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[C++知识库]QT+OpenGL实现Hello Triangle，绘制三角形

使用QT将learningopengl入门篇的Hello Triangle代码在QT框架下实现，绘制一个静态三角形

一、环境

操作系统：Windows 11
IDE：Microsoft Visual Studio Community 2022 (64 位)
QT：5.12.12

二、代码实现

learningopengl Hello Triangle章节教程：https://learnopengl-cn.github.io/01%20Getting%20started/04%20Hello%20Triangle/#_10

GLFW框架下的代码：https://learnopengl.com/code_viewer_gh.php?code=src/1.getting_started/2.1.hello_triangle/hello_triangle.cpp

对照GLFW框架下代码进行以下修改

着色器

着色器代码和glfw或qt无关，无论使用哪个框架，这部分代码都是一样的，作为全局变量直接粘贴过来

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

GLFW初始化

main函数中的第一部代码，是进行glfw初始化和窗体配置的，在QT框架下，窗体已经由QT创建，这部分代码直接删除即可。

    // 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管理OpenGL函数指针

接下来glad的部分，在QT中被initializeOpenGLFunctions方法取代，在initializeGL()方法中直接调用即可

    // glad: load all OpenGL function pointers
    // ---------------------------------------
    if (!gladLoadGLLoader((GLADloadproc)glfwGetProcAddress))
    {
        std::cout << "Failed to initialize GLAD" << std::endl;
        return -1;
    }

替换为

    this->initializeOpenGLFunctions();    //为当前上下文初始化提供OpenGL函数解析

编译着色器代码

这部分也没有什么不同，直接粘贴到initializeGL()中，只需要将std::cout替换为qDebug即可，std::cout在QT中无法正常输出，原因和解决方法我还没有研究。

    // 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\n" << 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\n" << 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\n" << infoLog << std::endl;
    }
    glDeleteShader(vertexShader);
    glDeleteShader(fragmentShader);

设置顶点数据

    // set up vertex data (and buffer(s)) and configure vertex attributes
    // ------------------------------------------------------------------
    float vertices[] = {
        -0.5f, -0.5f, 0.0f, // left  
         0.5f, -0.5f, 0.0f, // right 
         0.0f,  0.5f, 0.0f  // 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);

    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); 

    // 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);

在这部分代码中，和VAO相关的创建、绑定、解绑的方法glBindVertexArray()和glBindVertexArray()报错，这两个函数好像是第三方库gl3w中的，我们将他们替换成qt中的方法

	unsigned int VAO; // 声明
	glGenVertexArrays(1, &VAO); // 创建
	glBindVertexArray(VAO); // 绑定
	glBindVertexArray(0); // 解绑

分别替换为

    QOpenGLVertexArrayObject VAO; // 声明
    VAO.create(); // 创建
    if(!VAO.isCreated()){
        qDebug()<<"vao is not created.";
    }
    VAO.bind(); // 绑定
    VAO.release(); // 解绑

并添加头文件

#include <QOpenGLVertexArrayObject>

render loop

    // 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, 3);
        // 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();
    }

这部分的while (!glfwWindowShouldClose(window))循环控制交给qt框架，循环体内的代码放在paintGL()方法中，由定时器和QT框架控制循环调用

processInput(window);是自定义的函数，用于处理按键事件，先去掉这个功能，后续用qt实现。

glUseProgram(shaderProgram);和glBindVertexArray(VAO);用到了前面定义的变量，将他们修改定义到类中，glBindVertexArray(VAO)改为VAO.bind();，VO是否需要每次解绑暂不清楚（使用glBindVertexArray进行绑定是不需要每次都解绑），保险起见先加上解绑的代码VAO.release();

glfwSwapBuffers(window);和glfwPollEvents();分别是glut交换缓冲区进行绘制和glfw响应键盘鼠标时间的函数，直接删掉

修改后代码为

void HelloTriangleWidget::paintGL()
{
	// render
	// ------
	glClearColor(0.2f, 0.3f, 0.3f, 1.0f);
	glClear(GL_COLOR_BUFFER_BIT);

	// draw our first triangle
	glUseProgram(shaderProgram);
	VAO.bind();
	glDrawArrays(GL_TRIANGLES, 0, 3);
	VAO.release();
}

此时，已经可以成功显示Hello Triangle示例中的三角形，但是不会自动刷新，对于静态显示已经可以达到效果，对于动态显示还需要让程序循环调用paintGL()方法，就像glfw框架下的while (!glfwWindowShouldClose(window))

timer刷新

gl绘制的代码写在paintGL()方法中，这个方法只有在移动窗口或显示/隐藏窗口时会被触发，因此还需新建一个定时器定时调用update()方法，主动触发paintGL()，定时刷新。

将下面代码加在initializeGL()方法最末尾，开启定时器每33ms刷新一次界面，刷新频率30Hz左右

	timer = new QTimer();
	timer->setInterval(33);
	connect(timer, SIGNAL(timeout()), this, SLOT(update()));
	timer->start();

此时opengl控件已经在不断刷新，但是因为绘制的内容只有一个静态三角形，看不出效果，我们修改一些paintGl()方法让背景颜色产生变化，代码如下

void HelloTriangleWidget::paintGL()
{
	// render
	// ------
	static float r = 0.0, g = 0.0, b = 0.0;
	r += 0.01;
	g += 0.02;
	b += 0.03;
	if (r > 1)r = 0;
	if (g > 1)g = 0;
	if (b > 1)b = 0;
	glClearColor(r, g, b, 1.0f);    //清屏
	glClear(GL_COLOR_BUFFER_BIT);

	// draw our first triangle
	glUseProgram(shaderProgram);
	VAO.bind();
	glDrawArrays(GL_TRIANGLES, 0, 3);
	VAO.release();
}

