Don't render when scene is not changing

Reduce CPU and GPU load by checking if the scene has changed and render
only if it has.

Task-number: QT3DS-4042
Change-Id: I54e447760f04fdad8d64319f326245175b471331
Reviewed-by: Miikka Heikkinen <miikka.heikkinen@qt.io>
Reviewed-by: Tomi Korpipää <tomi.korpipaa@qt.io>

2 files changed, 9 insertions, 6 deletions

diff --git a/src/runtime/Qt3DSApplication.cpp b/src/runtime/Qt3DSApplication.cpp
index cb8f732..79866c5 100644
--- a/src/runtime/Qt3DSApplication.cpp
+++ b/src/runtime/Qt3DSApplication.cpp
@@ -1031,7 +1031,7 @@ struct SApp : public IApplication
         } // End QT3DS_PERF_SCOPED_TIMER scope
     }
 
-    void UpdateScenes() { m_RuntimeFactory->GetSceneManager().Update(); }
+    bool UpdateScenes() { return m_RuntimeFactory->GetSceneManager().Update(); }
 
     bool LazyLoadSubPresentations()
     {
@@ -1113,7 +1113,7 @@ struct SApp : public IApplication
     void ResetDirtyCounter() { m_DirtyCountdown = 5; }
 
     // Update all the presentations and render them.
-    void UpdateAndRender() override
+    bool UpdateAndRender() override
     {
         QT3DS_ASSERT(m_AppLoadContext.mPtr == NULL);
         m_ThisFrameStartTime = qt3ds::foundation::Time::getCurrentCounterValue();
@@ -1147,15 +1147,17 @@ struct SApp : public IApplication
         }
 
         UpdatePresentations();
-        UpdateScenes();
+        bool dirty = UpdateScenes();
 
         // If subpresentations changed we need to check if any of them needs to be loaded.
         if (LazyLoadSubPresentations()) {
             // Just redo all
             UpdatePresentations();
-            UpdateScenes();
+            dirty |= UpdateScenes();
         }
-
+        bool renderNextFrame = false;
+        if (m_LastRenderWasDirty || dirty || m_initialFrame)
+            renderNextFrame = true;
         Render();
 
         m_InputEnginePtr->ClearInputFrame();
@@ -1185,6 +1187,7 @@ struct SApp : public IApplication
             ResetDirtyCounter();
         else
             m_DirtyCountdown = NVMax(0, m_DirtyCountdown - 1);
+        return renderNextFrame;
     }
 
     // hook this up to -layer-caching.
diff --git a/src/runtime/Qt3DSApplication.h b/src/runtime/Qt3DSApplication.h
index 72d88a1..ad447f7 100644
--- a/src/runtime/Qt3DSApplication.h
+++ b/src/runtime/Qt3DSApplication.h
@@ -174,7 +174,7 @@ public:
     virtual QList<Q3DStudio::CPresentation *> GetPresentationList() = 0;
 
     // Update all the presentations and render them.  Called exactly once per frame.
-    virtual void UpdateAndRender() = 0;
+    virtual bool UpdateAndRender() = 0;
 
     virtual bool IsApplicationDirty() = 0;