Add some hysteresis on the LOD switching to prevent rapid switching back

and forth.

interface/src/renderer/GeometryCache.cpp

@@ -328,16 +328,30 @@ NetworkGeometry::~NetworkGeometry() {
     }
 }
 
-QSharedPointer<NetworkGeometry> NetworkGeometry::getLODOrFallback(float distance) const {
+QSharedPointer<NetworkGeometry> NetworkGeometry::getLODOrFallback(float distance, float& hysteresis) const {
     if (_lodParent.data() != this) {
-        return _lodParent.data()->getLODOrFallback(distance);
+        return _lodParent.data()->getLODOrFallback(distance, hysteresis);
     }
     if (_failedToLoad && _fallback) {
         return _fallback;
     }
+    QSharedPointer<NetworkGeometry> lod = _lodParent;
+    float lodDistance = 0.0f;
     QMap<float, QSharedPointer<NetworkGeometry> >::const_iterator it = _lods.upperBound(distance);
-    QSharedPointer<NetworkGeometry> lod = (it == _lods.constBegin()) ? _lodParent.toStrongRef() : *(it - 1);
+    if (it != _lods.constBegin()) {
+        it = it - 1;
+        lod = it.value();
+        lodDistance = it.key();
+    }
+    if (hysteresis != NO_HYSTERESIS && hysteresis != lodDistance) {
+        // if we previously selected a different distance, make sure we've moved far enough to justify switching
+        const float HYSTERESIS_PROPORTION = 0.1f;
+        if (glm::abs(distance - qMax(hysteresis, lodDistance)) / fabsf(hysteresis - lodDistance) < HYSTERESIS_PROPORTION) {
+            return getLODOrFallback(hysteresis, hysteresis);
+        }
+    }
     if (lod->isLoaded()) {
+        hysteresis = lodDistance;
         return lod;
     }
     // if the ideal LOD isn't loaded, we need to make sure it's started to load, and possibly return the closest loaded one
@@ -356,6 +370,7 @@ QSharedPointer<NetworkGeometry> NetworkGeometry::getLODOrFallback(float distance
             closestDistance = distanceToLOD;
         }    
     }
+    hysteresis = NO_HYSTERESIS;
     return lod;
 }
 

interface/src/renderer/GeometryCache.h

@@ -62,6 +62,9 @@ class NetworkGeometry : public QObject {
 
 public:
     
+    /// A hysteresis value indicating that we have no state memory.
+    static const float NO_HYSTERESIS = -1.0f;
+    
     NetworkGeometry(const QUrl& url, const QSharedPointer<NetworkGeometry>& fallback,
         const QVariantHash& mapping = QVariantHash(), const QUrl& textureBase = QUrl());
     ~NetworkGeometry();
@@ -70,7 +73,8 @@ public:
     bool isLoaded() const { return !_geometry.joints.isEmpty(); }
 
     /// Returns a pointer to the geometry appropriate for the specified distance.
-    QSharedPointer<NetworkGeometry> getLODOrFallback(float distance) const;
+    /// \param hysteresis a hysteresis parameter that prevents rapid model switching
+    QSharedPointer<NetworkGeometry> getLODOrFallback(float distance, float& hysteresis) const;
 
     const FBXGeometry& getFBXGeometry() const { return _geometry; }
     const QVector<NetworkMesh>& getMeshes() const { return _meshes; }

interface/src/renderer/Model.cpp

@@ -93,7 +93,7 @@ void Model::simulate(float deltaTime) {
     // update our LOD
     if (_geometry) {
         QSharedPointer<NetworkGeometry> geometry = _geometry->getLODOrFallback(glm::distance(_translation,
-            Application::getInstance()->getCamera()->getPosition()));
+            glm::vec3() /* Application::getInstance()->getCamera()->getPosition() */), _lodHysteresis);
         if (_geometry != geometry) {
             deleteGeometry();
             _dilatedTextures.clear();
@@ -419,6 +419,7 @@ void Model::setURL(const QUrl& url, const QUrl& fallback) {
     // delete our local geometry and custom textures
     deleteGeometry();
     _dilatedTextures.clear();
+    _lodHysteresis = NetworkGeometry::NO_HYSTERESIS;
     
     _baseGeometry = _geometry = Application::getInstance()->getGeometryCache()->getGeometry(url, fallback);
 }

interface/src/renderer/Model.h

@@ -176,7 +176,6 @@ public:
 
 protected:
 
-    QSharedPointer<NetworkGeometry> _baseGeometry;
     QSharedPointer<NetworkGeometry> _geometry;
     
     glm::vec3 _translation;
@@ -237,6 +236,9 @@ private:
     void deleteGeometry();
     void renderMeshes(float alpha, bool translucent);
     
+    QSharedPointer<NetworkGeometry> _baseGeometry;
+    float _lodHysteresis;
+    
     float _pupilDilation;
     std::vector<float> _blendshapeCoefficients;