opengl---基本绘制（一）

基本绘制（一）

OpenGL有很多扩展库，下面我们一一了解一下，现在开发过程中，我们推荐使用opengl+glew+glfw组合方案。

opengl32.lib：（DLL)是MS为openGL能够在window环境下工作设计的函数库，这个库的工作很简单。如果你的机器上存在硬件加速的GL驱动，调用该驱动，否则用软件的方法实现。
包含gl.h文件并使用了里面的函数必须将opengl32.lib加入你的工程。

glew：不同的显卡公司，也会发布一些只有自家显卡才支 持的扩展函数，你要想用这数涵数，不得不去寻找最新的glext.h,有了GLEW扩展库，你就再也不用为找不到函数的接口而烦恼，因为GLEW能自动识别你的平台所支持的全部OpenGL高级扩展函数。也就是说，只要包含一个glew.h头文件，你就能使用gl,glu,glext,wgl,glx的全部函数。

glfw：一个轻量级的，开源的，跨平台的library。支持OpenGL及OpenGL ES，用来管理窗口，读取输入，处理事件等。因为OpenGL没有窗口管理的功能，所以很多热心的人写了工具来支持这些功能，比如早期的glut，现在的freeglut等。那么GLFW有何优势呢？glut太老了，最后一个版本还是90年代的。freeglut完全兼容glut，算是glut的代替品，功能齐全，但是bug太多。稳定性也不好（不是我说的啊），GLFW应运而生。

Equalizer（均衡器）是一个用于可伸缩OpenGL应用程序的开源编程接口和资源管理系统。均衡器应用程序可以部署在任何可视化系统上，从单管工作站到大型图形集群。

GLee是一个免费的跨平台扩展加载库，可以减轻应用程序的负担。GLee使得检查OpenGL扩展和核心版本的可用性变得容易，无需您的努力就可以自动设置入口点。

GLUS是一个开源的C库，它提供了一个硬件和操作系统抽象，以及使用OpenGL、opengles或OpenVG进行图形编程所需的许多函数。

OpenGL Mathematics （OpenGL 数学，GLM）是基于OpenGL着色语言（GLSL）规范的三维软件的C++数学库。

libktx是KTX工具集的一部分，是一个函数库，用于编写KTX格式的文件并从中实例化GL纹理。

OpenSceneGraph是一个高级的3D图形工具包，它在提供许多自己的功能的同时，还公开了OpenGL的功能。OpenSceneGraph拥有一个庞大的用户社区，已经被用于可视化仿真、游戏、虚拟现实、科学可视化和建模。

glu则是在gl基础上的扩展，如上面所说，他实际上已经是OpenGL的一部分了。他的函数都是以glu开头的（区别于gl函数族），你使用的gluPerspective就是这样的函数。要使用这些函数，必须将glu32.lib链接到你的工程中。

glut是另外一个opengl的扩展库，现在被广泛的使用，其中函数都以glut打头，使用时要连接glut32.lib。顺便说一句，SGI也有在windows环境下的opengl驱动，里面的库文件名改了一下，去掉了32:opengl.lib和glu.lib。这个库只支持32位，已经被淘汰了，替代品是freeglut。

如果我们现在做开发的话，推荐使用opengl+glew+glfw组合方案。

 https://www.khronos.org/registry/OpenGL-Refpages/gl4/html/

代码：

#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;

const char *vertexShaderSource = "#version 330 core
"
    "layout (location = 0) in vec3 aPos;
"
    "void main()
"
    "{
"
    "   gl_Position = vec4(aPos.x, aPos.y, aPos.z, 1.0);
"
    "}";
const char *fragmentShaderSource = "#version 330 core
"
    "out vec4 FragColor;
"
    "void main()
"
    "{
"
    "   FragColor = vec4(1.0f, 0.5f, 0.2f, 1.0f);
"
    "}
";

