Add visual indication for the rotation center

src/Gui/NavigationStyle.cpp

@@ -862,10 +862,9 @@ void NavigationStyle::doRotate(SoCamera * camera, float angle, const SbVec2f& po
 
 }
 
-SbVec3f NavigationStyle::getRotationCenter(SbBool* ok) const
+SbVec3f NavigationStyle::getRotationCenter(SbBool& found) const
 {
-    if (ok)
-        *ok = PRIVATE(this)->rotationCenterFound;
+    found = PRIVATE(this)->rotationCenterFound;
     return PRIVATE(this)->rotationCenter;
 }
 
@@ -1412,12 +1411,14 @@ void NavigationStyle::setViewingMode(const ViewerMode newmode)
     case DRAGGING:
         // Set up initial projection point for the projector object when
         // first starting a drag operation.
+        viewer->showRotationCenter(true);
         this->spinprojector->project(this->lastmouseposition);
         this->interactiveCountInc();
         this->clearLog();
         break;
 
     case SPINNING:
+        viewer->showRotationCenter(true);
         this->interactiveCountInc();
         viewer->getSoRenderManager()->scheduleRedraw();
         break;
@@ -1442,6 +1443,8 @@ void NavigationStyle::setViewingMode(const ViewerMode newmode)
     switch (oldmode) {
     case SPINNING:
     case DRAGGING:
+        viewer->showRotationCenter(false);
+        [[fallthrough]];
     case PANNING:
     case ZOOMING:
     case BOXZOOM:

src/Gui/NavigationStyle.h

@@ -172,6 +172,8 @@ public:
     void setOrbitStyle(OrbitStyle style);
     OrbitStyle getOrbitStyle() const;
 
+    SbVec3f getRotationCenter(SbBool&) const;
+
 protected:
     void initialize();
     void finalize();
@@ -187,7 +189,6 @@ protected:
     SbBool seekToPoint(const SbVec2s screenpos);
     void seekToPoint(const SbVec3f& scenepos);
     SbBool lookAtPoint(const SbVec2s screenpos);
-    SbVec3f getRotationCenter(SbBool*) const;
 
     void reorientCamera(SoCamera * camera, const SbRotation & rot);
     void panCamera(SoCamera * camera,

src/Gui/View3DInventorViewer.cpp

@@ -52,6 +52,7 @@
 # include <Inventor/events/SoKeyboardEvent.h>
 # include <Inventor/events/SoMotion3Event.h>
 # include <Inventor/manips/SoClipPlaneManip.h>
+# include <Inventor/nodes/SoAnnotation.h>
 # include <Inventor/nodes/SoBaseColor.h>
 # include <Inventor/nodes/SoCallback.h>
 # include <Inventor/nodes/SoCube.h>
@@ -62,8 +63,10 @@
 # include <Inventor/nodes/SoOrthographicCamera.h>
 # include <Inventor/nodes/SoPerspectiveCamera.h>
 # include <Inventor/nodes/SoPickStyle.h>
+# include <Inventor/nodes/SoScale.h>
 # include <Inventor/nodes/SoSelection.h>
 # include <Inventor/nodes/SoSeparator.h>
+# include <Inventor/nodes/SoSphere.h>
 # include <Inventor/nodes/SoSwitch.h>
 # include <Inventor/nodes/SoTransform.h>
 # include <Inventor/nodes/SoTranslation.h>
@@ -337,6 +340,7 @@ View3DInventorViewer::View3DInventorViewer(QWidget* parent, const QtGLWidget* sh
     , framebuffer(nullptr)
     , axisCross(nullptr)
     , axisGroup(nullptr)
+    , rotationCenterGroup(nullptr)
     , editing(false)
     , redirected(false)
     , allowredir(false)
@@ -355,6 +359,7 @@ View3DInventorViewer::View3DInventorViewer(const QtGLFormat& format, QWidget* pa
     , framebuffer(nullptr)
     , axisCross(nullptr)
     , axisGroup(nullptr)
+    , rotationCenterGroup(nullptr)
     , editing(false)
     , redirected(false)
     , allowredir(false)
@@ -1268,6 +1273,65 @@ bool View3DInventorViewer::hasAxisCross()
     return axisGroup;
 }
 
+void View3DInventorViewer::showRotationCenter(bool show)
+{
+    SoNode* scene = getSceneGraph();
+    auto sep = static_cast<SoSeparator*>(scene);
+
+    if (show) {
+        SbBool found;
+        SbVec3f center = navigation->getRotationCenter(found);
+
+        if (!found) {
+            return;
+        }
+
+        if (!rotationCenterGroup) {
+            rotationCenterGroup = new SoSkipBoundingGroup();
+
+            auto sphere = new SoSphere();
+
+            // There needs to be a non-transparent object to ensure the transparent sphere works when opening an new empty document
+            auto hidden = new SoSeparator();
+            auto hiddenScale = new SoScale();
+            hiddenScale->scaleFactor = SbVec3f(0, 0, 0);
+            hidden->addChild(hiddenScale);
+            hidden->addChild(sphere);
+
+            auto complexity = new SoComplexity();
+            complexity->value = 1;
+
+            auto material = new SoMaterial();
+            material->emissiveColor = SbColor(1, 0, 0);
+            material->transparency = 0.8;
+
+            auto translation = new SoTranslation();
+            translation->translation.setValue(center);
+
+            auto annotation = new SoAnnotation();
+            annotation->addChild(complexity);
+            annotation->addChild(material);
+            annotation->addChild(sphere);
+
+            auto scaledSphere = new SoShapeScale();
+            scaledSphere->setPart("shape", annotation);
+            scaledSphere->scaleFactor = 4.0;
+
+            rotationCenterGroup->addChild(translation);
+            rotationCenterGroup->addChild(hidden);
+            rotationCenterGroup->addChild(scaledSphere);
+
+            sep->addChild(rotationCenterGroup);
+        }
+    }
+    else {
+        if (rotationCenterGroup) {
+            sep->removeChild(rotationCenterGroup);
+            rotationCenterGroup = nullptr;
+        }
+    }
+}
+
 void View3DInventorViewer::setNavigationType(Base::Type t)
 {
     if (this->navigation && this->navigation->getTypeId() == t)

src/Gui/View3DInventorViewer.h

@@ -413,6 +413,8 @@ public:
     void setAxisCross(bool b);
     bool hasAxisCross();
 
+    void showRotationCenter(bool show);
+
     void setEnabledFPSCounter(bool b);
     void setEnabledNaviCube(bool b);
     bool isEnabledNaviCube() const;
@@ -509,6 +511,8 @@ private:
     SoShapeScale* axisCross;
     SoGroup* axisGroup;
 
+    SoGroup* rotationCenterGroup;
+
     //stuff needed to draw the fps counter
     bool fpsEnabled;
     bool vboEnabled;