openGL基础概念复习

重读LearnOpenGL CN,巩固下基础知识.
感谢Joey de Vries的LearnOpenGL,也感谢LearnOpenGL CN上各位大佬的翻译.

概念:

• OpenGL本身并不是一个API，仅仅是一个规范，由Khronos组织制定并维护。 实际的OpenGL库的开发者通常是显卡的生产商。
• 扩展(Extension): windows下opengl32.dll(opengl32.lib是导入库)中只包含openGL1.1以前的实现,新版本功能由显卡驱动实现,要想使用新版本功能,必须使用扩展机制,查找到相应的函数地址.
• 立即渲染模式 (Immediate mode): 固定管线;核心模式(Core-profile): 可编程管线.前者慢慢废弃,后者更加灵活高效.
• 状态机: OpenGL自身是一个巨大的状态机,OpenGL的状态通常被称为OpenGL上下文(Context)。我们通常使用如下途径去更改OpenGL 状态：设置一些选项，操作一些缓冲。最后，我们使用当前OpenGL上下文来渲染。
• 对象(Object): vbo是最常用的对象,但除了vbo还有很多对象,例如下面的例子:

// 创建对象 
GLuint objectId = 0; 
glGenObject(1, &objectId); 
// 绑定对象至上下文 
glBindObject(GL_WINDOW_TARGET, objectId); 
// 设置GL_WINDOW_TARGET对象的一些选项 
glSetObjectOption(GL_WINDOW_TARGET, GL_OPTION_WINDOW_WIDTH, 800); 
glSetObjectOption(GL_WINDOW_TARGET, GL_OPTION_WINDOW_HEIGHT, 600); 
// 将上下文的GL_WINDOW_TARGET对象设回默认 
glBindObject(GL_WINDOW_TARGET, 0);

窗口系统:
最流行的几 个库有GLUT，SDL，SFML和GLFW。学习建议使用GLFW。

函数寻址:
openGL的函数及扩展中的函数,使用动态调用dll的方式使用,因此函数寻址比较麻烦,如下:

// 定义函数类型 
typedef void (*GL_GENBUFFERS) (GLsizei, GLuint*); 
// 找到正确的函数并赋值给函数指针 
GL_GENBUFFERS glGenBuffers = (GL_GENBUFFERS)wglGetProcAddress("glGenBuffers"); 
// 现在函数可以被正常调用了 
GLuint buffer; 
glGenBuffers(1, &buffer);

glew和glad都可以帮助方便的使用openGL及其扩展的函数,现在glad比较流行.
glad.h必须在glfw3.h前被包含.

一个最简单的框架,仅仅刷新背景颜色:

#include <glad/glad.h>
#include <GLFW/glfw3.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);
#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;
    }    

    // render loop
    // -----------
    while (!glfwWindowShouldClose(window))
    {
        // input
        // -----
        processInput(window);

        // render
        // ------
        glClearColor(0.2f, 0.3f, 0.3f, 1.0f);
        glClear(GL_COLOR_BUFFER_BIT);

        // 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);
}