Small examples for game development in Qt, OpenGL, Box2D, Bullet Physics, and OpenAL

8Observer8

@Bondrusiek said in Small examples for game development in Qt, OpenGL, Box2D, Bullet Physics, and OpenAL:

I think it can be similar on Windows.

I confirm that it works on Windows. I'm guessing that the openglwidgets module includes the opengl and widgets modules. I've modified the examples to be the same for QOpenGLWindow and QOpenGLWidget. Thank you very much!

QT += core gui openglwidgets

win32: LIBS += -lopengl32

CONFIG += c++17

SOURCES += \
    main.cpp

win32: means that it is used on Windows only and it will be ignored on other OS like Android, iOS, macOS and so on. When I use glDrawArrays in separated file and if I don't add this line win32: LIBS += -lopengl32, I have this error on Windows:

line_drawer.cpp:74: error: undefined reference to `__imp_glDrawArrays'

It can be solved by adding this line of code to pro-file:

win32: LIBS += -lopengl32

What is interesting, I can delete the line above after recompilation and it will run. But when I change something in the code and recompile it I will have the same error. So the win32: LIBS += -lopengl32 must be in pro-file if you don't call glDrawArrays in the same file.

8Observer8

I have made the next demo for WebAssembly, Android, and Desktop using: Qt C++, OpenGL ES 2.0, OpenAL-Soft (this is a library for music and sounds), Box2D (for jumps, collision detections, and ray casting), Hiero (this is an application to create a font with distance field from TTF), Free Texture Packer (to pack images to one texture atlas), and Tiled map editor (to position sprites and Box2D static colliders).

Click to run in your browser (it is a link to itch where you can download EXE for Windows 10 64 bit and APK for Android 7-14)
Click to run in your browser (it is a link to the Netlify free hosting)

All resources (sprites, music and sounds) have been replaced with free ones. You can see a list of free resources here. For example, I took the sprites here: https://webfussel.itch.io/more-bit-8-bit-mario

I have made a custom joystick for Android in pure OpenGL ES 2.0 (this is an animation from the real phone that I made using scrcpy):

8Observer8

Setting English as system language

If a user system language is not English it will not work:

void OpenGLWindow::keyPressEvent(QKeyEvent *event)
{
    switch (event->key())
    {
        case Qt::Key::Key_W:
        case Qt::Key::Key_Up:
        {
                break;
        {

The following command is a solution for Windows. You can set English as system language:

OpenGLWindow::OpenGLWindow()
{
    setTitle("OpenGL ES 2.0, Qt6, C++");
    resize(400, 400);

// Set English as system language
#ifdef _WIN32
    PostMessage(GetForegroundWindow(), WM_INPUTLANGCHANGEREQUEST, 1, 0x04090409);
#endif

You don't need to include <windows.h> because Qt is included it by default:

#ifdef _WIN32
#include <windows.h>
#endif

8Observer8

Printing OpenGL version to the Debug console

The following commands print OpenGL and GLSL version and vendor:

qDebug() << "OpenGL version:" << (const char*) glGetString(GL_VERSION);
qDebug() << "GLSL version: " << (const char*) glGetString(GL_SHADING_LANGUAGE_VERSION);
qDebug() << "Vendor: " << (const char*) glGetString(GL_VENDOR);

or

std::cout << "OpenGL version: " << glGetString(GL_VERSION) << std::endl;
std::cout << "GLSL version: " << glGetString(GL_SHADING_LANGUAGE_VERSION) << std::endl;
std::cout << "Vendor: " << glGetString(GL_VENDOR) << std::endl;

Qt by default runs programs with integrated video card on laptop:

OpenGL version: 3.1.0 - Build 9.17.10.4459
GLSL version:  1.40 - Intel Build 9.17.10.4459
Vendor:  Intel

8Observer8

Loading 3D models from DAE COLLADA

This example shows how to parse XML (.dae) using QDomDocument. It was tested on Android 7.1.1, Windows 10, and WebAssembly.

Click this link to run the demo in your browser
QOpenGLWindow: Source code on GitHub
QOpenGLWidget: Source code on GitHub

Note. antialiasing works for WebAssembly with QOpenGLWindow for Qt 6.6.3 but it doesn't work for QOpenGLWidget. It was fixed in Qt 6.7.0. See the bug report: https://bugreports.qt.io/browse/QTBUG-123816

8Observer8

Pick color of a pixel with glReadPixels by mouse click or by touching on Mobile

glReadPixels requires the following parameters:

GLubyte pixel[4];
glReadPixels(m_mouseX, m_mouseY, 1, 1, GL_RGBA, GL_UNSIGNED_BYTE, pixel);

Where m_mouseX and m_mouseY are the mouse click coordinates. You should recalculate them before of calling glReadPixels because glReadPixels uses bottom left corner as (0, 0) but Qt uses top left corner as (0, 0). You should add QWindow::devicePixelRatio() because Windows has devicePixelRatio = 1 but Android and macOS (maybe) has devicePixelRatio = 2:

void mousePressEvent(QMouseEvent *event) override
{
    m_mouseX = event->pos().x() * devicePixelRatio();
    m_mouseY = (height() - event->pos().y() - 1) * devicePixelRatio();
    m_mouseClicked = true;
    update();
}

pick-color-of-simple-triangle-qopenglwindow-qt6-cpp.pro

QT += core gui openglwidgets

CONFIG += c++17

SOURCES += \
    main.cpp

main.cpp

#include <QtGui/QMouseEvent>
#include <QtGui/QOpenGLFunctions>
#include <QtOpenGL/QOpenGLBuffer>
#include <QtOpenGL/QOpenGLShaderProgram>
#include <QtOpenGL/QOpenGLWindow>
#include <QtWidgets/QApplication>

class OpenGLWindow : public QOpenGLWindow, private QOpenGLFunctions
{
public:
    OpenGLWindow()
    {
        setTitle("OpenGL ES 2.0, Qt6, C++");
        resize(380, 380);
    }

private:
    void initializeGL() override
    {
        initializeOpenGLFunctions();
        glClearColor(0.2f, 0.2f, 0.2f, 1.f);
        qDebug() << "Device pixel ratio:" << devicePixelRatio();

        QString vertexShaderSource =
            "attribute vec2 aPosition;\n"
            "void main()\n"
            "{\n"
            "    gl_Position = vec4(aPosition, 0.0, 1.0);\n"
            "}\n";

        QString fragmentShaderSource =
            "#ifdef GL_ES\n"
            "precision mediump float;\n"
            "#endif\n"
            "void main()\n"
            "{\n"
            "    gl_FragColor = vec4(1.0, 0.0, 0.0, 1.0);\n"
            "}\n";

        m_program.create();
        m_program.addShaderFromSourceCode(QOpenGLShader::ShaderTypeBit::Vertex,
                                          vertexShaderSource);
        m_program.addShaderFromSourceCode(QOpenGLShader::ShaderTypeBit::Fragment,
                                          fragmentShaderSource);
        m_program.link();
        m_program.bind();

        float vertPositions[] = {
            -0.5f, -0.5f,
            0.5f, -0.5f,
            0.f, 0.5f
        };
        m_vertPosBuffer.create();
        m_vertPosBuffer.bind();
        m_vertPosBuffer.allocate(vertPositions, sizeof(vertPositions));

        m_program.setAttributeBuffer("aPosition", GL_FLOAT, 0, 2);
        m_program.enableAttributeArray("aPosition");
    }

    void paintGL() override
    {
        glClear(GL_COLOR_BUFFER_BIT);
        glDrawArrays(GL_TRIANGLES, 0, 3);

        if (m_mouseClicked)
        {
            // Read the pixel
            GLubyte pixel[4];
            glReadPixels(m_mouseX, m_mouseY, 1, 1, GL_RGBA, GL_UNSIGNED_BYTE, pixel);
            // Print a color
            qDebug() << pixel[0] / 255.f << pixel[1] / 255.f << pixel[2] / 255.f;
            m_mouseClicked = false;
        }
    }

    void mousePressEvent(QMouseEvent *event) override
    {
        m_mouseX = event->pos().x() * devicePixelRatio();
        m_mouseY = (height() - event->pos().y() - 1) * devicePixelRatio();
        m_mouseClicked = true;
        update();
    }

private:
    int m_mouseX;
    int m_mouseY;
    bool m_mouseClicked = false;
    QOpenGLBuffer m_vertPosBuffer;
    QOpenGLShaderProgram m_program;
};

int main(int argc, char *argv[])
{
    QApplication::setAttribute(Qt::ApplicationAttribute::AA_UseDesktopOpenGL);
    QApplication app(argc, argv);
    OpenGLWindow w;
    w.show();
    return app.exec();
}

8Observer8

Perspective camera

perspective-camera-opengles2-qt6-cpp.pro

QT += core gui openglwidgets

CONFIG += c++17

SOURCES += \
    main.cpp

main.cpp

#include <QtGui/QMatrix4x4>
#include <QtGui/QOpenGLFunctions>
#include <QtGui/QSurfaceFormat>
#include <QtGui/QVector3D>
#include <QtOpenGL/QOpenGLBuffer>
#include <QtOpenGL/QOpenGLShader>
#include <QtOpenGL/QOpenGLShaderProgram>
#include <QtOpenGL/QOpenGLWindow>
#include <QtWidgets/QApplication>

class OpenGLWindow : public QOpenGLWindow, private QOpenGLFunctions
{
public:
    OpenGLWindow()
    {
        setTitle("OpenGL ES 2.0, Qt6, C++");
        resize(500, 500);

        QSurfaceFormat surfaceFormat;
        surfaceFormat.setDepthBufferSize(24);
        surfaceFormat.setSamples(4);
        setFormat(surfaceFormat);
    }

    void initializeGL() override
    {
        initializeOpenGLFunctions();
        glClearColor(153.f/255.f, 220.f/255.f, 236.f/255.f, 1.f);

        QString vertShaderSrc =
            "attribute vec2 aPosition;\n"
            "uniform mat4 uMvpMatrix;"
            "void main()\n"
            "{\n"
            "    gl_Position = uMvpMatrix * vec4(aPosition, 0.0, 1.0);\n"
            "}\n";

        QString fragShaderSrc =
            "#ifdef GL_ES\n"
            "precision mediump float;\n"
            "#endif\n"
            "void main()\n"
            "{\n"
            "    gl_FragColor = vec4(0.058, 0.615, 0.345, 1.0);\n"
            "}\n";

        m_program.create();
        m_program.addShaderFromSourceCode(QOpenGLShader::ShaderTypeBit::Vertex, vertShaderSrc);
        m_program.addShaderFromSourceCode(QOpenGLShader::ShaderTypeBit::Fragment, fragShaderSrc);
        m_program.link();
        m_program.bind();

        float vertPositions[] = {
            -0.5f, -0.5f,
            0.5f, -0.5f,
            -0.5f, 0.5f,
            0.5f, 0.5f
        };
        m_vertPosBuffer.create();
        m_vertPosBuffer.bind();
        m_vertPosBuffer.allocate(vertPositions, sizeof(vertPositions));
        m_program.setAttributeBuffer("aPosition", GL_FLOAT, 0, 2);
        m_program.enableAttributeArray("aPosition");

        m_uMvpMatrixLocation = m_program.uniformLocation("uMvpMatrix");
        m_viewMatrix.lookAt(QVector3D(0, 3, 5), QVector3D(0, 0, 0), QVector3D(0, 1, 0));
    }

    void resizeGL(int w, int h) override
    {
        m_projMatrix.setToIdentity();
        m_projMatrix.perspective(50.f, w / (float) h, 0.1f, 100.f);
        m_projViewMatrix = m_projMatrix * m_viewMatrix;
    }

    void paintGL() override
    {
        glClear(GL_COLOR_BUFFER_BIT);
        m_modelMatrix.setToIdentity();
        m_modelMatrix.translate(QVector3D(0, 0, 0));
        m_modelMatrix.rotate(90, QVector3D(1, 0, 0));
        m_modelMatrix.scale(QVector3D(3, 3, 1));
        m_mvpMatrix = m_projViewMatrix * m_modelMatrix;
        m_program.setUniformValue(m_uMvpMatrixLocation, m_mvpMatrix);
        glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);
    }

private:
    QOpenGLBuffer m_vertPosBuffer;
    QOpenGLShaderProgram m_program;
    int m_uMvpMatrixLocation;
    QMatrix4x4 m_mvpMatrix;
    QMatrix4x4 m_projMatrix;
    QMatrix4x4 m_viewMatrix;
    QMatrix4x4 m_projViewMatrix;
    QMatrix4x4 m_modelMatrix;
};

int main(int argc, char *argv[])
{
    QApplication::setAttribute(Qt::ApplicationAttribute::AA_UseDesktopOpenGL);
    QApplication app(argc, argv);
    OpenGLWindow w;
    w.show();
    return app.exec();
}

8Observer8

Rotate and zoom camera

Click here to test WebAssembly demo in the browser

On Android only rotation works. Some work must be made with touch events for zooming:

rotate-and-zoom-camera-opengles2-qt6-cpp.pro

QT += core gui openglwidgets

CONFIG += c++17

SOURCES += \
    main.cpp

main.cpp

#include <QtGui/QMatrix4x4>
#include <QtGui/QOpenGLFunctions>
#include <QtGui/QSurfaceFormat>
#include <QtGui/QVector3D>
#include <QtGui/QMouseEvent>
#include <QtOpenGL/QOpenGLBuffer>
#include <QtOpenGL/QOpenGLShader>
#include <QtOpenGL/QOpenGLShaderProgram>
#include <QtOpenGL/QOpenGLWindow>
#include <QtWidgets/QApplication>

class OpenGLWindow : public QOpenGLWindow, private QOpenGLFunctions
{
public:
    OpenGLWindow()
    {
        setTitle("OpenGL ES 2.0, Qt6, C++");
        resize(500, 500);
        m_degreesPerPixelX = 90.f / (float) width();
        m_degreesPerPixelY = 180.f / (float) height();

