diff -r aeb5f203b3eb -r a5111f4e6412 src/gl/partrenderer.cpp
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/gl/partrenderer.cpp	Fri Dec 13 15:55:56 2019 +0200
@@ -0,0 +1,118 @@
+#include <QMouseEvent>
+#include <GL/glut.h> // teapot
+#include "partrenderer.h"
+
+PartRenderer::PartRenderer(QWidget* parent) :
+	QOpenGLWidget{parent}
+{
+	this->setMouseTracking(true);
+}
+
+void PartRenderer::initializeGL()
+{
+	this->initializeOpenGLFunctions();
+	if (this->glGetError() != GL_NO_ERROR)
+	{
+		abort();
+	}
+	this->initializeLighting();
+	this->initialized = true;
+	this->rotation = QQuaternion::fromAxisAndAngle({1, 0, 0}, 30);
+	this->rotation *= QQuaternion::fromAxisAndAngle({0, 1, 0}, 330);
+}
+
+// https://stackoverflow.com/a/12943456
+static void perspectiveGL(GLdouble fovY, GLdouble aspect, GLdouble zNear, GLdouble zFar)
+{
+	//fH = tan( (fovY / 2) / 180 * pi ) * zNear;
+	const GLdouble fH = std::tan(fovY / 360 * pi) * zNear;
+	const GLdouble fW = fH * aspect;
+	glFrustum(-fW, fW, -fH, fH, zNear, zFar);
+}
+
+/*
+ * Pads a 3×3 matrix into a 4×4 one by adding cells from the identity matrix.
+ */
+static QMatrix4x4 padMatrix(const QMatrix3x3& stub)
+{
+	return {
+		stub(0, 0), stub(0, 1), stub(0, 2), 0,
+		stub(1, 0), stub(1, 1), stub(1, 2), 0,
+		stub(2, 0), stub(2, 1), stub(2, 2), 0,
+		0, 0, 0, 1
+	};
+}
+
+
+void PartRenderer::initializeLighting()
+{
+	GLfloat materialShininess[] = {5.0};
+	GLfloat lightPosition[] = {1.0, 1.0, 1.0, 0.0};
+	GLfloat ambientLightingLevel[] = {0.5, 0.5, 0.5, 1.0};
+	glShadeModel(GL_SMOOTH);
+	glMaterialfv(GL_FRONT, GL_SHININESS, materialShininess);
+	glLightfv(GL_LIGHT0, GL_AMBIENT, ambientLightingLevel);
+	glLightfv(GL_LIGHT0, GL_DIFFUSE, ambientLightingLevel);
+	glLightfv(GL_LIGHT0, GL_POSITION, lightPosition);
+	glEnable(GL_LIGHTING);
+	glEnable(GL_LIGHT0);
+	glEnable(GL_COLOR_MATERIAL);
+	glEnable(GL_DEPTH_TEST);
+}
+
+void PartRenderer::resizeGL(int width, int height)
+{
+	constexpr GLfloat near = 1.0f;
+	constexpr GLfloat far = 1.0e+05f;
+	glViewport (0, 0, width, height);
+	glMatrixMode(GL_PROJECTION);
+	glLoadIdentity();
+	::perspectiveGL(45.0f, static_cast<double>(width) / static_cast<double>(height), near, far);
+	glMatrixMode(GL_MODELVIEW);
+}
+
+// https://www.codemiles.com/c-opengl-examples/drawing-teapot-using-opengl-t9010.html?mobile=on
+void PartRenderer::paintGL()
+{
+	glMatrixMode(GL_MODELVIEW);
+	// clear the drawing buffer.
+	glClear(GL_COLOR_BUFFER_BIT);
+	// clear the identity matrix.
+	glLoadIdentity();
+	// traslate the draw by z = -4.0
+	// Note this when you decrease z like -8.0 the drawing will looks far , or smaller.
+	glTranslatef(0.0,0.0,-4.5);
+	// Red color used to draw.
+	glColor3f(0.8, 0.2, 0.1);
+	glMultMatrixf(padMatrix(this->rotation.toRotationMatrix()).constData());
+	glutSolidTeapot(1.0);
+	glFlush();
+}
+
+static QPointF pointToPointF(const QPoint& point)
+{
+	return {static_cast<qreal>(point.x()), static_cast<qreal>(point.y())};
+}
+
+/*
+static QPoint pointFToPoint(const QPointF& point)
+{
+	return {static_cast<int>(std::round(point.x())), static_cast<int>(std::round(point.y()))};
+}
+*/
+
+void PartRenderer::mouseMoveEvent(QMouseEvent* event)
+{
+	const bool left = event->buttons() & Qt::LeftButton;
+	const QPointF move = pointToPointF(event->pos()) - this->lastMousePosition;
+	if (left and not move.isNull())
+	{
+		const QQuaternion versor = QQuaternion::fromAxisAndAngle(
+			QVector3D{static_cast<float>(move.y()), static_cast<float>(move.x()), 0.0f},
+			0.6 * std::hypot(move.x(), move.y())
+		);
+		this->rotation = versor * this->rotation;
+		this->update();
+	}
+	this->lastMousePosition = pointToPointF(event->pos());
+}