Optimize scene graph rendering by reusing old nodes if possible

Previously, Qt WebEngine always dropped all existing scene graph
nodes that were created for previous frames and built up a whole
new tree. The main reason for this was that the render pass
structure is not guaranteed to remain the same across two frames.
This resulted in a full repaint of every new frame, even if only
a small area had changed.

We now check first if the structure of the frame data has changed
across two sequential frames. We only discard the old nodes if
there actually has been a change. Otherwise, we reuse the scene
graph nodes and only update the nodes that have changed.

A general exception to this at the moment is video. In case any
streaming video or yuv video nodes are found in the render pass,
all nodes get recreated.

Task-number: QTBUG-57720
Change-Id: I8998577af48a163d54144f205c316ee427ed0307
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>

1 files changed, 2 insertions, 0 deletions

diff --git a/src/core/delegated_frame_node.h b/src/core/delegated_frame_node.h
index 36ec20f1d..6178bd232 100644
--- a/src/core/delegated_frame_node.h
+++ b/src/core/delegated_frame_node.h
@@ -74,6 +74,7 @@ public:
     ChromiumCompositorData() : frameDevicePixelRatio(1) { }
     QHash<unsigned, QSharedPointer<ResourceHolder> > resourceHolders;
     std::unique_ptr<cc::DelegatedFrameData> frameData;
+    std::unique_ptr<cc::DelegatedFrameData> previousFrameData;
     qreal frameDevicePixelRatio;
 };
 
@@ -100,6 +101,7 @@ private:
         QVector<QSharedPointer<QSGRootNode> > renderPassRootNodes;
         QVector<QSharedPointer<QSGTexture> > textureStrongRefs;
     } m_sgObjects;
+    QVector<QSGNode*> m_sceneGraphNodes;
     int m_numPendingSyncPoints;
     QWaitCondition m_mailboxesFetchedWaitCond;
     QMutex m_mutex;