        QSurfaceFormat surfaceFormat;
        surfaceFormat.setDepthBufferSize(24);
        surfaceFormat.setSamples(4);
        setFormat(surfaceFormat);
    }

    void initializeGL() override
    {
        initializeOpenGLFunctions();
        glClearColor(153.f/255.f, 220.f/255.f, 236.f/255.f, 1.f);

        QString vertShaderSrc =
            "attribute vec2 aPosition;\n"
            "uniform mat4 uMvpMatrix;"
            "void main()\n"
            "{\n"
            "    gl_Position = uMvpMatrix * vec4(aPosition, 0.0, 1.0);\n"
            "}\n";

        QString fragShaderSrc =
            "#ifdef GL_ES\n"
            "precision mediump float;\n"
            "#endif\n"
            "void main()\n"
            "{\n"
            "    gl_FragColor = vec4(0.058, 0.615, 0.345, 1.0);\n"
            "}\n";

        m_program.create();
        m_program.addShaderFromSourceCode(QOpenGLShader::ShaderTypeBit::Vertex, vertShaderSrc);
        m_program.addShaderFromSourceCode(QOpenGLShader::ShaderTypeBit::Fragment, fragShaderSrc);
        m_program.link();
        m_program.bind();

        float vertPositions[] = {
            -0.5f, -0.5f,
            0.5f, -0.5f,
            -0.5f, 0.5f,
            0.5f, 0.5f
        };
        m_vertPosBuffer.create();
        m_vertPosBuffer.bind();
        m_vertPosBuffer.allocate(vertPositions, sizeof(vertPositions));
        m_program.setAttributeBuffer("aPosition", GL_FLOAT, 0, 2);
        m_program.enableAttributeArray("aPosition");

        m_uMvpMatrixLocation = m_program.uniformLocation("uMvpMatrix");
        m_viewMatrix = getViewMatrix();
    }

    void resizeGL(int w, int h) override
    {
        m_projMatrix.setToIdentity();
        m_projMatrix.perspective(50.f, w / (float)h, 0.1f, 100.f);
    }

    void paintGL() override
    {
        glClear(GL_COLOR_BUFFER_BIT);
        m_modelMatrix.setToIdentity();
        m_modelMatrix.translate(QVector3D(0, 0, 0));
        m_modelMatrix.rotate(90, QVector3D(1, 0, 0));
        m_modelMatrix.scale(QVector3D(3, 3, 1));
        m_projViewMatrix = m_projMatrix * m_viewMatrix;
        m_mvpMatrix = m_projViewMatrix * m_modelMatrix;
        m_program.setUniformValue(m_uMvpMatrixLocation, m_mvpMatrix);
        glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);
    }

    void mousePressEvent(QMouseEvent *event) override
    {
        switch (event->button())
        {
            case Qt::MouseButton::LeftButton:
            {
                if (m_mouseHolding)
                    return;

                m_mouseHolding = true;
                m_mousePrevX = (float) event->pos().x();
                m_mousePrevY = (float) event->pos().y();
                break;
            }
            default:
                break;
        }
    }

    void mouseMoveEvent(QMouseEvent *event) override
    {
        if (!m_mouseHolding)
            return;

        float x = (float) event->pos().x();
        float y = (float) event->pos().y();

        float newCameraRotX = m_cameraRotX + m_degreesPerPixelX * (y - m_mousePrevY);
        newCameraRotX = qMax(-85.f, qMin(85.f, newCameraRotX));
        float newCameraRotY = m_cameraRotY + m_degreesPerPixelY * (x - m_mousePrevX);
        m_mousePrevX = x;
        m_mousePrevY = y;
        m_cameraRotX = newCameraRotX;
        m_cameraRotY = newCameraRotY;

        m_viewMatrix = getViewMatrix();
        update();
    }

    void mouseReleaseEvent(QMouseEvent *event) override
    {
        Q_UNUSED(event);
        m_mouseHolding = false;
    }

    void wheelEvent(QWheelEvent *event) override
    {
        float delta = event->angleDelta().y();
        m_viewDistance += -delta / 500.f;
        m_viewMatrix = getViewMatrix();
        update();
    }

    QMatrix4x4 getViewMatrix()
    {
        QMatrix4x4 mat;

        const float cosX = qCos(m_cameraRotX / 180.f * M_PI);
        const float sinX = qSin(m_cameraRotX / 180.f * M_PI);
        const float cosY = qCos(m_cameraRotY / 180.f * M_PI);
        const float sinY = qSin(m_cameraRotY / 180.f * M_PI);

        mat.setColumn(0, QVector4D(cosY, sinX * sinY, -cosX * sinY, 0.f));
        mat.setColumn(1, QVector4D(0.f, cosX, sinX, 0.f));
        mat.setColumn(2, QVector4D(sinY, -sinX * cosY, cosX * cosY, 0.f));
        mat.setColumn(3, QVector4D(0, 0, -m_viewDistance, 1.f));

        return mat;
    }

private:
    QOpenGLBuffer m_vertPosBuffer;
    QOpenGLShaderProgram m_program;
    int m_uMvpMatrixLocation;
    QMatrix4x4 m_mvpMatrix;
    QMatrix4x4 m_projMatrix;
    QMatrix4x4 m_viewMatrix;
    QMatrix4x4 m_projViewMatrix;
    QMatrix4x4 m_modelMatrix;
    bool m_mouseHolding = false;
    float m_mousePrevX = 0.f;
    float m_mousePrevY = 0.f;
    float m_cameraRotX = 30.f;
    float m_cameraRotY = 0.f;
    float m_viewDistance = 5.f;
    float m_degreesPerPixelX;
    float m_degreesPerPixelY;
};

int main(int argc, char *argv[])
{
    QApplication::setAttribute(Qt::ApplicationAttribute::AA_UseDesktopOpenGL);
    QApplication app(argc, argv);
    OpenGLWindow w;
    w.show();
    return app.exec();
}

8Observer8

A circle (disk) is inscribed in a square

WebAssembly demo on free Netlify hosting

disk-opengles2-qt6-cpp.pro

QT += core gui openglwidgets

CONFIG += c++17

SOURCES += \
    main.cpp

main.cpp

#include <QtMath>
#include <QtGui/QMatrix4x4>
#include <QtGui/QOpenGLFunctions>
#include <QtGui/QSurfaceFormat>
#include <QtGui/QVector3D>
#include <QtOpenGL/QOpenGLBuffer>
#include <QtOpenGL/QOpenGLShader>
#include <QtOpenGL/QOpenGLShaderProgram>
#include <QtOpenGL/QOpenGLWindow>
#include <QtWidgets/QApplication>

class OpenGLWindow : public QOpenGLWindow, private QOpenGLFunctions
{
public:
    OpenGLWindow()
    {
        setTitle("OpenGL ES 2.0, Qt6, C++");
        resize(380, 380);

        QSurfaceFormat surfaceFormat;
        surfaceFormat.setDepthBufferSize(24);
        surfaceFormat.setSamples(4);
        setFormat(surfaceFormat);
    }

    void initializeGL() override
    {
        initializeOpenGLFunctions();
        glClearColor(0.04f, 0.62f, 0.48f, 1.f);

        QString vertShaderSrc =
            "attribute vec2 aPosition;\n"
            "uniform mat4 uMvpMatrix;"
            "void main()\n"
            "{\n"
            "    gl_Position = uMvpMatrix * vec4(aPosition, 0.0, 1.0);\n"
            "}\n";

        QString fragShaderSrc =
            "#ifdef GL_ES\n"
            "precision mediump float;\n"
            "#endif\n"
            "uniform vec3 uColor;\n"
            "void main()\n"
            "{\n"
            "    gl_FragColor = vec4(uColor, 1.0);\n"
            "}\n";

        m_program.create();
        m_program.addShaderFromSourceCode(QOpenGLShader::ShaderTypeBit::Vertex, vertShaderSrc);
        m_program.addShaderFromSourceCode(QOpenGLShader::ShaderTypeBit::Fragment, fragShaderSrc);
        m_program.link();
        m_program.bind();

        float vertPositions[m_amountOfDiskVertices * 2 + 8]; // Disk + Square
        float radius = 0.5f;
        float angle = 0.f;

        int index = 0;
        // Disk center
        vertPositions[index++] = 0.f;
        vertPositions[index++] = 0.f;

        // Disk
        for (int i = 0; i < m_amountOfDiskVertices - 1; i++)
        {
            float radians = qDegreesToRadians(angle);
            float x = radius * qCos(radians);
            float y = radius * qSin(radians);
            vertPositions[index++] = x;
            vertPositions[index++] = y;
            angle += m_angleStep;
        }
        // Square
        vertPositions[index++] = -0.5f;
        vertPositions[index++] = -0.5f;
        vertPositions[index++] = 0.5f;
        vertPositions[index++] = -0.5f;
        vertPositions[index++] = -0.5f;
        vertPositions[index++] = 0.5f;
        vertPositions[index++] = 0.5f;
        vertPositions[index++] = 0.5f;
        m_vertPosBuffer.create();
        m_vertPosBuffer.bind();
        m_vertPosBuffer.allocate(vertPositions, sizeof(vertPositions));
        m_program.setAttributeBuffer("aPosition", GL_FLOAT, 0, 2);
        m_program.enableAttributeArray("aPosition");

        m_uColorLocation = m_program.uniformLocation("uColor");
        m_uMvpMatrixLocation = m_program.uniformLocation("uMvpMatrix");
        m_viewMatrix.lookAt(QVector3D(0, 0, 1), QVector3D(0, 0, 0), QVector3D(0, 1, 0));
    }

    void resizeGL(int w, int h) override
    {
        int deviceW = w * devicePixelRatio();
        int deviceH = h * devicePixelRatio();
        float deviceAspect = deviceH / (float) deviceW;

        if (deviceAspect > m_worldAspect)
        {
            m_viewportWidth = deviceW;
            m_viewportHeight = (int) deviceW * m_worldAspect;
            m_viewportX = 0;
            m_viewportY = (int) (deviceH - m_viewportHeight) / 2.f;
        }
        else
        {
            m_viewportWidth = (int) deviceH / m_worldAspect;
            m_viewportHeight = deviceH;
            m_viewportX = (int) (deviceW - m_viewportWidth) / 2.f;
            m_viewportY = 0;
        }
        m_projMatrix.setToIdentity();
        m_projMatrix.ortho(0.f, m_worldWidth, 0.f, m_worldHeight, 1.f, -1.f);
        m_projViewMatrix = m_projMatrix * m_viewMatrix;
    }

    void paintGL() override
    {
        glClear(GL_COLOR_BUFFER_BIT);
        glClearColor(0.2, 0.2, 0.2, 1);
        glClear(GL_COLOR_BUFFER_BIT);
        glViewport(m_viewportX, m_viewportY, m_viewportWidth, m_viewportHeight);
        glScissor(m_viewportX, m_viewportY, m_viewportWidth, m_viewportHeight);
        glClearColor(0.04, 0.62, 0.48, 1);
        glEnable(GL_SCISSOR_TEST);
        glClear(GL_COLOR_BUFFER_BIT);
        glDisable(GL_SCISSOR_TEST);

        // Square
        drawRectangle(100, 50, 50, 50, 0, QVector3D(0.3, 0.07, 0.5));
        // Disk
        drawDisk(100, 50, 50, QVector3D(0.8, 0.8, 0.5));
        // Left border
        drawRectangle(5, 50, 5, 85, 0, QVector3D(0.62, 0.04, 0.18));
        // Right border
        drawRectangle(195, 50, 5, 85, 0, QVector3D(0.62, 0.04, 0.18));
        // Top border
        drawRectangle(100, 95, 185, 5, 0, QVector3D(0.62, 0.04, 0.18));
        // Bottom border
        drawRectangle(100, 5, 185, 5, 0, QVector3D(0.62, 0.04, 0.18));
    }

    void drawDisk(float x, float y, float diameter, const QVector3D& color)
    {
        m_modelMatrix.setToIdentity();
        m_modelMatrix.translate(QVector3D(x, y, 0));
        m_modelMatrix.scale(QVector3D(diameter, diameter, 1));
        m_mvpMatrix = m_projViewMatrix * m_modelMatrix;
        m_program.setUniformValue(m_uMvpMatrixLocation, m_mvpMatrix);
        m_program.setUniformValue(m_uColorLocation, color);
        glDrawArrays(GL_TRIANGLE_FAN, 0, m_amountOfDiskVertices);
    }

    void drawRectangle(float x, float y, float w, float h,
        float angle, const QVector3D& color)
    {
        m_modelMatrix.setToIdentity();
        m_modelMatrix.translate(QVector3D(x, y, 0));
        m_modelMatrix.rotate(angle, QVector3D(0, 0, 1));
        m_modelMatrix.scale(QVector3D(w, h, 1));
        m_mvpMatrix = m_projViewMatrix * m_modelMatrix;
        m_program.setUniformValue(m_uMvpMatrixLocation, m_mvpMatrix);
        m_program.setUniformValue(m_uColorLocation, color);
        glDrawArrays(GL_TRIANGLE_STRIP, m_amountOfDiskVertices, 4);
    }

private:
    QOpenGLBuffer m_vertPosBuffer;
    QOpenGLShaderProgram m_program;
    int m_uColorLocation;
    int m_uMvpMatrixLocation;
    QMatrix4x4 m_mvpMatrix;
    QMatrix4x4 m_projMatrix;
    QMatrix4x4 m_viewMatrix;
    QMatrix4x4 m_projViewMatrix;
    QMatrix4x4 m_modelMatrix;
    const float m_worldWidth = 200.f;
    const float m_worldHeight = 100.f;
    float m_worldAspect = m_worldHeight / m_worldWidth;
    int m_viewportX;
    int m_viewportY;
    int m_viewportWidth;
    int m_viewportHeight;
    float m_angleStep = 10.f;
    int m_amountOfDiskVertices = 360.f / m_angleStep + 2;
};

int main(int argc, char *argv[])
{
    QApplication::setAttribute(Qt::ApplicationAttribute::AA_UseDesktopOpenGL);
    QApplication app(argc, argv);
    OpenGLWindow w;
    w.show();
    return app.exec();
}

