Simple QOpenGLWidget (Qt6/Qt5) test, draw an rgb triangle, handle mouse events (press, move and wheel)

main.cpp

#include <QtWidgets/QApplication>
#include <QtWidgets/QMainWindow>

#include "TriangleWidget.h"


int main(int argc, char *argv[])
{
    QApplication app(argc, argv);

    QMainWindow mainWindow;

    TriangleWidget centralWidget(&mainWindow);
    mainWindow.setCentralWidget(&centralWidget);

    mainWindow.show();

    return app.exec();
}

Makefile

all: main


USE_QT = 5


ifeq ($(USE_QT), 6)
	QT6_CXXFLAGS = -I/usr/include/x86_64-linux-gnu/qt6
	QT_CXXFLAGS = $(QT6_CXXFLAGS)
	QT6_LDFLAGS = -lQt6Core -lQt6Gui -lQt6Widgets -lQt6OpenGL -lQt6OpenGLWidgets
	QT_LDFLAGS = $(QT6_LDFLAGS)
else
	QT5_CXXFLAGS = -I/usr/include/x86_64-linux-gnu/qt5
	QT_CXXFLAGS = $(QT5_CXXFLAGS)
	QT5_LDFLAGS = -lQt5Core -lQt5Gui -lQt5Widgets -lQt5OpenGL
	QT_LDFLAGS = $(QT5_LDFLAGS)
endif

CXXFLAGS += $(QT_CXXFLAGS)

# Qt::Widgets Qt::OpenGL Qt::Gui Qt::OpenGLWidgets Qt::Core
LDFLAGS += -L/usr/lib/x86_64-linux-gnu/ $(QT_LDFLAGS) -lGL -lGLU

main: main.cpp TriangleWidget.cpp TriangleWidget.h
	c++ -Wall -O0 $(CXXFLAGS) -fPIC -o main main.cpp TriangleWidget.cpp $(LDFLAGS)

.PHONY: clean
clean:
	-rm -f main

TriangleWidget.cpp

#include "TriangleWidget.h"

// #include <QApplication>
#include <cmath>

#include <QtCore/QDebug>
#include <GL/glu.h>

TriangleWidget::TriangleWidget(QWidget *parent)
: QOpenGLWidget(parent)
{
    _azimuth = 0.0f; // Start facing the positive X axis
    _declination = 0.0f; // Start looking straight ahead
    _distance = 3.0f; // Start 3 units away from the origin
}


void TriangleWidget::initializeGL()
{
    // Initialize OpenGL functions
    initializeOpenGLFunctions();

    // Set clear color to black
    glClearColor(0.0f, 0.0f, 0.0f, 1.0f);
}

void TriangleWidget::resizeGL(int w, int h)
{
    glViewport(0, 0, w, h);

    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();

    float aspect = static_cast<float>(w) / static_cast<float>(h);
    gluPerspective(45.0, aspect, 0.1, 100.0);

    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();
}

void TriangleWidget::mousePressEvent(QMouseEvent *event)
{
    qDebug() << "press ev :" << mouseEventPosition(event);
    if (event->button() == Qt::LeftButton) {
        _lastPos = event->pos();
    }
}

void TriangleWidget::mouseMoveEvent(QMouseEvent *event)
{
    qDebug() << "move ev :" << mouseEventPosition(event);
    if (event->buttons() & Qt::LeftButton) {
        int dx = mouseEventPosition(event).x() - _lastPos.x();
        int dy = mouseEventPosition(event).y() - _lastPos.y();
        _azimuth -= dx * 0.5 * 0.01;
        _declination += dy * 0.5 * 0.01;
        _lastPos = event->pos();
        update();
    }
}

void TriangleWidget::wheelEvent(QWheelEvent *event)
{
    qDebug() << "wheel ev angle delta:" << event->angleDelta();
    qDebug() << "wheel ev pixel delta:" << event->pixelDelta();
    int numDegrees = event->angleDelta().y() / 8;
    int numSteps = numDegrees / 15;
    _distance += numSteps;
    update();
}

void TriangleWidget::paintGL()
{
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    glEnable(GL_DEPTH_TEST);

    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();

    GLfloat camX = _distance * sin(_azimuth) * cos(_declination);
    GLfloat camY = _distance * sin(_declination);
    GLfloat camZ = _distance * cos(_azimuth) * cos(_declination);
    gluLookAt(camX, camY, camZ, 0, 0, 0, 0, 1, 0);

    glBegin(GL_TRIANGLES);
    glColor3f(1.0f, 0.0f, 0.0f); // red
    glVertex3f(-1.0f, -1.0f, 0.0f);
    glColor3f(0.0f, 1.0f, 0.0f); // green
    glVertex3f(0.0f, 1.0f, 0.0f);
    glColor3f(0.0f, 0.0f, 1.0f); // blue
    glVertex3f(1.0f, -1.0f, 0.0f);
    glEnd();
}

TriangleWidget.h

#pragma once

#include <QtCore/qconfig.h>

#if QT_VERSION_MAJOR < 6
#warning "QT_VERSION_MAJOR < 6"
// #if QT_VERSION < QT_VERSION_CHECK(5, 0, 0)
// #error "Qt version not supported"
// #else
#include <QtWidgets/QOpenGLWidget>
// #endif
#else
#warning "QT_VERSION_MAJOR >= 6"
#include <QtOpenGLWidgets/QOpenGLWidget>
#endif

#include <QtGui/QOpenGLFunctions>

#include <QtGui/QMouseEvent>
#include <QtCore/QPoint>

inline QPointF mouseEventPosition(QMouseEvent *event)
{
#if QT_VERSION >= QT_VERSION_CHECK(6, 0, 0)
    return event->position();
#else
    return event->pos();
#endif
}

class TriangleWidget : public QOpenGLWidget, protected QOpenGLFunctions
{
public:
    TriangleWidget(QWidget *parent = nullptr);
    void mousePressEvent(QMouseEvent *event);
    void mouseMoveEvent(QMouseEvent *event);
    void wheelEvent(QWheelEvent *event);

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

private:
    QPoint _lastPos;
    double _azimuth;
    double _declination;
    double _distance;
};