>
  #include <GLFW/glfw3.h>
  #include "shader.h"
  #include <iostream>
  
  void framebuffer_size_callback(GLFWwindow* window, int width, int height);
  void processInput(GLFWwindow *window);
  
  // settings
  const unsigned int SCR_WIDTH = 800;
  const unsigned int SCR_HEIGHT = 600;
  
  int main()
  {
      // glfw: initialize and configure
      // ------------------------------
      glfwInit();
      glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 3);
      glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 3);
      glfwWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE);
  
  #ifdef __APPLE__
      glfwWindowHint(GLFW_OPENGL_FORWARD_COMPAT, GL_TRUE); // uncomment this statement to fix compilation on OS X
  #endif
  
      // glfw window creation
      // --------------------
      GLFWwindow* window = glfwCreateWindow(SCR_WIDTH, SCR_HEIGHT, "LearnOpenGL", NULL, NULL);
      if (window == NULL)
      {
          std::cout << "Failed to create GLFW window" << std::endl;
          glfwTerminate();
          return -1;
      }
      glfwMakeContextCurrent(window);
      glfwSetFramebufferSizeCallback(window, framebuffer_size_callback);
  
      // glad: load all OpenGL function pointers
      // ---------------------------------------
      if (!gladLoadGLLoader((GLADloadproc)glfwGetProcAddress))
      {
          std::cout << "Failed to initialize GLAD" << std::endl;
          return -1;
      }    
  
      Shader ourShader("/Users/zhongweixie/workspace/learn_opengl/opengl/shader.vs", "/Users/zhongweixie/workspace/learn_opengl/opengl/shader.fs");
      
      float vertices[] = {
          // positions         // colors
          0.5f, -0.5f, 0.0f,  1.0f, 0.0f, 0.0f,  // bottom right
          -0.5f, -0.5f, 0.0f,  0.0f, 1.0f, 0.0f,  // bottom left
          0.0f,  0.5f, 0.0f,  0.0f, 0.0f, 1.0f   // top
      };
      
      unsigned int VBO, VAO;
      glGenVertexArrays(1, &VAO);
      glGenBuffers(1, &VBO);
      // bind the Vertex Array Object first, then bind and set vertex buffer(s), and then configure vertex attributes(s).
      glBindVertexArray(VAO);
      
      glBindBuffer(GL_ARRAY_BUFFER, VBO);
      glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW);
      
      // position attribute
      glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 6 * sizeof(float), (void*)0);
      glEnableVertexAttribArray(0);
      // color attribute
      glVertexAttribPointer(1, 3, GL_FLOAT, GL_FALSE, 6 * sizeof(float), (void*)(3 * sizeof(float)));
      glEnableVertexAttribArray(1);
      
      // render loop
      // -----------
      while (!glfwWindowShouldClose(window))
      {
          // input
          // -----
          processInput(window);
  
          // render
          // ------
          glClearColor(0.2f, 0.3f, 0.3f, 1.0f);
          glClear(GL_COLOR_BUFFER_BIT);
  
          float offset = 0.5f;
          ourShader.setFloat("xOffset", offset);
          ourShader.use();
  
          
          glBindVertexArray(VAO);
          glDrawArrays(GL_TRIANGLES, 0, 3);
  
          // glfw: swap buffers and poll IO events (keys pressed/released, mouse moved etc.)
          // -------------------------------------------------------------------------------
          glfwSwapBuffers(window);
          glfwPollEvents();
      }
  
      // glfw: terminate, clearing all previously allocated GLFW resources.
      // ------------------------------------------------------------------
      glfwTerminate();
      return 0;
  }
  
  // process all input: query GLFW whether relevant keys are pressed/released this frame and react accordingly
  // ---------------------------------------------------------------------------------------------------------
  void processInput(GLFWwindow *window)
  {
      if(glfwGetKey(window, GLFW_KEY_ESCAPE) == GLFW_PRESS)
          glfwSetWindowShouldClose(window, true);
  }
  
  // glfw: whenever the window size changed (by OS or user resize) this callback function executes
  // ---------------------------------------------------------------------------------------------
  void framebuffer_size_callback(GLFWwindow* window, int width, int height)
  {
      // make sure the viewport matches the new window dimensions; note that width and 
      // height will be significantly larger than specified on retina displays.
      glViewport(0, 0, width, height);
  }
  