8Observer8

Moving a disk by mouse and touching

WebAssembly demo on free Netlify hosting

move-disk-by-mouse-and-touching-opengles2-qt6-cpp-android.gif

move-disk-by-mouse-and-touching-opengles2-qt6-cpp-desktop.gif

move-disk-by-mouse-and-touching-opengles2-qt6-cpp.pro

QT += core gui openglwidgets

CONFIG += c++17

SOURCES += \
    main.cpp

main.cpp

#include <QtMath>
#include <QtGui/QMatrix4x4>
#include <QtGui/QMouseEvent>
#include <QtGui/QOpenGLFunctions>
#include <QtGui/QSurfaceFormat>
#include <QtGui/QVector3D>
#include <QtOpenGL/QOpenGLBuffer>
#include <QtOpenGL/QOpenGLShader>
#include <QtOpenGL/QOpenGLShaderProgram>
#include <QtOpenGL/QOpenGLWindow>
#include <QtWidgets/QApplication>

class OpenGLWindow : public QOpenGLWindow, private QOpenGLFunctions
{
public:
    OpenGLWindow()
    {
        setTitle("OpenGL ES 2.0, Qt6, C++");
        resize(380, 380);

        QSurfaceFormat surfaceFormat;
        surfaceFormat.setDepthBufferSize(24);
        surfaceFormat.setSamples(4);
        setFormat(surfaceFormat);
    }

    void initializeGL() override
    {
        initializeOpenGLFunctions();
        glClearColor(0.04f, 0.62f, 0.48f, 1.f);

        QString vertShaderSrc =
            "attribute vec2 aPosition;\n"
            "uniform mat4 uMvpMatrix;"
            "void main()\n"
            "{\n"
            "    gl_Position = uMvpMatrix * vec4(aPosition, 0.0, 1.0);\n"
            "}\n";

        QString fragShaderSrc =
            "#ifdef GL_ES\n"
            "precision mediump float;\n"
            "#endif\n"
            "uniform vec3 uColor;\n"
            "void main()\n"
            "{\n"
            "    gl_FragColor = vec4(uColor, 1.0);\n"
            "}\n";

        m_program.create();
        m_program.addShaderFromSourceCode(QOpenGLShader::ShaderTypeBit::Vertex, vertShaderSrc);
        m_program.addShaderFromSourceCode(QOpenGLShader::ShaderTypeBit::Fragment, fragShaderSrc);
        m_program.link();
        m_program.bind();

        float vertPositions[m_amountOfDiskVertices * 2 + 8]; // Disk + Square
        float radius = 0.5f;
        float angle = 0.f;

        int index = 0;
        // Disk center
        vertPositions[index++] = 0.f;
        vertPositions[index++] = 0.f;

        // Disk
        for (int i = 0; i < m_amountOfDiskVertices - 1; i++)
        {
            float radians = qDegreesToRadians(angle);
            float x = radius * qCos(radians);
            float y = radius * qSin(radians);
            vertPositions[index++] = x;
            vertPositions[index++] = y;
            angle += m_angleStep;
        }
        // Square
        vertPositions[index++] = -0.5f;
        vertPositions[index++] = -0.5f;
        vertPositions[index++] = 0.5f;
        vertPositions[index++] = -0.5f;
        vertPositions[index++] = -0.5f;
        vertPositions[index++] = 0.5f;
        vertPositions[index++] = 0.5f;
        vertPositions[index++] = 0.5f;
        m_vertPosBuffer.create();
        m_vertPosBuffer.bind();
        m_vertPosBuffer.allocate(vertPositions, sizeof(vertPositions));
        m_program.setAttributeBuffer("aPosition", GL_FLOAT, 0, 2);
        m_program.enableAttributeArray("aPosition");

        m_uColorLocation = m_program.uniformLocation("uColor");
        m_uMvpMatrixLocation = m_program.uniformLocation("uMvpMatrix");
        m_viewMatrix.lookAt(QVector3D(0, 0, 1), QVector3D(0, 0, 0), QVector3D(0, 1, 0));
    }

    void resizeGL(int w, int h) override
    {
        int deviceW = w * devicePixelRatio();
        int deviceH = h * devicePixelRatio();
        float deviceAspect = deviceH / (float) deviceW;

        if (deviceAspect > m_worldAspect)
        {
            m_viewportWidth = deviceW;
            m_viewportHeight = (int) deviceW * m_worldAspect;
            m_viewportX = 0;
            m_viewportY = (int) (deviceH - m_viewportHeight) / 2.f;
        }
        else
        {
            m_viewportWidth = (int) deviceH / m_worldAspect;
            m_viewportHeight = deviceH;
            m_viewportX = (int) (deviceW - m_viewportWidth) / 2.f;
            m_viewportY = 0;
        }
        m_projMatrix.setToIdentity();
        m_projMatrix.ortho(0.f, m_worldWidth, 0.f, m_worldHeight, 1.f, -1.f);
        m_projViewMatrix = m_projMatrix * m_viewMatrix;
    }

    void paintGL() override
    {
        glClear(GL_COLOR_BUFFER_BIT);
        glClearColor(0.2, 0.2, 0.2, 1);
        glClear(GL_COLOR_BUFFER_BIT);
        glViewport(m_viewportX, m_viewportY, m_viewportWidth, m_viewportHeight);
        glScissor(m_viewportX, m_viewportY, m_viewportWidth, m_viewportHeight);
        glClearColor(0.04, 0.62, 0.48, 1);
        glEnable(GL_SCISSOR_TEST);
        glClear(GL_COLOR_BUFFER_BIT);
        glDisable(GL_SCISSOR_TEST);

        // Square
        drawRectangle(100, 50, 50, 50, 0, QVector3D(0.3, 0.07, 0.5));
        // Disk
        drawDisk(m_diskPosX, m_diskPosY, 50, QVector3D(0.8, 0.8, 0.5));
        // Left border
        drawRectangle(5, 50, 5, 85, 0, QVector3D(0.62, 0.04, 0.18));
        // Right border
        drawRectangle(195, 50, 5, 85, 0, QVector3D(0.62, 0.04, 0.18));
        // Top border
        drawRectangle(100, 95, 185, 5, 0, QVector3D(0.62, 0.04, 0.18));
        // Bottom border
        drawRectangle(100, 5, 185, 5, 0, QVector3D(0.62, 0.04, 0.18));
    }

    void drawDisk(float x, float y, float diameter, const QVector3D& color)
    {
        m_modelMatrix.setToIdentity();
        m_modelMatrix.translate(QVector3D(x, y, 0));
        m_modelMatrix.scale(QVector3D(diameter, diameter, 1));
        m_mvpMatrix = m_projViewMatrix * m_modelMatrix;
        m_program.setUniformValue(m_uMvpMatrixLocation, m_mvpMatrix);
        m_program.setUniformValue(m_uColorLocation, color);
        glDrawArrays(GL_TRIANGLE_FAN, 0, m_amountOfDiskVertices);
    }

    void drawRectangle(float x, float y, float w, float h,
        float angle, const QVector3D& color)
    {
        m_modelMatrix.setToIdentity();
        m_modelMatrix.translate(QVector3D(x, y, 0));
        m_modelMatrix.rotate(angle, QVector3D(0, 0, 1));
        m_modelMatrix.scale(QVector3D(w, h, 1));
        m_mvpMatrix = m_projViewMatrix * m_modelMatrix;
        m_program.setUniformValue(m_uMvpMatrixLocation, m_mvpMatrix);
        m_program.setUniformValue(m_uColorLocation, color);
        glDrawArrays(GL_TRIANGLE_STRIP, m_amountOfDiskVertices, 4);
    }

    void mousePressEvent(QMouseEvent *event) override
    {
        switch (event->button()) {
            case Qt::MouseButton::LeftButton:
            {
                m_mouseHolding = true;
                int x = event->pos().x() * devicePixelRatio();
                int y = (height() - event->pos().y() - 1) * devicePixelRatio();
                if ((m_viewportX <= x && x < (m_viewportX + m_viewportWidth)) &&
                    (m_viewportY <= y && y < (m_viewportY + m_viewportHeight)))
                {
                    float normalizedX = (x - m_viewportX) / (float) m_viewportWidth;
                    float normalizedY = (y - m_viewportY) / (float) m_viewportHeight;
                    m_diskPosX = m_worldWidth * normalizedX;
                    m_diskPosY = m_worldHeight * normalizedY;
                }
                update();
                break;
            }
            default:
                break;
        }
    }

    void mouseMoveEvent(QMouseEvent *event) override
    {
        if (!m_mouseHolding)
            return;

        int x = event->pos().x() * devicePixelRatio();
        int y = (height() - event->pos().y() - 1) * devicePixelRatio();
        if ((m_viewportX <= x && x < (m_viewportX + m_viewportWidth)) &&
            (m_viewportY <= y && y < (m_viewportY + m_viewportHeight)))
        {
            float normalizedX = (x - m_viewportX) / (float) m_viewportWidth;
            float normalizedY = (y - m_viewportY) / (float) m_viewportHeight;
            m_diskPosX = m_worldWidth * normalizedX;
            m_diskPosY = m_worldHeight * normalizedY;
        }
        update();
    }

    void mouseReleaseEvent(QMouseEvent *event) override
    {
        switch (event->button()) {
            case Qt::MouseButton::LeftButton:
            {
                m_mouseHolding = false;
                break;
            }
            default:
                break;
        }
    }

private:
    QOpenGLBuffer m_vertPosBuffer;
    QOpenGLShaderProgram m_program;
    int m_uColorLocation;
    int m_uMvpMatrixLocation;
    QMatrix4x4 m_mvpMatrix;
    QMatrix4x4 m_projMatrix;
    QMatrix4x4 m_viewMatrix;
    QMatrix4x4 m_projViewMatrix;
    QMatrix4x4 m_modelMatrix;
    const float m_worldWidth = 200.f;
    const float m_worldHeight = 100.f;
    float m_worldAspect = m_worldHeight / m_worldWidth;
    int m_viewportX;
    int m_viewportY;
    int m_viewportWidth;
    int m_viewportHeight;
    float m_angleStep = 10.f;
    int m_amountOfDiskVertices = 360.f / m_angleStep + 2;
    bool m_mouseHolding = false;
    float m_diskPosX = 100.f;
    float m_diskPosY = 50.f;
};

int main(int argc, char *argv[])
{
    QApplication::setAttribute(Qt::ApplicationAttribute::AA_UseDesktopOpenGL);
    QApplication app(argc, argv);
    OpenGLWindow w;
    w.show();
    return app.exec();
}

8Observer8

Custom collision detection between two disks

WebAssembly demo on free Netlify hosting

custom-collision-detection-disks-opengles2-qt6-cpp-android.gif

custom-collision-detection-disks-opengles2-qt6-cpp-desktop.gif

custom-collision-detection-disks-opengles2-qt6-cpp.pro

QT += core gui openglwidgets

CONFIG += c++17

SOURCES += \
    main.cpp

main.cpp


#include <QtMath>
#include <QtGui/QMatrix4x4>
#include <QtGui/QMouseEvent>
#include <QtGui/QOpenGLFunctions>
#include <QtGui/QSurfaceFormat>
#include <QtGui/QVector3D>
#include <QtGui/QVector4D>
#include <QtOpenGL/QOpenGLBuffer>
#include <QtOpenGL/QOpenGLShader>
#include <QtOpenGL/QOpenGLShaderProgram>
#include <QtOpenGL/QOpenGLWindow>
#include <QtWidgets/QApplication>

class OpenGLWindow : public QOpenGLWindow, private QOpenGLFunctions
{
public:
    OpenGLWindow()
    {
        setTitle("OpenGL ES 2.0, Qt6, C++");
        resize(500, 500);

        QSurfaceFormat surfaceFormat;
        surfaceFormat.setDepthBufferSize(24);
        surfaceFormat.setSamples(4);
        setFormat(surfaceFormat);
    }

    void initializeGL() override
    {
        initializeOpenGLFunctions();
        glClearColor(0.04f, 0.62f, 0.48f, 1.f);
        glEnable(GL_BLEND);
        glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);

        QString vertShaderSrc =
            "attribute vec2 aPosition;\n"
            "uniform mat4 uMvpMatrix;"
            "void main()\n"
            "{\n"
            "    gl_Position = uMvpMatrix * vec4(aPosition, 0.0, 1.0);\n"
            "}\n";

