Qt OpenGL Minimal QGLWidget Implementation for QPainter Painting, Drawing OpenGL Triangle, QPainter Text and Circle

main.cpp

#include <QApplication>
#include <QGLWidget>

class GLWidget : public QGLWidget 
{
public:
	GLWidget(QWidget *parent = 0);
	~GLWidget();
	QSize sizeHint() const { return QSize(400, 400); }

protected:
	void initializeGL() override;
	void paintEvent(QPaintEvent *e) override;
	void resizeGL(int width, int height) override;
};

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


GLWidget::GLWidget(QWidget *parent) : QGLWidget(parent) 
{
	// requied for QPainter operations after OpenGL drawing
	// otherwise QPainter fills image with solid color
	setAutoFillBackground(false);
}

GLWidget::~GLWidget() 
{
}

void GLWidget::initializeGL() 
{
}

void GLWidget::paintEvent(QPaintEvent *e)
{
	makeCurrent();

	QSize viewport_size = size();
	glViewport(0, 0, viewport_size.width(), viewport_size.height());

	glMatrixMode(GL_PROJECTION);
	glLoadIdentity();
	glFrustum(-1, 1, -1, 1, 5, 7); // near and far match your triangle Z distance

	glMatrixMode(GL_MODELVIEW);

	glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
	glLoadIdentity();
	glTranslatef(-1.5f, 0.0f, -6.0f);

	glBegin(GL_TRIANGLES);
	glColor3f(1, 0, 0);
	glVertex3f(0.0f, 1.0f, 0.0f);
	glColor3f(0.0, 1, 0);
	glVertex3f(-1.0f, -1.0f, 0.0f);
	glColor3f(0.0, 0.0, 1);
	glVertex3f(1.0f, -1.0f, 0.0f);
	glEnd();

	QPainter painter(this);
	painter.setRenderHint(QPainter::Antialiasing);
	painter.setPen(Qt::white);

	QString text = tr("Text drawn with QPainter");
	QFontMetrics metrics = QFontMetrics(font());
	int border = qMax(4, metrics.leading());

	QRect rect = metrics.boundingRect(0, 0, width() - 2 * border, int(height()*0.125),
		Qt::AlignCenter | Qt::TextWordWrap, text);
	painter.setRenderHint(QPainter::TextAntialiasing);
	painter.fillRect(QRect(0, 0, width(), rect.height() + 2 * border),
		QColor(0, 0, 0, 127));
	painter.setPen(Qt::white);
	painter.fillRect(QRect(0, 0, width(), rect.height() + 2 * border),
		QColor(0, 0, 0, 127));
	painter.drawText((width() - rect.width()) / 2, border,
		rect.width(), rect.height(),
		Qt::AlignCenter | Qt::TextWordWrap, text);

	painter.setPen(Qt::red);
	painter.drawEllipse(QPoint(width() / 2, height() / 2), 30, 30);

	painter.end();	// calls swapBuffers();
	// it is necessary to call swapBuffers() manually if QPainter is not used in paintEvent()
}

void GLWidget::resizeGL(int w, int h)
{
	QGLWidget::resize(w, h);
}