OpenGL入门：窗口开启、改变窗口背景颜色

OpenGL入门

• 开启一个窗口：
• 运用OpenGL在窗口中绘制图形

开启一个窗口：

// Include for GLFW headers - library handling the window
#include <GLFW/glfw3.h>
#include <iostream>

int main()
{
    glfwInit(); // 初始化GLFW，在创建窗口前一定要做
    
    // 创建一个名为My Window的500*500的窗口
    // 第一个参数用于指定是否是全屏，第二个参数用于指向一个窗口用于共享资源。
    // 返回一个创建好的窗口的指针，如果失败则返回nullptr
    GLFWwindow* window = glfwCreateWindow(500, 500, "My Window", nullptr, nullptr);

    if( window==nullptr ) {
        std::cerr<<"GLFW Failed to create a Window"<<std::endl;
        glfwTerminate();
        exit(1);
    }

	// 如果窗口没有关闭
    while( !glfwWindowShouldClose(window) ) {
        glfwSwapBuffers(window); // Double buffering (will be used to avoid flickering when animating a scene)
        glfwPollEvents();        // Handle GLFW events (ex. mouse clicks, etc)
    }

    glfwTerminate(); // Close the GLFW Window

    return 0;
}

关键代码：

glfwInit();
GLFWwindow* window = glfwCreateWindow(500,500,"My Window",nullptr,nullptr);
glfwWindowShouldClose(window); //True or False
glfwSwapBuffers(window); //减少闪烁
glfwPollEvents(); //用于检测鼠标点击
glfwTerminate();

Rq:

• OpenGL 会直接和GPU对话。
• OpenGL 是一个库。

运用OpenGL在窗口中绘制图形

显示一个窗口

#include "../external/glad/include/glad/glad.hpp" // glad.h should be included before glfw or any OpenGL related include

#include <GLFW/glfw3.h>
#include <iostream>
#include <cmath>

int main()
{
    glfwInit();

    // 告诉GLFW OpenGL的环境，这里是OpenGL 3.3 core profile
    glfwWindowHint(GLFW_CLIENT_API, GLFW_OPENGL_API);
    glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 3);
    glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 3);
    glfwWindowHint(GLFW_OPENGL_FORWARD_COMPAT, 1);
    glfwWindowHint(GLFW_OPENGL_DEBUG_CONTEXT, 1);
    glfwWindowHint(GLFW_OPENGL_PROFILE,GLFW_OPENGL_CORE_PROFILE);
	
	// =====跟之前一样，初始化一个窗口=====
    GLFWwindow* window = glfwCreateWindow(500, 500, "My Window", nullptr, nullptr);

    if( window==nullptr ) {
        std::cerr<<"GLFW Failed to create a Window"<<std::endl;
        glfwTerminate();
        exit(1);
    }
	// ===============================
	
    // 使得窗口能接受OpenGL的指令
    glfwMakeContextCurrent(window);

    // Load OpenGL Functions
    gladLoadGL();

    // Print OpenGL Information，与具体的显卡有关
    std::cout << "OpenGL information: VENDOR      : " << glGetString(GL_VENDOR)   <<std::endl;
    std::cout << "                    RENDERDER   : " << glGetString(GL_RENDERER) <<std::endl;
    std::cout << "                    VERSION     : " << glGetString(GL_VERSION)  <<std::endl;
    std::cout << "                    GLSL VERSION: " << glGetString(GL_SHADING_LANGUAGE_VERSION)<<std::endl;

	// =====跟之前一样，对一个窗口的关闭进行判断=====
    while( !glfwWindowShouldClose(window) ) {
        glfwSwapBuffers(window);
        glfwPollEvents();
    }
    glfwTerminate();
	// ========================================
    
    return 0;
}

关键代码：

#include "../external/glad/include/glad/glad.hpp" // glad.h should be included before glfw or any OpenGL related include
// 告诉GLFW OpenGL的环境，这里是OpenGL 3.3 core profile
    glfwWindowHint(GLFW_CLIENT_API, GLFW_OPENGL_API);
    glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 3);
    glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 3);
    glfwWindowHint(GLFW_OPENGL_FORWARD_COMPAT, 1);
    glfwWindowHint(GLFW_OPENGL_DEBUG_CONTEXT, 1);
    glfwWindowHint(GLFW_OPENGL_PROFILE,GLFW_OPENGL_CORE_PROFILE);
// 使得窗口能接受OpenGL的指令
    glfwMakeContextCurrent(window);
// Load OpenGL Functions
    gladLoadGL();

Rq:

• 为了得到一个良好的动画的体验，至少要有25fps。

利用OpenGL修改窗口背景颜色

在while( !glfwWindowShouldClose(window) )的循环中加入

// Set the (R,G,B,A) color to clear the screen
// RGBA (Red-Green-Blue-Alpha), A表示透明度,每一个值在0和1之间。
glClearColor(1.0f, 1.0f, 0.5f, 1.0f);
// Clear the screen (designated by the color buffer)，如果不删除不能改变颜色
glClear(GL_COLOR_BUFFER_BIT);

改变颜色的闪烁

我们用余弦函数的形式改变图片中红色的组成多少

int counter = 0;
while( !glfwWindowShouldClose(window) ) {
    counter = (counter+1)%100;
    float u = counter/99.0f; // 不能写成/99，因为需要一个float的除法！
    // Set the (R,G,B,A) color to clear the screen
    glClearColor(0.5+std::cos(2*3.14f*u)/2.0f, 1.0f, 0.5f, 1.0f);
    // Clear the screen (designated by the color buffer)
    glClear(GL_COLOR_BUFFER_BIT);
    glfwSwapBuffers(window);
    glfwPollEvents();
}