        QString fragShaderSrc =
            "#ifdef GL_ES\n"
            "precision mediump float;\n"
            "#endif\n"
            "uniform vec4 uColor;\n"
            "void main()\n"
            "{\n"
            "    gl_FragColor = uColor;\n"
            "}\n";

        m_program.create();
        m_program.addShaderFromSourceCode(QOpenGLShader::ShaderTypeBit::Vertex, vertShaderSrc);
        m_program.addShaderFromSourceCode(QOpenGLShader::ShaderTypeBit::Fragment, fragShaderSrc);
        m_program.link();
        m_program.bind();

        float vertPositions[m_amountOfDiskVertices * 2 + 8]; // Disk + Square
        float radius = 0.5f;
        float angle = 0.f;

        int index = 0;
        // Disk center
        vertPositions[index++] = 0.f;
        vertPositions[index++] = 0.f;

               // Disk
        for (int i = 0; i < m_amountOfDiskVertices - 1; i++)
        {
            float radians = qDegreesToRadians(angle);
            float x = radius * qCos(radians);
            float y = radius * qSin(radians);
            vertPositions[index++] = x;
            vertPositions[index++] = y;
            angle += m_angleStep;
        }
        // Square
        vertPositions[index++] = -0.5f;
        vertPositions[index++] = -0.5f;
        vertPositions[index++] = 0.5f;
        vertPositions[index++] = -0.5f;
        vertPositions[index++] = -0.5f;
        vertPositions[index++] = 0.5f;
        vertPositions[index++] = 0.5f;
        vertPositions[index++] = 0.5f;
        m_vertPosBuffer.create();
        m_vertPosBuffer.bind();
        m_vertPosBuffer.allocate(vertPositions, sizeof(vertPositions));
        m_program.setAttributeBuffer("aPosition", GL_FLOAT, 0, 2);
        m_program.enableAttributeArray("aPosition");

        m_uColorLocation = m_program.uniformLocation("uColor");
        m_uMvpMatrixLocation = m_program.uniformLocation("uMvpMatrix");
        m_viewMatrix.lookAt(QVector3D(0, 0, 1), QVector3D(0, 0, 0), QVector3D(0, 1, 0));
    }

    void resizeGL(int w, int h) override
    {
        int deviceW = w * devicePixelRatio();
        int deviceH = h * devicePixelRatio();
        float deviceAspect = deviceH / (float) deviceW;

        if (deviceAspect > m_worldAspect)
        {
            m_viewportWidth = deviceW;
            m_viewportHeight = (int) deviceW * m_worldAspect;
            m_viewportX = 0;
            m_viewportY = (int) (deviceH - m_viewportHeight) / 2.f;
        }
        else
        {
            m_viewportWidth = (int) deviceH / m_worldAspect;
            m_viewportHeight = deviceH;
            m_viewportX = (int) (deviceW - m_viewportWidth) / 2.f;
            m_viewportY = 0;
        }
        m_projMatrix.setToIdentity();
        m_projMatrix.ortho(0.f, m_worldWidth, 0.f, m_worldHeight, 1.f, -1.f);
        m_projViewMatrix = m_projMatrix * m_viewMatrix;
    }

    void paintGL() override
    {
        glClear(GL_COLOR_BUFFER_BIT);
        glClearColor(0.2, 0.2, 0.2, 1);
        glClear(GL_COLOR_BUFFER_BIT);
        glViewport(m_viewportX, m_viewportY, m_viewportWidth, m_viewportHeight);
        glScissor(m_viewportX, m_viewportY, m_viewportWidth, m_viewportHeight);
        glClearColor(0.04, 0.62, 0.48, 1);
        glEnable(GL_SCISSOR_TEST);
        glClear(GL_COLOR_BUFFER_BIT);
        glDisable(GL_SCISSOR_TEST);

        m_disk2Color.setW(0.2f);
        if (dist(m_disk1PosX, m_disk1PosY, m_disk2PosX, m_disk2PosY) < 30.f)
            m_disk2Color.setW(1.f);

        // Disk
        drawDisk(m_disk2PosX, m_disk2PosY, 30.f, m_disk2Color);
        // Disk
        drawDisk(m_disk1PosX, m_disk1PosY, 30.f, QVector4D(0.f, 0.f, 0.5f, 1.f));
        // Left border
        drawRectangle(5.f, 50.f, 5.f, 85.f, 0.f, QVector4D(0.62f, 0.04f, 0.18f, 1.f));
        // Right border
        drawRectangle(195.f, 50.f, 5.f, 85.f, 0.f, QVector4D(0.62f, 0.04f, 0.18f, 1.f));
        // Top border
        drawRectangle(100.f, 95.f, 185.f, 5.f, 0.f, QVector4D(0.62f, 0.04f, 0.18f, 1.f));
        // Bottom border
        drawRectangle(100.f, 5.f, 185.f, 5.f, 0.f, QVector4D(0.62f, 0.04f, 0.18f, 1.f));
    }

    float dist(float x1, float y1, float x2, float y2)
    {
        float deltaX = qFabs(x1 - x2);
        float deltaY = qFabs(y1 - y2);
        return qSqrt(qPow(deltaX, 2) + qPow(deltaY, 2));
    }

    void drawDisk(float x, float y, float diameter, const QVector4D& color)
    {
        m_modelMatrix.setToIdentity();
        m_modelMatrix.translate(QVector3D(x, y, 0));
        m_modelMatrix.scale(QVector3D(diameter, diameter, 1));
        m_mvpMatrix = m_projViewMatrix * m_modelMatrix;
        m_program.setUniformValue(m_uMvpMatrixLocation, m_mvpMatrix);
        m_program.setUniformValue(m_uColorLocation, color);
        glDrawArrays(GL_TRIANGLE_FAN, 0, m_amountOfDiskVertices);
    }

    void drawRectangle(float x, float y, float w, float h,
        float angle, const QVector4D& color)
    {
        m_modelMatrix.setToIdentity();
        m_modelMatrix.translate(QVector3D(x, y, 0));
        m_modelMatrix.rotate(angle, QVector3D(0, 0, 1));
        m_modelMatrix.scale(QVector3D(w, h, 1));
        m_mvpMatrix = m_projViewMatrix * m_modelMatrix;
        m_program.setUniformValue(m_uMvpMatrixLocation, m_mvpMatrix);
        m_program.setUniformValue(m_uColorLocation, color);
        glDrawArrays(GL_TRIANGLE_STRIP, m_amountOfDiskVertices, 4);
    }

    void mousePressEvent(QMouseEvent *event) override
    {
        switch (event->button()) {
            case Qt::MouseButton::LeftButton:
            {
                m_mouseHolding = true;
                int x = event->pos().x() * devicePixelRatio();
                int y = (height() - event->pos().y() - 1) * devicePixelRatio();
                if ((m_viewportX <= x && x < (m_viewportX + m_viewportWidth)) &&
                    (m_viewportY <= y && y < (m_viewportY + m_viewportHeight)))
                {
                    float normalizedX = (x - m_viewportX) / (float) m_viewportWidth;
                    float normalizedY = (y - m_viewportY) / (float) m_viewportHeight;
                    m_disk1PosX = m_worldWidth * normalizedX;
                    m_disk1PosY = m_worldHeight * normalizedY;
                }
                update();
                break;
            }
            default:
                break;
        }
    }

    void mouseMoveEvent(QMouseEvent *event) override
    {
        if (!m_mouseHolding)
            return;

        int x = event->pos().x() * devicePixelRatio();
        int y = (height() - event->pos().y() - 1) * devicePixelRatio();
        if ((m_viewportX <= x && x < (m_viewportX + m_viewportWidth)) &&
            (m_viewportY <= y && y < (m_viewportY + m_viewportHeight)))
        {
            float normalizedX = (x - m_viewportX) / (float) m_viewportWidth;
            float normalizedY = (y - m_viewportY) / (float) m_viewportHeight;
            m_disk1PosX = m_worldWidth * normalizedX;
            m_disk1PosY = m_worldHeight * normalizedY;
        }
        update();
    }

    void mouseReleaseEvent(QMouseEvent *event) override
    {
        switch (event->button()) {
            case Qt::MouseButton::LeftButton:
            {
                m_mouseHolding = false;
                break;
            }
            default:
                break;
        }
    }

private:
    QOpenGLBuffer m_vertPosBuffer;
    QOpenGLShaderProgram m_program;
    int m_uColorLocation;
    int m_uMvpMatrixLocation;
    QMatrix4x4 m_mvpMatrix;
    QMatrix4x4 m_projMatrix;
    QMatrix4x4 m_viewMatrix;
    QMatrix4x4 m_projViewMatrix;
    QMatrix4x4 m_modelMatrix;
    const float m_worldWidth = 200.f;
    const float m_worldHeight = 100.f;
    float m_worldAspect = m_worldHeight / m_worldWidth;
    int m_viewportX;
    int m_viewportY;
    int m_viewportWidth;
    int m_viewportHeight;
    float m_angleStep = 10.f;
    int m_amountOfDiskVertices = 360.f / m_angleStep + 2;
    bool m_mouseHolding = false;
    float m_disk1PosX = 150.f;
    float m_disk1PosY = 50.f;
    QVector4D m_disk2Color = QVector4D(0.8f, 0.f, 0.f, 0.2f);
    float m_disk2PosX = 70.f;
    float m_disk2PosY = 50.f;
};

int main(int argc, char *argv[])
{
    QApplication::setAttribute(Qt::ApplicationAttribute::AA_UseDesktopOpenGL);
    QApplication app(argc, argv);
    OpenGLWindow w;
    w.show();
    return app.exec();
}

8Observer8

Lines with thickness with disks between them

This example draws two lines from "start" to "finish" with thickness:

void drawLine(const QVector2D &from, const QVector2D &to, float thickness,
    const QVector3D& color)

It draws a rectangle. I place three disks: in the start point, between lines (where lines are connected), and in the finish point:

        // Strip line
        // Disk
        QVector3D lineColor(0.25f, 0.31f, 0.85f);
        drawDisk(20.f, 50.f, 10.f, lineColor);
        // Line
        drawLine(QVector2D(20.f, 50.f), QVector2D(50.f, 70.f), 10.f, lineColor);
        // Disk
        drawDisk(50.f, 70.f, 10.f, lineColor);
        // Line
        drawLine(QVector2D(50.f, 70.f), QVector2D(60.f, 20.f), 10.f, lineColor);
        // Disk
        drawDisk(60.f, 20.f, 10.f, lineColor);

WebAssembly demo on free Netlify hosting

lines-with-thickness-opengles2-qt6-cpp.pro

QT += core gui openglwidgets

CONFIG += c++17

SOURCES += \
    main.cpp

main.cpp

#include <QtMath>
#include <QtGui/QMatrix4x4>
#include <QtGui/QOpenGLFunctions>
#include <QtGui/QQuaternion>
#include <QtGui/QSurfaceFormat>
#include <QtGui/QVector2D>
#include <QtGui/QVector3D>
#include <QtOpenGL/QOpenGLBuffer>
#include <QtOpenGL/QOpenGLShader>
#include <QtOpenGL/QOpenGLShaderProgram>
#include <QtOpenGL/QOpenGLWindow>
#include <QtWidgets/QApplication>

class OpenGLWindow : public QOpenGLWindow, private QOpenGLFunctions
{
public:
    OpenGLWindow()
    {
        setTitle("OpenGL ES 2.0, Qt6, C++");
        resize(450, 450);

        QSurfaceFormat surfaceFormat;
        surfaceFormat.setDepthBufferSize(24);
        surfaceFormat.setSamples(4);
        setFormat(surfaceFormat);
    }

    void initializeGL() override
    {
        initializeOpenGLFunctions();
        glClearColor(0.04f, 0.62f, 0.48f, 1.f);

        QString vertShaderSrc =
            "attribute vec2 aPosition;\n"
            "uniform mat4 uMvpMatrix;"
            "void main()\n"
            "{\n"
            "    gl_Position = uMvpMatrix * vec4(aPosition, 0.0, 1.0);\n"
            "}\n";

        QString fragShaderSrc =
            "#ifdef GL_ES\n"
            "precision mediump float;\n"
            "#endif\n"
            "uniform vec3 uColor;\n"
            "void main()\n"
            "{\n"
            "    gl_FragColor = vec4(uColor, 1.0);\n"
            "}\n";

        m_program.create();
        m_program.addShaderFromSourceCode(QOpenGLShader::ShaderTypeBit::Vertex, vertShaderSrc);
        m_program.addShaderFromSourceCode(QOpenGLShader::ShaderTypeBit::Fragment, fragShaderSrc);
        m_program.link();
        m_program.bind();

        float vertPositions[m_amountOfDiskVertices * 2 + 8]; // Disk + Square
        float radius = 0.5f;
        float angle = 0.f;

        int index = 0;
        // Disk center
        vertPositions[index++] = 0.f;
        vertPositions[index++] = 0.f;