至此全部代码已完成，编译运行，窗口中绘制一个橙色三角形，背景颜色在不断变化

三、总结

将glfw框架下的OpenGL代码移植到qt框架下，需要进行以下修改：

删除glfw相关的窗体绘制的代码和键盘、鼠标响应的代码，在qt中重新实现
将glad的gladLoadGLLoader方法替换为qt的initializeOpenGLFunctions方法
顶点数组对象（VAO）相关的方法glBindVertexArray()和glBindVertexArray()替换为QOpenGLVertexArrayObject::create()、QOpenGLVertexArrayObject::bind()和QOpenGLVertexArrayObject::release()
将render loop主循环中渲染部分的代码移动到paintGL()方法中，并用定时器定时调用update()，让paintGL()方法定时执行

四、全部代码

HelloTriangle.h

// HelloTriangle.h
#pragma once

#ifndef __HELLO_TRIANGLE_H__
#define __HELLO_TRIANGLE_H__

#include <QOpenGLWidget>
#include <QOpenGLFunctions>
#include <QTimer>
#include <QOpenGLVertexArrayObject>

class HelloTriangleWidget : public QOpenGLWidget, protected QOpenGLFunctions
{
    Q_OBJECT

public:
    HelloTriangleWidget(QWidget* parent = 0);
    ~HelloTriangleWidget();

protected:
    void initializeGL() override;
    void resizeGL(int w, int h) override;
    void paintGL() override;

private:
    QTimer* timer;
    unsigned int shaderProgram;
    QOpenGLVertexArrayObject VAO;
};

#endif // WIDGET_H

HelloTriangle.cpp

// HelloTriangle.cpp
#include "HelloTriangle.h"
#include <qDebug>

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

HelloTriangleWidget::HelloTriangleWidget(QWidget* parent)
	: QOpenGLWidget(parent)
{
}

HelloTriangleWidget::~HelloTriangleWidget()
{

}

void HelloTriangleWidget::initializeGL()
{
	this->initializeOpenGLFunctions();    //为当前上下文初始化提供OpenGL函数解析

	// 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);
		qDebug() << "ERROR::SHADER::VERTEX::COMPILATION_FAILED\n" << infoLog;
	}
	// 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);
		qDebug() << "ERROR::SHADER::FRAGMENT::COMPILATION_FAILED\n" << infoLog;
	}
	// link shaders
	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);
		qDebug() << "ERROR::SHADER::PROGRAM::LINKING_FAILED\n" << infoLog;
	}
	glDeleteShader(vertexShader);
	glDeleteShader(fragmentShader);


	// set up vertex data (and buffer(s)) and configure vertex attributes
// ------------------------------------------------------------------
	float vertices[] = {
		-0.5f, -0.5f, 0.0f, // left  
		 0.5f, -0.5f, 0.0f, // right 
		 0.0f,  0.5f, 0.0f  // top   
	};

	unsigned int VBO;
	VAO.create();
	if (!VAO.isCreated()) {
		qDebug() << "vao is not created.";
	}
	glGenBuffers(1, &VBO);
	// bind the Vertex Array Object first, then bind and set vertex buffer(s), and then configure vertex attributes(s).
	VAO.bind();

	glBindBuffer(GL_ARRAY_BUFFER, VBO);
	glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, 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);

	// 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.
	VAO.release();

	timer = new QTimer();
	timer->setInterval(33);
	connect(timer, SIGNAL(timeout()), this, SLOT(update()));
	timer->start();
}

void HelloTriangleWidget::resizeGL(int w, int h)
{
	glViewport(0.0f, 0.0f, w, h);    //调整视口
}

void HelloTriangleWidget::paintGL()
{
	// render
	// ------
	static float r = 0.0, g = 0.0, b = 0.0;
	r += 0.01;
	g += 0.02;
	b += 0.03;
	if (r > 1)r = 0;
	if (g > 1)g = 0;
	if (b > 1)b = 0;
	glClearColor(r, g, b, 1.0f);    //清屏
	glClear(GL_COLOR_BUFFER_BIT);

	// draw our first triangle
	glUseProgram(shaderProgram);
	VAO.bind();
	glDrawArrays(GL_TRIANGLES, 0, 3);
	VAO.release();
}

qt_test.h

// qt_test.h
#pragma once

#include <QtWidgets/QMainWindow>
#include <Qlabel>
#include <QPushButton>
#include <Qopenglwidget>
#include "ui_qt_test.h"
#include "GlWidget.h"
#include "HelloTriangle.h"

class qt_test : public QMainWindow
{
    Q_OBJECT

public:
    qt_test(QWidget *parent = Q_NULLPTR);
    void onButtonClick();

private:
    Ui::qt_testClass ui;
    QLabel* lab;
    QPushButton* button;

    HelloTriangleWidget* opengl_widget;
};

qt_test.cpp

// qt_test.cpp
#include "qt_test.h"

qt_test::qt_test(QWidget *parent)
    : QMainWindow(parent)
{
    ui.setupUi(this);
    this->opengl_widget = new HelloTriangleWidget(this);
    this->opengl_widget->setGeometry(this->geometry());
    this->lab = new QLabel("Hello,World!", this);
    this->lab->setGeometry(10, 20, 100, 20);
    this->button = new QPushButton("test", this);
    this->button->setGeometry(10, 40, 100, 20);
	QObject::connect(this->button, &QPushButton::clicked, this, &qt_test::onButtonClick);
}

void qt_test::onButtonClick()
{
    static int i = 0;
    this->lab->setText(QString::number(i++));
}

main.cpp

// main.cpp
#include "qt_test.h"
#include <QtWidgets/QApplication>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    qt_test w;
    w.show();
    return a.exec();
}