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;
    }


    // build and compile our shader program
    // ------------------------------------
    // vertex shader
    unsigned int vertexShader = glCreateShader(GL_VERTEX_SHADER);
    glShaderSource(vertexShader, 1, &vertexShaderSource, NULL);
    glCompileShader(vertexShader);
    // check for shader compile errors
    int success;
    char infoLog[512];
    glGetShaderiv(vertexShader, GL_COMPILE_STATUS, &success);
    if (!success)
    {
        glGetShaderInfoLog(vertexShader, 512, NULL, infoLog);
        std::cout << "ERROR::SHADER::VERTEX::COMPILATION_FAILED
" << infoLog << std::endl;
    }
    // fragment shader
    unsigned int fragmentShader = glCreateShader(GL_FRAGMENT_SHADER);
    glShaderSource(fragmentShader, 1, &fragmentShaderSource, NULL);
    glCompileShader(fragmentShader);
    // check for shader compile errors
    glGetShaderiv(fragmentShader, GL_COMPILE_STATUS, &success);
    if (!success)
    {
        glGetShaderInfoLog(fragmentShader, 512, NULL, infoLog);
        std::cout << "ERROR::SHADER::FRAGMENT::COMPILATION_FAILED
" << infoLog << std::endl;
    }
    // link shaders
    unsigned int 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
" << infoLog << std::endl;
    }
    glDeleteShader(vertexShader);
    glDeleteShader(fragmentShader);

    // set up vertex data (and buffer(s)) and configure vertex attributes
    // ------------------------------------------------------------------
    float vertices[] = {
         0.5f,  0.5f, 0.0f,  // top right
         0.5f, -0.5f, 0.0f,  // bottom right
        -0.5f, -0.5f, 0.0f,  // bottom left
        -0.5f,  0.5f, 0.0f   // top left 
    };
    unsigned int indices[] = {  // note that we start from 0!
        0, 1, 3,  // first Triangle
        1, 2, 3   // second Triangle
    };
    unsigned int VBO, VAO, EBO;
    glGenVertexArrays(1, &VAO);
    glGenBuffers(1, &VBO);
    glGenBuffers(1, &EBO);
    // 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);

    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, EBO);
    glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(indices), indices, GL_STATIC_DRAW);

    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 3 * sizeof(float), (void*)0);
    glEnableVertexAttribArray(0);

    // note that this is allowed, the call to glVertexAttribPointer registered VBO as the vertex attribute's bound vertex buffer object so afterwards we can safely unbind
    glBindBuffer(GL_ARRAY_BUFFER, 0); 

    // remember: do NOT unbind the EBO while a VAO is active as the bound element buffer object IS stored in the VAO; keep the EBO bound.
    //glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, 0);

    // You can unbind the VAO afterwards so other VAO calls won't accidentally modify this VAO, but this rarely happens. Modifying other
    // VAOs requires a call to glBindVertexArray anyways so we generally don't unbind VAOs (nor VBOs) when it's not directly necessary.
    glBindVertexArray(0); 


    // uncomment this call to draw in wireframe polygons.
    //glPolygonMode(GL_FRONT_AND_BACK, GL_LINE);

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

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

        // draw our first triangle
        glUseProgram(shaderProgram);
        glBindVertexArray(VAO); // seeing as we only have a single VAO there's no need to bind it every time, but we'll do so to keep things a bit more organized
        //glDrawArrays(GL_TRIANGLES, 0, 6);
        glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_INT, 0);
        // glBindVertexArray(0); // no need to unbind it every time 
 
        // glfw: swap buffers and poll IO events (keys pressed/released, mouse moved etc.)
        // -------------------------------------------------------------------------------
        glfwSwapBuffers(window);
        glfwPollEvents();
    }

    // optional: de-allocate all resources once they've outlived their purpose:
    // ------------------------------------------------------------------------
    glDeleteVertexArrays(1, &VAO);
    glDeleteBuffers(1, &VBO);
    glDeleteBuffers(1, &EBO);
    glDeleteProgram(shaderProgram);

    // 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);
}

参考：

LearnOpenGL - Hello Triangle  

glPolygonMode - OpenGL 4 Reference Pages (khronos.org)