        // Disk
        for (int i = 0; i < m_amountOfDiskVertices - 1; i++)
        {
            float radians = qDegreesToRadians(angle);
            float x = radius * qCos(radians);
            float y = radius * qSin(radians);
            vertPositions[index++] = x;
            vertPositions[index++] = y;
            angle += m_angleStep;
        }
        // Square
        vertPositions[index++] = -0.5f;
        vertPositions[index++] = -0.5f;
        vertPositions[index++] = 0.5f;
        vertPositions[index++] = -0.5f;
        vertPositions[index++] = -0.5f;
        vertPositions[index++] = 0.5f;
        vertPositions[index++] = 0.5f;
        vertPositions[index++] = 0.5f;
        m_vertPosBuffer.create();
        m_vertPosBuffer.bind();
        m_vertPosBuffer.allocate(vertPositions, sizeof(vertPositions));
        m_program.setAttributeBuffer("aPosition", GL_FLOAT, 0, 2);
        m_program.enableAttributeArray("aPosition");

        m_uColorLocation = m_program.uniformLocation("uColor");
        m_uMvpMatrixLocation = m_program.uniformLocation("uMvpMatrix");
        m_viewMatrix.lookAt(QVector3D(0, 0, 1), QVector3D(0, 0, 0), QVector3D(0, 1, 0));
    }

    void resizeGL(int w, int h) override
    {
        int deviceW = w * devicePixelRatio();
        int deviceH = h * devicePixelRatio();
        float deviceAspect = deviceH / (float) deviceW;

        if (deviceAspect > m_worldAspect)
        {
            m_viewportWidth = deviceW;
            m_viewportHeight = (int) deviceW * m_worldAspect;
            m_viewportX = 0;
            m_viewportY = (int) (deviceH - m_viewportHeight) / 2.f;
        }
        else
        {
            m_viewportWidth = (int) deviceH / m_worldAspect;
            m_viewportHeight = deviceH;
            m_viewportX = (int) (deviceW - m_viewportWidth) / 2.f;
            m_viewportY = 0;
        }
        m_projMatrix.setToIdentity();
        m_projMatrix.ortho(0.f, m_worldWidth, 0.f, m_worldHeight, 1.f, -1.f);
        m_projViewMatrix = m_projMatrix * m_viewMatrix;
    }

    void paintGL() override
    {
        glClear(GL_COLOR_BUFFER_BIT);
        glClearColor(0.82f, 0.87f, 0.97f, 1.f);
        glClear(GL_COLOR_BUFFER_BIT);
        glViewport(m_viewportX, m_viewportY, m_viewportWidth, m_viewportHeight);
        glScissor(m_viewportX, m_viewportY, m_viewportWidth, m_viewportHeight);
        glClearColor(0.82f, 0.87f, 0.97f, 1.f);
        glEnable(GL_SCISSOR_TEST);
        glClear(GL_COLOR_BUFFER_BIT);
        glDisable(GL_SCISSOR_TEST);

        // Strip line
        // Disk
        QVector3D lineColor(0.25f, 0.31f, 0.85f);
        drawDisk(20.f, 50.f, 10.f, lineColor);
        // Line
        drawLine(QVector2D(20.f, 50.f), QVector2D(50.f, 70.f), 10.f, lineColor);
        // Disk
        drawDisk(50.f, 70.f, 10.f, lineColor);
        // Line
        drawLine(QVector2D(50.f, 70.f), QVector2D(60.f, 20.f), 10.f, lineColor);
        // Disk
        drawDisk(60.f, 20.f, 10.f, lineColor);

        // Square
        drawRectangle(100.f, 50.f, 50.f, 50.f, 0.f, QVector3D(0.3f, 0.07f, 0.5f));
        // Disk
        drawDisk(100.f, 50.f, 50.f, QVector3D(0.8f, 0.8f, 0.5f));

        // Borders
        QVector3D borderColor(0.62f, 0.04f, 0.18f);
        // Left border
        drawRectangle(5.f, 50.f, 5.f, 85.f, 0.f, borderColor);
        // Right border
        drawRectangle(195.f, 50.f, 5.f, 85.f, 0.f, borderColor);
        // Top border
        drawRectangle(100.f, 95.f, 185.f, 5.f, 0.f, borderColor);
        // Bottom border
        drawRectangle(100.f, 5.f, 185.f, 5.f, 0.f, borderColor);
    }

    void drawDisk(float x, float y, float diameter, const QVector3D& color)
    {
        m_modelMatrix.setToIdentity();
        m_modelMatrix.translate(QVector3D(x, y, 0.f));
        m_modelMatrix.scale(QVector3D(diameter, diameter, 1.f));
        m_mvpMatrix = m_projViewMatrix * m_modelMatrix;
        m_program.setUniformValue(m_uMvpMatrixLocation, m_mvpMatrix);
        m_program.setUniformValue(m_uColorLocation, color);
        glDrawArrays(GL_TRIANGLE_FAN, 0, m_amountOfDiskVertices);
    }

    void drawRectangle(float x, float y, float w, float h,
        float angle, const QVector3D& color)
    {
        m_modelMatrix.setToIdentity();
        m_modelMatrix.translate(QVector3D(x, y, 0.f));
        m_modelMatrix.rotate(angle, QVector3D(0.f, 0.f, 1.f));
        m_modelMatrix.scale(QVector3D(w, h, 1.f));
        m_mvpMatrix = m_projViewMatrix * m_modelMatrix;
        m_program.setUniformValue(m_uMvpMatrixLocation, m_mvpMatrix);
        m_program.setUniformValue(m_uColorLocation, color);
        glDrawArrays(GL_TRIANGLE_STRIP, m_amountOfDiskVertices, 4);
    }

    void drawLine(const QVector2D &from, const QVector2D &to, float thickness,
        const QVector3D& color)
    {
        QVector2D center;

        if (from.x() > to.x())
        {
            center.setX(to.x() + qFabs(from.x() - to.x()) / 2.f);
        }
        else
        {
            center.setX(from.x() + qFabs(to.x() - from.x()) / 2.f);
        }

        if (from.y() > to.y())
        {
            center.setY(to.y() + qFabs(from.y() - to.y()) / 2.f);
        }
        else
        {
            center.setY(from.y() + qFabs(to.y() - from.y()) / 2.f);
        }

        QVector3D tempVec;
        tempVec.setX(to.x() - from.x());
        tempVec.setY(to.y() - from.y());
        float length = tempVec.length();
        tempVec.normalize();
        QQuaternion quat = QQuaternion::rotationTo(QVector3D(1.f, 0.f, 0.f), tempVec);

        m_modelMatrix.setToIdentity();
        m_modelMatrix.translate(QVector3D(center.x(), center.y(), 0.f));
        m_modelMatrix.rotate(quat);
        m_modelMatrix.scale(QVector3D(length, thickness, 1.f));
        m_mvpMatrix = m_projViewMatrix * m_modelMatrix;
        m_program.setUniformValue(m_uMvpMatrixLocation, m_mvpMatrix);
        m_program.setUniformValue(m_uColorLocation, color);
        glDrawArrays(GL_TRIANGLE_STRIP, m_amountOfDiskVertices, 4);
    }

private:
    QOpenGLBuffer m_vertPosBuffer;
    QOpenGLShaderProgram m_program;
    int m_uColorLocation;
    int m_uMvpMatrixLocation;
    QMatrix4x4 m_mvpMatrix;
    QMatrix4x4 m_projMatrix;
    QMatrix4x4 m_viewMatrix;
    QMatrix4x4 m_projViewMatrix;
    QMatrix4x4 m_modelMatrix;
    const float m_worldWidth = 200.f;
    const float m_worldHeight = 100.f;
    float m_worldAspect = m_worldHeight / m_worldWidth;
    int m_viewportX;
    int m_viewportY;
    int m_viewportWidth;
    int m_viewportHeight;
    int m_angleStep = 10;
    int m_amountOfDiskVertices = 360 / m_angleStep + 2;
};

int main(int argc, char *argv[])
{
    QApplication::setAttribute(Qt::ApplicationAttribute::AA_UseDesktopOpenGL);
    QApplication app(argc, argv);
    OpenGLWindow w;
    w.show();
    return app.exec();
}

8Observer8

Line drawer

Created a separate class for drawing a line.

WebAssembly demo on free Netlify hosting

line-drawer-opengles2-qt6-cpp.pro

QT += core gui openglwidgets

win32: LIBS += -lopengl32

CONFIG += c++17

SOURCES += \
    line_drawer.cpp \
    main.cpp \
    opengl_window.cpp

HEADERS += \
    line_drawer.h \
    opengl_window.h

main.cpp

#include <QtWidgets/QApplication>
#include "opengl_window.h"

int main(int argc, char *argv[])
{
    QApplication::setAttribute(Qt::ApplicationAttribute::AA_UseDesktopOpenGL);
    QApplication app(argc, argv);
    OpenGLWindow w;
    w.show();
    return app.exec();
}

line_drawer.h

#ifndef LINE_DRAWER
#define LINE_DRAWER

#include <QtGui/QMatrix4x4>
#include <QtGui/QVector2D>
#include <QtGui/QVector3D>
#include <QtOpenGL/QOpenGLBuffer>
#include <QtOpenGL/QOpenGLShaderProgram>

class LineDrawer
{
public:
    LineDrawer(QOpenGLShaderProgram *program);

    void draw(const QVector2D &from, const QVector2D &to,
        const QVector3D &color, float thickness = 1.f);
    void setProjViewMatrix(const QMatrix4x4 &projViewMatrix);

private:
    void bind();

private:
    QOpenGLShaderProgram *m_program;
    QOpenGLBuffer m_vertPosBuffer;
    int m_aPositionLocation;
    int m_uMvpMatrixLocation;
    int m_uColorLocation;
    QMatrix4x4 m_projViewMatrix;
    QMatrix4x4 m_modelMatrix;
    QMatrix4x4 m_mvpMatrix;
};

#endif // LINE_DRAWER

line_drawer.cpp

#include "line_drawer.h"

LineDrawer::LineDrawer(QOpenGLShaderProgram *program)
{
    m_program = program;
    m_program->bind();

    m_aPositionLocation = m_program->attributeLocation("aPosition");
    m_uMvpMatrixLocation = m_program->uniformLocation("uMvpMatrix");
    m_uColorLocation = m_program->uniformLocation("uColor");

    float vertPositions[] = {
        -0.5, -0.5,
        0.5, -0.5,
        -0.5, 0.5,
        0.5, 0.5
    };
    m_vertPosBuffer.create();
    m_vertPosBuffer.bind();
    m_vertPosBuffer.allocate(vertPositions, sizeof(vertPositions));
}

void LineDrawer::setProjViewMatrix(const QMatrix4x4 &projViewMatrix)
{
    m_projViewMatrix = projViewMatrix;
}

void LineDrawer::bind()
{
    m_program->bind();
    m_vertPosBuffer.bind();
    m_program->setAttributeBuffer(m_aPositionLocation, GL_FLOAT, 0, 2);
    m_program->enableAttributeArray(m_aPositionLocation);
}

void LineDrawer::draw(const QVector2D &from, const QVector2D &to,
    const QVector3D &color, float thickness)
{
    bind();
    float centerX, centerY;
    QVector3D tempVec;

    if (from.x() > to.x())
    {
        centerX = to.x() + std::abs(from.x() - to.x()) / 2.f;
    }
    else
    {
        centerX = from.x() + std::abs(to.x() - from.x()) / 2.f;
    }

    if (from.y() > to.y())
    {
        centerY = to.y() + std::abs(from.y() - to.y()) / 2.f;
    }
    else
    {
        centerY = from.y() + std::abs(to.y() - from.y()) / 2.f;
    }

    tempVec.setX(to.x() - from.x());
    tempVec.setY(to.y() - from.y());
    float length = tempVec.length();
    tempVec.normalize();
    QQuaternion quat = QQuaternion::rotationTo(QVector3D(1, 0, 0), tempVec);

    m_modelMatrix.setToIdentity();
    m_modelMatrix.translate(QVector3D(centerX, centerY, 0));
    m_modelMatrix.rotate(quat);
    m_modelMatrix.scale(QVector3D(length, thickness, 1));
    m_mvpMatrix = m_projViewMatrix * m_modelMatrix;
    m_program->setUniformValue(m_uMvpMatrixLocation, m_mvpMatrix);
    m_program->setUniformValue(m_uColorLocation, color);
    glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);
}

opengl_window.h

#ifndef OPENGL_WINDOW_H
#define OPENGL_WINDOW_H

#include <QtMath>
#include <QtGui/QMatrix4x4>
#include <QtGui/QOpenGLFunctions>
#include <QtGui/QQuaternion>
#include <QtGui/QSurfaceFormat>
#include <QtGui/QVector2D>
#include <QtGui/QVector3D>
#include <QtOpenGL/QOpenGLShader>
#include <QtOpenGL/QOpenGLShaderProgram>
#include <QtOpenGL/QOpenGLWindow>

#include "line_drawer.h"

class OpenGLWindow : public QOpenGLWindow, private QOpenGLFunctions
{
public:
    OpenGLWindow();

    ~OpenGLWindow();

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

private:
    QOpenGLShaderProgram m_program;
    QMatrix4x4 m_projMatrix;
    QMatrix4x4 m_viewMatrix;
    QMatrix4x4 m_projViewMatrix;
    const float m_worldWidth = 200.f;
    const float m_worldHeight = 100.f;
    float m_worldAspect = m_worldHeight / m_worldWidth;
    int m_viewportX;
    int m_viewportY;
    int m_viewportWidth;
    int m_viewportHeight;
    LineDrawer *m_lineDrawer;
};

#endif // OPENGL_WINDOW_H

opengl_window.cpp

#include "opengl_window.h"

OpenGLWindow::OpenGLWindow()
{
    setTitle("OpenGL ES 2.0, Qt6, C++");
    resize(450, 450);

