feat: Implement middle-mouse rotation around grid intersection with visual pivot

Co-authored-by: aider (gemini/gemini-2.5-pro) <aider@aider.chat>

src/Camera.cpp

@@ -88,8 +88,15 @@ QMatrix4x4 Camera::modelViewMatrix() const
 {
     QMatrix4x4 model;
     model.translate(m_panX, m_panY, m_zoom);
+
+    if (m_isRotating) {
+        model.translate(m_rotationPivot);
+    }
     model.rotate(m_xRot / 16.0f, 1, 0, 0);
     model.rotate(m_yRot / 16.0f, 0, 1, 0);
+    if (m_isRotating) {
+        model.translate(-m_rotationPivot);
+    }
     return model;
 }
 
@@ -217,6 +224,44 @@ void Camera::animateToPlaneView(int plane)
     animGroup->start(QAbstractAnimation::DeleteWhenStopped);
 }
 
+void Camera::startRotation(const QVector3D& pivot)
+{
+    m_rotationPivot = pivot;
+
+    QMatrix4x4 rotation;
+    rotation.rotate(m_xRot / 16.0f, 1, 0, 0);
+    rotation.rotate(m_yRot / 16.0f, 0, 1, 0);
+
+    QVector3D p_rotated = rotation.map(m_rotationPivot);
+    QVector3D p_diff = p_rotated - m_rotationPivot;
+
+    setPanX(m_panX + p_diff.x());
+    setPanY(m_panY + p_diff.y());
+    setZoom(m_zoom + p_diff.z());
+
+    m_isRotating = true;
+}
+
+void Camera::stopRotation()
+{
+    if (!m_isRotating) {
+        return;
+    }
+
+    QMatrix4x4 rotation;
+    rotation.rotate(m_xRot / 16.0f, 1, 0, 0);
+    rotation.rotate(m_yRot / 16.0f, 0, 1, 0);
+
+    QVector3D p_rotated = rotation.map(m_rotationPivot);
+    QVector3D p_diff = p_rotated - m_rotationPivot;
+
+    setPanX(m_panX - p_diff.x());
+    setPanY(m_panY - p_diff.y());
+    setZoom(m_zoom - p_diff.z());
+
+    m_isRotating = false;
+}
+
 void Camera::animateRestoreState()
 {
     auto* animGroup = new QParallelAnimationGroup(this);

src/Camera.h

@@ -42,6 +42,11 @@ public:
     void animateToPlaneView(int plane);
     void animateRestoreState();
 
+    void startRotation(const QVector3D& pivot);
+    void stopRotation();
+    bool isRotating() const { return m_isRotating; }
+    const QVector3D& rotationPivot() const { return m_rotationPivot; }
+
     float savedXRot() const { return m_savedXRot; }
     float savedYRot() const { return m_savedYRot; }
     float savedZoom() const { return m_savedZoom; }
@@ -60,6 +65,9 @@ private:
     float m_panX;
     float m_panY;
 
+    QVector3D m_rotationPivot;
+    bool m_isRotating = false;
+
     float m_savedXRot = 0;
     float m_savedYRot = 0;
     float m_savedZoom = -5.0f;

src/ViewportWidget.cpp

@@ -153,6 +153,29 @@ void ViewportWidget::paintGL()
 
     m_snapping->paintGL();
 
+    if (m_camera->isRotating()) {
+        const float radius = 0.004f * -m_camera->zoom();
+        const int numSegments = 16;
+        QMatrix4x4 invModelView = m_camera->modelViewMatrix().inverted();
+        QVector3D rightVec = invModelView.column(0).toVector3D();
+        QVector3D upVec = invModelView.column(1).toVector3D();
+
+        QVector<GLfloat> circleFillVertices;
+        QVector3D center = m_camera->rotationPivot();
+
+        circleFillVertices << center.x() << center.y() << center.z(); // Center vertex for fan
+        for (int i = 0; i <= numSegments; ++i) { // <= to close the circle
+            float angle = (2.0f * M_PI * float(i)) / float(numSegments);
+            QVector3D p = center + radius * (cos(angle) * rightVec + sin(angle) * upVec);
+            circleFillVertices << p.x() << p.y() << p.z();
+        }
+
+        m_shaderProgram->setUniformValue(m_colorLoc, QVector4D(0.0f, 0.0f, 1.0f, 1.0f)); // Blue
+        m_vbo.bind();
+        m_vbo.allocate(circleFillVertices.constData(), circleFillVertices.size() * sizeof(GLfloat));
+        glDrawArrays(GL_TRIANGLE_FAN, 0, circleFillVertices.size() / 3);
+    }
+
     if (m_activeSketchTool) {
         m_activeSketchTool->paintGL();
     }
@@ -196,6 +219,11 @@ void ViewportWidget::mousePressEvent(QMouseEvent *event)
 {
     m_camera->mousePressEvent(event);
 
+    if (event->button() == Qt::MiddleButton && !(QApplication::keyboardModifiers() & Qt::ShiftModifier)) {
+        m_camera->startRotation(unproject(event->pos(), m_currentPlane));
+        update();
+    }
+
     if (event->button() == Qt::LeftButton) {
         if (m_isSelectingPlane) {
             if (m_highlightedPlane != SketchPlane::NONE) {
@@ -270,6 +298,15 @@ void ViewportWidget::keyPressEvent(QKeyEvent *event)
     QOpenGLWidget::keyPressEvent(event);
 }
 
+void ViewportWidget::mouseReleaseEvent(QMouseEvent *event)
+{
+    if (event->button() == Qt::MiddleButton) {
+        m_camera->stopRotation();
+        update();
+    }
+    QOpenGLWidget::mouseReleaseEvent(event);
+}
+
 void ViewportWidget::addLine(const gp_Pnt& start, const gp_Pnt& end)
 {
     emit lineAdded(start, end);