3 files changed, 38 insertions, 7 deletions

diff --git a/src/quick3d/imports/scene3d/scene3ditem.cpp b/src/quick3d/imports/scene3d/scene3ditem.cpp
index 5a1cec495..f0ebfbc38 100644
--- a/src/quick3d/imports/scene3d/scene3ditem.cpp
+++ b/src/quick3d/imports/scene3d/scene3ditem.cpp
@@ -433,12 +433,26 @@ void Scene3DItem::applyRootEntityChange()
 
 bool Scene3DItem::needsRender()
 {
+    // We need the dirty flag which is connected to the change arbiter
+    // receiving updates to know whether something in the scene has changed
+
+    // Ideally we would use shouldRender() alone but given that it becomes true
+    // only after the arbiter has sync the changes and might be reset before
+    // process jobs is completed, we cannot fully rely on it. It would require
+    // splitting processFrame in 2 parts.
+
+    // We only use it for cases where Qt3D render may require several loops of
+    // the simulation to fully process a frame (e.g shaders are loaded in frame
+    // n and we can only build render commands for the new shader at frame n +
+    // This is where renderer->shouldRender() comes into play as it knows
+    // whether some states remain dirty or not (even after processFrame is
+    // called)
+
     auto renderAspectPriv = static_cast<QRenderAspectPrivate*>(QRenderAspectPrivate::get(m_renderAspect));
     const bool dirty = m_dirty
             || (renderAspectPriv
                 && renderAspectPriv->m_renderer
-                && renderAspectPriv->m_renderer->settings()
-                && renderAspectPriv->m_renderer->settings()->renderPolicy() == QRenderSettings::Always);
+                && renderAspectPriv->m_renderer->shouldRender());
     m_dirty = false;
     return dirty;
 }
@@ -466,10 +480,6 @@ void Scene3DItem::onBeforeSync()
     if (!isVisible())
         return;
 
-    // Has anything in the 3D scene actually changed that requires us to render?
-    if (!needsRender())
-        return;
-
     Q_ASSERT(QThread::currentThread() == thread());
 
     // Since we are in manual mode, trigger jobs for the next frame
@@ -510,6 +520,7 @@ void Scene3DItem::onBeforeSync()
     // start rendering before this function has been called
     // We add in a safety to skip such frames as this could otherwise
     // make Qt3D enter a locked state
+    m_renderer->setSkipFrame(!needsRender());
     m_renderer->allowRender();
 
     // Note: it's too early to request an update at this point as
diff --git a/src/quick3d/imports/scene3d/scene3drenderer.cpp b/src/quick3d/imports/scene3d/scene3drenderer.cpp
index 1e322a615..fafeeedf4 100644
--- a/src/quick3d/imports/scene3d/scene3drenderer.cpp
+++ b/src/quick3d/imports/scene3d/scene3drenderer.cpp
@@ -161,6 +161,7 @@ Scene3DRenderer::Scene3DRenderer(Scene3DItem *item, Qt3DCore::QAspectEngine *asp
     , m_forceRecreate(false)
     , m_shouldRender(false)
     , m_dirtyViews(false)
+    , m_skipFrame(false)
     , m_allowRendering(0)
     , m_compositingMode(Scene3DItem::FBO)
 {
@@ -278,6 +279,19 @@ void Scene3DRenderer::beforeSynchronize()
         // We could otherwise enter a deadlock state
         if (!m_allowRendering.tryAcquire(std::max(m_allowRendering.available(), 1)))
             return;
+
+        // In the case of OnDemand rendering, we still need to get to this
+        // point to ensure we have processed jobs for all aspects.
+        // We also still need to call render() to allow proceeding with the
+        // next frame. However it won't be performing any 3d rendering at all
+        // so we do it here and return early. This prevents a costly QtQuick
+        // SceneGraph update for nothing
+        if (m_skipFrame) {
+            m_skipFrame = false;
+            static_cast<QRenderAspectPrivate*>(QRenderAspectPrivate::get(m_renderAspect))->renderSynchronous(false);
+            return;
+        }
+
         m_shouldRender = true;
 
         // Check size / multisampling
@@ -360,6 +374,11 @@ void Scene3DRenderer::setCompositingMode(Scene3DItem::CompositingMode mode)
     m_compositingMode = mode;
 }
 
+void Scene3DRenderer::setSkipFrame(bool skip)
+{
+    m_skipFrame = skip;
+}
+
 // Main Thread, Render Thread locked
 void Scene3DRenderer::setScene3DViews(const QVector<Scene3DView *> views)
 {
diff --git a/src/quick3d/imports/scene3d/scene3drenderer_p.h b/src/quick3d/imports/scene3d/scene3drenderer_p.h
index 4f3651cd3..08a2c60a3 100644
--- a/src/quick3d/imports/scene3d/scene3drenderer_p.h
+++ b/src/quick3d/imports/scene3d/scene3drenderer_p.h
@@ -87,7 +87,7 @@ public:
     void setCleanerHelper(Scene3DCleaner *cleaner);
     void allowRender();
     void setCompositingMode(Scene3DItem::CompositingMode mode);
-
+    void setSkipFrame(bool skip);
     void setScene3DViews(const QVector<Scene3DView *> views);
 
 public Q_SLOTS:
@@ -119,6 +119,7 @@ private:
     bool m_forceRecreate;
     bool m_shouldRender;
     bool m_dirtyViews;
+    bool m_skipFrame;
     QSemaphore m_allowRendering;
     Scene3DItem::CompositingMode m_compositingMode;
     QVector<Scene3DView *> m_views;