    QSurfaceFormat surfaceFormat;
    surfaceFormat.setDepthBufferSize(24);
    surfaceFormat.setSamples(4);
    setFormat(surfaceFormat);
}

OpenGLWindow::~OpenGLWindow()
{
    delete m_lineDrawer;
}

void OpenGLWindow::initializeGL()
{
    initializeOpenGLFunctions();

    QString vertShaderSrc =
        "attribute vec2 aPosition;\n"
        "uniform mat4 uMvpMatrix;"
        "void main()\n"
        "{\n"
        "    gl_Position = uMvpMatrix * vec4(aPosition, 0.0, 1.0);\n"
        "}\n";

    QString fragShaderSrc =
        "#ifdef GL_ES\n"
        "precision mediump float;\n"
        "#endif\n"
        "uniform vec3 uColor;\n"
        "void main()\n"
        "{\n"
        "    gl_FragColor = vec4(uColor, 1.0);\n"
        "}\n";

    m_program.create();
    m_program.addShaderFromSourceCode(QOpenGLShader::ShaderTypeBit::Vertex, vertShaderSrc);
    m_program.addShaderFromSourceCode(QOpenGLShader::ShaderTypeBit::Fragment, fragShaderSrc);
    m_program.link();
    m_program.bind();

    m_lineDrawer = new LineDrawer(&m_program);

    m_viewMatrix.lookAt(QVector3D(0, 0, 1), QVector3D(0, 0, 0), QVector3D(0, 1, 0));
}

void OpenGLWindow::resizeGL(int w, int h)
{
    int deviceW = w * devicePixelRatio();
    int deviceH = h * devicePixelRatio();
    float deviceAspect = deviceH / (float) deviceW;

    if (deviceAspect > m_worldAspect)
    {
        m_viewportWidth = deviceW;
        m_viewportHeight = (int) deviceW * m_worldAspect;
        m_viewportX = 0;
        m_viewportY = (int) (deviceH - m_viewportHeight) / 2.f;
    }
    else
    {
        m_viewportWidth = (int) deviceH / m_worldAspect;
        m_viewportHeight = deviceH;
        m_viewportX = (int) (deviceW - m_viewportWidth) / 2.f;
        m_viewportY = 0;
    }
    m_projMatrix.setToIdentity();
    m_projMatrix.ortho(0.f, m_worldWidth, 0.f, m_worldHeight, 1.f, -1.f);
    m_projViewMatrix = m_projMatrix * m_viewMatrix;

    m_lineDrawer->setProjViewMatrix(m_projViewMatrix);
}

void OpenGLWindow::paintGL()
{
    glClear(GL_COLOR_BUFFER_BIT);
    glClearColor(0.82f, 0.87f, 0.97f, 1.f);
    glClear(GL_COLOR_BUFFER_BIT);
    glViewport(m_viewportX, m_viewportY, m_viewportWidth, m_viewportHeight);
    glScissor(m_viewportX, m_viewportY, m_viewportWidth, m_viewportHeight);
    glClearColor(0.82f, 0.87f, 0.97f, 1.f);
    glEnable(GL_SCISSOR_TEST);
    glClear(GL_COLOR_BUFFER_BIT);
    glDisable(GL_SCISSOR_TEST);

    // X axis
    QVector3D xAxisColor(0.80f, 0.15f, 0.18f);
    m_lineDrawer->draw(QVector2D(30.f, 20.f), QVector2D(170.f, 20.f), xAxisColor);
    // Arrow
    m_lineDrawer->draw(QVector2D(165.f, 23.f), QVector2D(170.f, 20.f), xAxisColor);
    m_lineDrawer->draw(QVector2D(165.f, 17.f), QVector2D(170.f, 20.f),xAxisColor);
    // Divisions
    m_lineDrawer->draw(QVector2D(50.f, 17.f), QVector2D(50.f, 23.f), xAxisColor);
    m_lineDrawer->draw(QVector2D(70.f, 17.f), QVector2D(70.f, 23.f), xAxisColor);
    m_lineDrawer->draw(QVector2D(90.f, 17.f), QVector2D(90.f, 23.f), xAxisColor);
    m_lineDrawer->draw(QVector2D(110.f, 17.f), QVector2D(110.f, 23.f), xAxisColor);
    m_lineDrawer->draw(QVector2D(130.f, 17.f), QVector2D(130.f, 23.f), xAxisColor);
    m_lineDrawer->draw(QVector2D(150.f, 17.f), QVector2D(150.f, 23.f), xAxisColor);

    // Y axis
    QVector3D yAxisColor(0.20f, 0.60f, 0.30f);
    m_lineDrawer->draw(QVector2D(30.f, 20.f), QVector2D(30.f, 95.f), yAxisColor);
    // Arrow
    m_lineDrawer->draw(QVector2D(27.f, 90.f), QVector2D(30.f, 95.f), yAxisColor);
    m_lineDrawer->draw(QVector2D(33.f, 90.f), QVector2D(30.f, 95.f), yAxisColor);
    // Divisions
    m_lineDrawer->draw(QVector2D(27.f, 40.f), QVector2D(33.f, 40.f), yAxisColor);
    m_lineDrawer->draw(QVector2D(27.f, 60.f), QVector2D(33.f, 60.f), yAxisColor);
    m_lineDrawer->draw(QVector2D(27.f, 80.f), QVector2D(33.f, 80.f), yAxisColor);

    // Graph
    QVector3D graphColor(0.25f, 0.31f, 0.85f);
    m_lineDrawer->draw(QVector2D(50.f, 40.f), QVector2D(70.f, 60.f), graphColor);
    m_lineDrawer->draw(QVector2D(70.f, 60.f), QVector2D(130.f, 60.f), graphColor);
    m_lineDrawer->draw(QVector2D(130.f, 60.f), QVector2D(150.f, 80.f), graphColor);
}

8Observer8

Rectangle drawer

WebAssembly demo on free Netlify hosting

rectangle-drawer-opengles2-qt6-cpp.pro

QT += core gui openglwidgets

win32: LIBS += -lopengl32

CONFIG += c++17

SOURCES += \
    main.cpp \
    opengl_window.cpp \
    rectangle_drawer.cpp

HEADERS += \
    opengl_window.h \
    rectangle_drawer.h

main.cpp

#include <QtWidgets/QApplication>
#include "opengl_window.h"

int main(int argc, char *argv[])
{
    QApplication::setAttribute(Qt::ApplicationAttribute::AA_UseDesktopOpenGL);
    QApplication app(argc, argv);
    OpenGLWindow w;
    w.show();
    return app.exec();
}

rectangle_drawer.h

#ifndef RECTANGLE_DRAWER
#define RECTANGLE_DRAWER

#include <QtGui/QMatrix4x4>
#include <QtGui/QVector2D>
#include <QtGui/QVector3D>
#include <QtOpenGL/QOpenGLBuffer>
#include <QtOpenGL/QOpenGLShaderProgram>

class RectangleDrawer
{
public:
    RectangleDrawer(QOpenGLShaderProgram *program);

    void draw(const QVector2D &position, const QVector2D &size,
        const QVector3D &color, float angle = 0.f);
    void setProjViewMatrix(const QMatrix4x4 &projViewMatrix);

private:
    void bind();

private:
    QOpenGLShaderProgram *m_program;
    QOpenGLBuffer m_vertPosBuffer;
    int m_aPositionLocation;
    int m_uMvpMatrixLocation;
    int m_uColorLocation;
    QMatrix4x4 m_projViewMatrix;
    QMatrix4x4 m_modelMatrix;
    QMatrix4x4 m_mvpMatrix;
};

#endif // RECTANGLE_DRAWER

rectangle_drawer.cpp

#include "rectangle_drawer.h"

RectangleDrawer::RectangleDrawer(QOpenGLShaderProgram *program)
{
    m_program = program;
    m_program->bind();

    m_aPositionLocation = m_program->attributeLocation("aPosition");
    m_uMvpMatrixLocation = m_program->uniformLocation("uMvpMatrix");
    m_uColorLocation = m_program->uniformLocation("uColor");

    float vertPositions[] = {
        -0.5, -0.5,
        0.5, -0.5,
        -0.5, 0.5,
        0.5, 0.5
    };
    m_vertPosBuffer.create();
    m_vertPosBuffer.bind();
    m_vertPosBuffer.allocate(vertPositions, sizeof(vertPositions));
}

void RectangleDrawer::setProjViewMatrix(const QMatrix4x4 &projViewMatrix)
{
    m_projViewMatrix = projViewMatrix;
}

void RectangleDrawer::bind()
{
    m_program->bind();
    m_vertPosBuffer.bind();
    m_program->setAttributeBuffer(m_aPositionLocation, GL_FLOAT, 0, 2);
    m_program->enableAttributeArray(m_aPositionLocation);
}

void RectangleDrawer::draw(const QVector2D &position, const QVector2D &size,
    const QVector3D &color, float angle)
{
    bind();
    m_modelMatrix.setToIdentity();
    m_modelMatrix.translate(QVector3D(position.x(), position.y(), 0.f));
    m_modelMatrix.rotate(angle, QVector3D(0.f, 0.f, 1.f));
    m_modelMatrix.scale(QVector3D(size.x(), size.y(), 1.f));
    m_mvpMatrix = m_projViewMatrix * m_modelMatrix;
    m_program->setUniformValue(m_uMvpMatrixLocation, m_mvpMatrix);
    m_program->setUniformValue(m_uColorLocation, color);
    glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);
}

opengl_window.h

#ifndef OPENGL_WINDOW
#define OPENGL_WINDOW

#include <QtMath>
#include <QtGui/QMatrix4x4>
#include <QtGui/QOpenGLFunctions>
#include <QtGui/QQuaternion>
#include <QtGui/QSurfaceFormat>
#include <QtGui/QVector2D>
#include <QtGui/QVector3D>
#include <QtOpenGL/QOpenGLShader>
#include <QtOpenGL/QOpenGLShaderProgram>
#include <QtOpenGL/QOpenGLWindow>

#include "rectangle_drawer.h"

class OpenGLWindow : public QOpenGLWindow, private QOpenGLFunctions
{
public:
    OpenGLWindow();

    ~OpenGLWindow();

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

private:
    QOpenGLShaderProgram m_program;
    QMatrix4x4 m_projMatrix;
    QMatrix4x4 m_viewMatrix;
    QMatrix4x4 m_projViewMatrix;
    const float m_worldWidth = 200.f;
    const float m_worldHeight = 100.f;
    float m_worldAspect = m_worldHeight / m_worldWidth;
    int m_viewportX;
    int m_viewportY;
    int m_viewportWidth;
    int m_viewportHeight;
    RectangleDrawer *m_rectangleDrawer;
};

#endif // OPENGL_WINDOW

opengl_window.cpp

#include "opengl_window.h"

OpenGLWindow::OpenGLWindow()
{
    setTitle("OpenGL ES 2.0, Qt6, C++");
    resize(600, 300);

    QSurfaceFormat surfaceFormat;
    surfaceFormat.setDepthBufferSize(24);
    surfaceFormat.setSamples(4);
    setFormat(surfaceFormat);
}

OpenGLWindow::~OpenGLWindow()
{
    delete m_rectangleDrawer;
}

void OpenGLWindow::initializeGL()
{
    initializeOpenGLFunctions();

    QString vertShaderSrc =
        "attribute vec2 aPosition;\n"
        "uniform mat4 uMvpMatrix;"
        "void main()\n"
        "{\n"
        "    gl_Position = uMvpMatrix * vec4(aPosition, 0.0, 1.0);\n"
        "}\n";

    QString fragShaderSrc =
        "#ifdef GL_ES\n"
        "precision mediump float;\n"
        "#endif\n"
        "uniform vec3 uColor;\n"
        "void main()\n"
        "{\n"
        "    gl_FragColor = vec4(uColor, 1.0);\n"
        "}\n";

    m_program.create();
    m_program.addShaderFromSourceCode(QOpenGLShader::ShaderTypeBit::Vertex, vertShaderSrc);
    m_program.addShaderFromSourceCode(QOpenGLShader::ShaderTypeBit::Fragment, fragShaderSrc);
    m_program.link();
    m_program.bind();

    m_rectangleDrawer = new RectangleDrawer(&m_program);

    m_viewMatrix.lookAt(QVector3D(0, 0, 1), QVector3D(0, 0, 0), QVector3D(0, 1, 0));
}

void OpenGLWindow::resizeGL(int w, int h)
{
    int deviceW = w * devicePixelRatio();
    int deviceH = h * devicePixelRatio();
    float deviceAspect = deviceH / (float) deviceW;

    if (deviceAspect > m_worldAspect)
    {
        m_viewportWidth = deviceW;
        m_viewportHeight = (int) deviceW * m_worldAspect;
        m_viewportX = 0;
        m_viewportY = (int) (deviceH - m_viewportHeight) / 2.f;
    }
    else
    {
        m_viewportWidth = (int) deviceH / m_worldAspect;
        m_viewportHeight = deviceH;
        m_viewportX = (int) (deviceW - m_viewportWidth) / 2.f;
        m_viewportY = 0;
    }
    m_projMatrix.setToIdentity();
    m_projMatrix.ortho(0.f, m_worldWidth, 0.f, m_worldHeight, 1.f, -1.f);
    m_projViewMatrix = m_projMatrix * m_viewMatrix;

    m_rectangleDrawer->setProjViewMatrix(m_projViewMatrix);
}

void OpenGLWindow::paintGL()
{
    glClear(GL_COLOR_BUFFER_BIT);
    glClearColor(0.2f, 0.2f, 0.2f, 1.f);
    // glClearColor(0.82f, 0.87f, 0.97f, 1.f);
    glClear(GL_COLOR_BUFFER_BIT);
    glViewport(m_viewportX, m_viewportY, m_viewportWidth, m_viewportHeight);
    glScissor(m_viewportX, m_viewportY, m_viewportWidth, m_viewportHeight);
    glClearColor(0.82f, 0.87f, 0.97f, 1.f);
    glEnable(GL_SCISSOR_TEST);
    glClear(GL_COLOR_BUFFER_BIT);
    glDisable(GL_SCISSOR_TEST);

    m_rectangleDrawer->draw(QVector2D(50.f, 70.f), QVector2D(72.f, 7.f),
        QVector3D(0.80f, 0.15f, 0.18f), -27.f);
    m_rectangleDrawer->draw(QVector2D(160.f, 50.f), QVector2D(70.f, 5.f),
        QVector3D(0.20f, 0.60f, 0.30f), 10.f);
    m_rectangleDrawer->draw(QVector2D(100.f, 15.f), QVector2D(150.f, 7.f),
        QVector3D(0.25f, 0.31f, 0.85f));
}

8Observer8

Disk drawer

WebAssembly demo on free Netlify hosting

disk-drawer-opengles2-qt6-cpp.pro

QT += core gui openglwidgets

win32: LIBS += -lopengl32

CONFIG += c++17

SOURCES += \
    disk_drawer.cpp \
    main.cpp \
    opengl_window.cpp

HEADERS += \
    disk_drawer.h \
    opengl_window.h

main.cpp

#include <QtWidgets/QApplication>
#include "opengl_window.h"

int main(int argc, char *argv[])
{
    QApplication::setAttribute(Qt::ApplicationAttribute::AA_UseDesktopOpenGL);
    QApplication app(argc, argv);
    OpenGLWindow w;
    w.show();
    return app.exec();
}

disk_drawer.h

#ifndef DISK_DRAWER
#define DISK_DRAWER

#include <QtGui/QMatrix4x4>
#include <QtGui/QVector2D>
#include <QtGui/QVector3D>
#include <QtOpenGL/QOpenGLBuffer>
#include <QtOpenGL/QOpenGLShaderProgram>

class DiskDrawer
{
public:
    DiskDrawer(QOpenGLShaderProgram *program);

    void draw(const QVector2D &position, float diameter,
        const QVector3D &color);
    void setProjViewMatrix(const QMatrix4x4 &projViewMatrix);

private:
    void bind();

private:
    QOpenGLShaderProgram *m_program;
    QOpenGLBuffer m_vertPosBuffer;
    int m_aPositionLocation;
    int m_uMvpMatrixLocation;
    int m_uColorLocation;
    QMatrix4x4 m_projViewMatrix;
    QMatrix4x4 m_modelMatrix;
    QMatrix4x4 m_mvpMatrix;
    int m_angleStep = 5;
    int m_amountOfDiskVertices = 360 / m_angleStep + 2;
};

#endif // DISK_DRAWER

disk_drawer.cpp

#include "disk_drawer.h"

DiskDrawer::DiskDrawer(QOpenGLShaderProgram *program)
{
    m_program = program;
    m_program->bind();

    m_aPositionLocation = m_program->attributeLocation("aPosition");
    m_uMvpMatrixLocation = m_program->uniformLocation("uMvpMatrix");
    m_uColorLocation = m_program->uniformLocation("uColor");

    float vertPositions[m_amountOfDiskVertices * 2];
    float radius = 0.5f;
    float angle = 0.f;
    int index = 0;

    // Disk center
    vertPositions[index++] = 0.f;
    vertPositions[index++] = 0.f;

    for (int i = 0; i < m_amountOfDiskVertices - 1; i++)
    {
        float radians = qDegreesToRadians(angle);
        float x = radius * qCos(radians);
        float y = radius * qSin(radians);
        vertPositions[index++] = x;
        vertPositions[index++] = y;
        angle += m_angleStep;
    }

    m_vertPosBuffer.create();
    m_vertPosBuffer.bind();
    m_vertPosBuffer.allocate(vertPositions, sizeof(vertPositions));
}

void DiskDrawer::setProjViewMatrix(const QMatrix4x4 &projViewMatrix)
{
    m_projViewMatrix = projViewMatrix;
}

void DiskDrawer::bind()
{
    m_program->bind();
    m_vertPosBuffer.bind();
    m_program->setAttributeBuffer(m_aPositionLocation, GL_FLOAT, 0, 2);
    m_program->enableAttributeArray(m_aPositionLocation);
}

void DiskDrawer::draw(const QVector2D &position, float diameter,
    const QVector3D &color)
{
    bind();
    m_modelMatrix.setToIdentity();
    m_modelMatrix.translate(QVector3D(position.x(), position.y(), 0.f));
    m_modelMatrix.scale(QVector3D(diameter, diameter, 1.f));
    m_mvpMatrix = m_projViewMatrix * m_modelMatrix;
    m_program->setUniformValue(m_uMvpMatrixLocation, m_mvpMatrix);
    m_program->setUniformValue(m_uColorLocation, color);
    glDrawArrays(GL_TRIANGLE_FAN, 0, m_amountOfDiskVertices);
}

opengl_window.h

#ifndef OPENGL_WINDOW
#define OPENGL_WINDOW

#include <QtMath>
#include <QtGui/QMatrix4x4>
#include <QtGui/QOpenGLFunctions>
#include <QtGui/QQuaternion>
#include <QtGui/QSurfaceFormat>
#include <QtGui/QVector2D>
#include <QtGui/QVector3D>
#include <QtOpenGL/QOpenGLShader>
#include <QtOpenGL/QOpenGLShaderProgram>
#include <QtOpenGL/QOpenGLWindow>

#include "disk_drawer.h"

class OpenGLWindow : public QOpenGLWindow, private QOpenGLFunctions
{
public:
    OpenGLWindow();

    ~OpenGLWindow();

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

private:
    QOpenGLShaderProgram m_program;
    QMatrix4x4 m_projMatrix;
    QMatrix4x4 m_viewMatrix;
    QMatrix4x4 m_projViewMatrix;
    const float m_worldWidth = 200.f;
    const float m_worldHeight = 100.f;
    float m_worldAspect = m_worldHeight / m_worldWidth;
    int m_viewportX;
    int m_viewportY;
    int m_viewportWidth;
    int m_viewportHeight;
    DiskDrawer *m_diskDrawer;
};

#endif // OPENGL_WINDOW

opengl_window.cpp

#include "opengl_window.h"

OpenGLWindow::OpenGLWindow()
{
    setTitle("OpenGL ES 2.0, Qt6, C++");
    resize(600, 300);

    QSurfaceFormat surfaceFormat;
    surfaceFormat.setDepthBufferSize(24);
    surfaceFormat.setSamples(4);
    setFormat(surfaceFormat);
}

OpenGLWindow::~OpenGLWindow()
{
    delete m_diskDrawer;
}

void OpenGLWindow::initializeGL()
{
    initializeOpenGLFunctions();

    QString vertShaderSrc =
        "attribute vec2 aPosition;\n"
        "uniform mat4 uMvpMatrix;"
        "void main()\n"
        "{\n"
        "    gl_Position = uMvpMatrix * vec4(aPosition, 0.0, 1.0);\n"
        "}\n";

    QString fragShaderSrc =
        "#ifdef GL_ES\n"
        "precision mediump float;\n"
        "#endif\n"
        "uniform vec3 uColor;\n"
        "void main()\n"
        "{\n"
        "    gl_FragColor = vec4(uColor, 1.0);\n"
        "}\n";

    m_program.create();
    m_program.addShaderFromSourceCode(QOpenGLShader::ShaderTypeBit::Vertex, vertShaderSrc);
    m_program.addShaderFromSourceCode(QOpenGLShader::ShaderTypeBit::Fragment, fragShaderSrc);
    m_program.link();
    m_program.bind();

    m_diskDrawer = new DiskDrawer(&m_program);

    m_viewMatrix.lookAt(QVector3D(0, 0, 1), QVector3D(0, 0, 0), QVector3D(0, 1, 0));
}

void OpenGLWindow::resizeGL(int w, int h)
{
    int deviceW = w * devicePixelRatio();
    int deviceH = h * devicePixelRatio();
    float deviceAspect = deviceH / (float) deviceW;

    if (deviceAspect > m_worldAspect)
    {
        m_viewportWidth = deviceW;
        m_viewportHeight = (int) deviceW * m_worldAspect;
        m_viewportX = 0;
        m_viewportY = (int) (deviceH - m_viewportHeight) / 2.f;
    }
    else
    {
        m_viewportWidth = (int) deviceH / m_worldAspect;
        m_viewportHeight = deviceH;
        m_viewportX = (int) (deviceW - m_viewportWidth) / 2.f;
        m_viewportY = 0;
    }
    m_projMatrix.setToIdentity();
    m_projMatrix.ortho(0.f, m_worldWidth, 0.f, m_worldHeight, 1.f, -1.f);
    m_projViewMatrix = m_projMatrix * m_viewMatrix;

    m_diskDrawer->setProjViewMatrix(m_projViewMatrix);
}

void OpenGLWindow::paintGL()
{
    glClear(GL_COLOR_BUFFER_BIT);
    // glClearColor(0.2f, 0.2f, 0.2f, 1.f);
    glClearColor(0.82f, 0.87f, 0.97f, 1.f);
    glClear(GL_COLOR_BUFFER_BIT);
    glViewport(m_viewportX, m_viewportY, m_viewportWidth, m_viewportHeight);
    glScissor(m_viewportX, m_viewportY, m_viewportWidth, m_viewportHeight);
    glClearColor(0.82f, 0.87f, 0.97f, 1.f);
    glEnable(GL_SCISSOR_TEST);
    glClear(GL_COLOR_BUFFER_BIT);
    glDisable(GL_SCISSOR_TEST);

    m_diskDrawer->draw(QVector2D(50.f, 70.f), 40.f,
        QVector3D(0.80f, 0.15f, 0.18f));
    m_diskDrawer->draw(QVector2D(160.f, 50.f), 50.f,
        QVector3D(0.20f, 0.60f, 0.30f));
    m_diskDrawer->draw(QVector2D(100.f, 15.f), 20.f,
        QVector3D(0.25f, 0.31f, 0.85f));
}

8Observer8

Ring drawer using line drawer

The RingDrawer class uses the LineDrawer class to draw the segments of a ring. For example, for number of segments equals to 10:

void RingDrawer::draw(const QVector2D &center, float radius,
    const QVector3D &color, int numberOfSegments, float thickness)
{
    float angle = 0.f;
    const float angleStep = 360.f / numberOfSegments;

    float x = radius * qCos(qDegreesToRadians(angle));
    float y = radius * qSin(qDegreesToRadians(angle));
    QVector2D from(center.x() + x, center.y() + y);
    angle += angleStep;

    for (int i = 0; i < numberOfSegments; i++)
    {
        x = radius * qCos(qDegreesToRadians(angle));
        y = radius * qSin(qDegreesToRadians(angle));
        QVector2D to(center.x() + x, center.y() + y);
        m_lineDrawer->draw(from, to, color, thickness);
        angle += angleStep;
        from = to;
    }
}

For number of segments equals to 50: WebAssembly demo on free Netlify hosting

ring-drawer-opengles2-qt6-cpp.pro

QT += core gui openglwidgets

win32: LIBS += -lopengl32

CONFIG += c++17

SOURCES += \
    line_drawer.cpp \
    main.cpp \
    opengl_window.cpp \
    ring_drawer.cpp

HEADERS += \
    line_drawer.h \
    opengl_window.h \
    ring_drawer.h

main.cpp

#include <QtWidgets/QApplication>
#include "opengl_window.h"

int main(int argc, char *argv[])
{
    QApplication::setAttribute(Qt::ApplicationAttribute::AA_UseDesktopOpenGL);
    QApplication app(argc, argv);
    OpenGLWindow w;
    w.show();
    return app.exec();
}

ring_drawer.h

#ifndef RING_DRAWER
#define RING_DRAWER

#include "line_drawer.h"

class RingDrawer
{
public:
    RingDrawer(QOpenGLShaderProgram *program);
    ~RingDrawer();

    void draw(const QVector2D &center, float radius,
        const QVector3D &color, int numberOfSegments = 50,
        float thickness = 1.f);
    void setProjViewMatrix(const QMatrix4x4 &projViewMatrix);

private:
    LineDrawer *m_lineDrawer;
};

#endif // RING_DRAWER

ring_drawer.cpp

#include <QtMath>

#include "ring_drawer.h"

RingDrawer::RingDrawer(QOpenGLShaderProgram *program)
{
    m_lineDrawer = new LineDrawer(program);
}

RingDrawer::~RingDrawer()
{
    delete m_lineDrawer;
}

void RingDrawer::setProjViewMatrix(const QMatrix4x4 &projViewMatrix)
{
    m_lineDrawer->setProjViewMatrix(projViewMatrix);
}

void RingDrawer::draw(const QVector2D &center, float radius,
    const QVector3D &color, int numberOfSegments, float thickness)
{
    float angle = 0.f;
    const float angleStep = 360.f / numberOfSegments;

    float x = radius * qCos(qDegreesToRadians(angle));
    float y = radius * qSin(qDegreesToRadians(angle));
    QVector2D from(center.x() + x, center.y() + y);
    angle += angleStep;

    for (int i = 0; i < numberOfSegments; i++)
    {
        x = radius * qCos(qDegreesToRadians(angle));
        y = radius * qSin(qDegreesToRadians(angle));
        QVector2D to(center.x() + x, center.y() + y);
        m_lineDrawer->draw(from, to, color, thickness);
        angle += angleStep;
        from = to;
    }
}

opengl_window.h

#ifndef OPENGL_WINDOW
#define OPENGL_WINDOW

#include <QtMath>
#include <QtGui/QMatrix4x4>
#include <QtGui/QOpenGLFunctions>
#include <QtGui/QQuaternion>
#include <QtGui/QSurfaceFormat>
#include <QtGui/QVector2D>
#include <QtGui/QVector3D>
#include <QtOpenGL/QOpenGLShader>
#include <QtOpenGL/QOpenGLShaderProgram>
#include <QtOpenGL/QOpenGLWindow>

#include "ring_drawer.h"

class OpenGLWindow : public QOpenGLWindow, private QOpenGLFunctions
{
public:
    OpenGLWindow();

    ~OpenGLWindow();

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

private:
    QOpenGLShaderProgram m_program;
    QMatrix4x4 m_projMatrix;
    QMatrix4x4 m_viewMatrix;
    QMatrix4x4 m_projViewMatrix;
    const float m_worldWidth = 200.f;
    const float m_worldHeight = 100.f;
    float m_worldAspect = m_worldHeight / m_worldWidth;
    int m_viewportX;
    int m_viewportY;
    int m_viewportWidth;
    int m_viewportHeight;
    RingDrawer *m_ringDrawer;
};

#endif // OPENGL_WINDOW

opengl_window.cpp

#include "opengl_window.h"

OpenGLWindow::OpenGLWindow()
{
    setTitle("OpenGL ES 2.0, Qt6, C++");
    resize(600, 300);

    QSurfaceFormat surfaceFormat;
    surfaceFormat.setDepthBufferSize(24);
    surfaceFormat.setSamples(4);
    setFormat(surfaceFormat);
}

OpenGLWindow::~OpenGLWindow()
{
    delete m_ringDrawer;
}

void OpenGLWindow::initializeGL()
{
    initializeOpenGLFunctions();


    QString vertShaderSrc =
        "attribute vec2 aPosition;\n"
        "uniform mat4 uMvpMatrix;"
        "void main()\n"
        "{\n"
        "    gl_Position = uMvpMatrix * vec4(aPosition, 0.0, 1.0);\n"
        "}\n";

    QString fragShaderSrc =
        "#ifdef GL_ES\n"
        "precision mediump float;\n"
        "#endif\n"
        "uniform vec3 uColor;\n"
        "void main()\n"
        "{\n"
        "    gl_FragColor = vec4(uColor, 1.0);\n"
        "}\n";

    m_program.create();
    m_program.addShaderFromSourceCode(QOpenGLShader::ShaderTypeBit::Vertex, vertShaderSrc);
    m_program.addShaderFromSourceCode(QOpenGLShader::ShaderTypeBit::Fragment, fragShaderSrc);
    m_program.link();
    m_program.bind();

    m_ringDrawer = new RingDrawer(&m_program);

    m_viewMatrix.lookAt(QVector3D(0, 0, 1), QVector3D(0, 0, 0), QVector3D(0, 1, 0));
}

void OpenGLWindow::resizeGL(int w, int h)
{
    int deviceW = w * devicePixelRatio();
    int deviceH = h * devicePixelRatio();
    float deviceAspect = deviceH / (float) deviceW;

    if (deviceAspect > m_worldAspect)
    {
        m_viewportWidth = deviceW;
        m_viewportHeight = (int) deviceW * m_worldAspect;
        m_viewportX = 0;
        m_viewportY = (int) (deviceH - m_viewportHeight) / 2.f;
    }
    else
    {
        m_viewportWidth = (int) deviceH / m_worldAspect;
        m_viewportHeight = deviceH;
        m_viewportX = (int) (deviceW - m_viewportWidth) / 2.f;
        m_viewportY = 0;
    }
    m_projMatrix.setToIdentity();
    m_projMatrix.ortho(0.f, m_worldWidth, 0.f, m_worldHeight, 1.f, -1.f);
    m_projViewMatrix = m_projMatrix * m_viewMatrix;

    m_ringDrawer->setProjViewMatrix(m_projViewMatrix);
}

void OpenGLWindow::paintGL()
{
    glClear(GL_COLOR_BUFFER_BIT);
    // glClearColor(0.2f, 0.2f, 0.2f, 1.f);
    glClearColor(0.82f, 0.87f, 0.97f, 1.f);
    glClear(GL_COLOR_BUFFER_BIT);
    glViewport(m_viewportX, m_viewportY, m_viewportWidth, m_viewportHeight);
    glScissor(m_viewportX, m_viewportY, m_viewportWidth, m_viewportHeight);
    glClearColor(0.82f, 0.87f, 0.97f, 1.f);
    glEnable(GL_SCISSOR_TEST);
    glClear(GL_COLOR_BUFFER_BIT);
    glDisable(GL_SCISSOR_TEST);

    m_ringDrawer->draw(QVector2D(50.f, 70.f), 20.f,
        QVector3D(0.80f, 0.15f, 0.18f));
    m_ringDrawer->draw(QVector2D(160.f, 50.f), 25.f,
        QVector3D(0.20f, 0.60f, 0.30f));
    m_ringDrawer->draw(QVector2D(100.f, 15.f), 10.f,
        QVector3D(0.25f, 0.31f, 0.85f));
}

line_drawer.h

#ifndef LINE_DRAWER
#define LINE_DRAWER

#include <QtGui/QMatrix4x4>
#include <QtGui/QVector2D>
#include <QtGui/QVector3D>
#include <QtOpenGL/QOpenGLBuffer>
#include <QtOpenGL/QOpenGLShaderProgram>

class LineDrawer
{
public:
    LineDrawer(QOpenGLShaderProgram *program);

    void draw(const QVector2D &from, const QVector2D &to,
        const QVector3D &color, float thickness = 1.f);
    void setProjViewMatrix(const QMatrix4x4 &projViewMatrix);

private:
    void bind();

private:
    QOpenGLShaderProgram *m_program;
    QOpenGLBuffer m_vertPosBuffer;
    int m_aPositionLocation;
    int m_uMvpMatrixLocation;
    int m_uColorLocation;
    QMatrix4x4 m_projViewMatrix;
    QMatrix4x4 m_modelMatrix;
    QMatrix4x4 m_mvpMatrix;
};

#endif // LINE_DRAWER

line_drawer.cpp

#include "line_drawer.h"

LineDrawer::LineDrawer(QOpenGLShaderProgram *program)
{
    m_program = program;
    m_program->bind();

    m_aPositionLocation = m_program->attributeLocation("aPosition");
    m_uMvpMatrixLocation = m_program->uniformLocation("uMvpMatrix");
    m_uColorLocation = m_program->uniformLocation("uColor");

    float vertPositions[] = {
        -0.5, -0.5,
        0.5, -0.5,
        -0.5, 0.5,
        0.5, 0.5
    };
    m_vertPosBuffer.create();
    m_vertPosBuffer.bind();
    m_vertPosBuffer.allocate(vertPositions, sizeof(vertPositions));
}

void LineDrawer::setProjViewMatrix(const QMatrix4x4 &projViewMatrix)
{
    m_projViewMatrix = projViewMatrix;
}

void LineDrawer::bind()
{
    m_program->bind();
    m_vertPosBuffer.bind();
    m_program->setAttributeBuffer(m_aPositionLocation, GL_FLOAT, 0, 2);
    m_program->enableAttributeArray(m_aPositionLocation);
}

void LineDrawer::draw(const QVector2D &from, const QVector2D &to,
    const QVector3D &color, float thickness)
{
    bind();
    float centerX, centerY;
    QVector3D tempVec;

    if (from.x() > to.x())
    {
        centerX = to.x() + std::abs(from.x() - to.x()) / 2.f;
    }
    else
    {
        centerX = from.x() + std::abs(to.x() - from.x()) / 2.f;
    }

    if (from.y() > to.y())
    {
        centerY = to.y() + std::abs(from.y() - to.y()) / 2.f;
    }
    else
    {
        centerY = from.y() + std::abs(to.y() - from.y()) / 2.f;
    }

    tempVec.setX(to.x() - from.x());
    tempVec.setY(to.y() - from.y());
    float length = tempVec.length();
    tempVec.normalize();
    QQuaternion quat = QQuaternion::rotationTo(QVector3D(1, 0, 0), tempVec);

    m_modelMatrix.setToIdentity();
    m_modelMatrix.translate(QVector3D(centerX, centerY, 0));
    m_modelMatrix.rotate(quat);
    m_modelMatrix.scale(QVector3D(length, thickness, 1));
    m_mvpMatrix = m_projViewMatrix * m_modelMatrix;
    m_program->setUniformValue(m_uMvpMatrixLocation, m_mvpMatrix);
    m_program->setUniformValue(m_uColorLocation, color);
    glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);
}

8Observer8

Orbit controls

I have created the OrbitControls class to pan a camera with right mouse button, rotate with the left mouse button, and zoom in/out with the wheel:

WebAssembly demo on free Netlify hosting

It is not ready to pan and zoom in/out on Android, rotation only:

Source code for QOpenGLWindow, Qt6, C++

8Observer8

How to set up Box2D in Qt Creator from sources for build for Android, Desktop, and WebAssembly

I have recorded this video today. But I don't like it. The voice is too low. I will make another video later and I will delete this one.

https://www.youtube.com/watch?v=OE4OOPQC7Nk

8Observer8

Detecting a mobile browser with Qt WebAssembly (topic)

It is important to determine whether a WebAssembly game is running on a mobile device or a personal computer in order to show or hide the joystick.

#ifdef Q_OS_WASM
#include <emscripten.h>
#endif

#include "widget.h"

#ifdef Q_OS_WASM
EM_JS(bool, isMobile, (), {
    return /iPhone|iPad|iPod|Android/i.test(navigator.userAgent);
})
#endif

Widget::Widget(QWidget *parent)
    : QWidget(parent)
{
    bool isMobile = false;

#ifdef Q_OS_WASM
    isMobile = isMobile();
#endif

#if defined Q_OS_ANDROID || defined Q_OS_IOS
    isMobile = true;
#endif

    qDebug() << "isMobile:" << isMobile;
}

Don't forget to add wasm: here:

QT += core gui widgets

CONFIG += c++17

wasm: INCLUDEPATH += "C:\emsdk\upstream\emscripten\cache\sysroot\include"

SOURCES += \
    main.cpp \
    widget.cpp

HEADERS += \
    widget.h

8Observer8

Get data with Qt client from deployed Node.js server using WebSockets (topic)

My following Qt 6.6.3 example works on Android, Desktop, and Web (with Qt WebAssembly). It prints data received from the server. The server contains the Box2D-WASM library. It sends the gravity value in JSON format when a client is connected. It is useful example to make multiplayer games with physics on the server side. I have deployed the example on free Glitch hosting: https://glitch.com/edit/#!/merciful-regal-soursop from the GitHub repository: send-gravity-from-server-to-client-box2d-wasm-js The client contains only one main.cpp file. It outputs the following information to the console:

connected
"{\"action\":\"scGravity\",\"data\":\"{\\\"x\\\":0,\\\"y\\\":-3}\"}"

You should download OpenSSL to run the following example on Android. Open the following window in Qt Creator (Edit > Preferences... > Devices > Android):

Add the following path to your pro-file:

QT += core gui websockets widgets

android: include(C:/Qt/Tools/OpenSSL-1.1.1j/Win_x64/bin/openssl.pri)

CONFIG += c++17

SOURCES += \
    main.cpp

Read how to add OpenSSL to your CMake project if you use CMake instead of QMake in the Qt documentaion: Adding OpenSSL Support for Android

Build the following example for Android, Desktop, and WebAssembly (I have tested it):

main.cpp

#include <QtNetwork/QNetworkRequest>
#include <QtWebSockets/QWebSocket>
#include <QtWidgets/QApplication>
#include <QtWidgets/QWidget>

class Widget : public QWidget
{
    Q_OBJECT

public:
    Widget()
    {
        setWindowTitle("Show gravity from server with Box2D-WASM");
        resize(420, 200);

        connect(&m_webSocket, &QWebSocket::connected,
            this, &Widget::onConnected);
        connect(&m_webSocket, &QWebSocket::textMessageReceived,
            this, &Widget::onMessageReceived);
        connect(&m_webSocket, &QWebSocket::errorOccurred,
            this, &Widget::onErrorOccurred);

        QUrl url("wss://merciful-regal-soursop.glitch.me");

        QNetworkRequest request;
        request.setUrl(url);
        request.setRawHeader(QByteArray("User-Agent"),
            QByteArray("Mozilla/5.0 "
                       "(Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) "
                       "Chrome/124.0.0.0 Safari/537.36"));

        m_webSocket.open(request);
    }

private slots:

    void onConnected()
    {
        qDebug() << "connected";
    }
    void onMessageReceived(const QString &message)
    {
        qDebug() << message;
    }

    void onErrorOccurred(QAbstractSocket::SocketError error)
    {
        qDebug() << "!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!";
        qDebug() << "Error:" << error;
        qDebug() << "Device supports OpenSSL:" << QSslSocket::supportsSsl();
        qDebug() << "!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!";
    }

private:
    QWebSocket m_webSocket;
};

#include "main.moc"

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