Simple prefetch scheme

Prefetches larger set of tiles when camera is stopped

QTBUG-25136

Prefetch implementation including zoom-level prefetching
Two prefetching options- pan buffer, or pan buffer for two nearest zoom levels
Code simplified by removing all three visibleTiles lists, so they can't go out of sync
Textures not promoted up the cache if they aren't visible when retrieved from server

Also: Prefetch on map initialize + optimisation

This ensures that the prefetch is called at the end of initialisation
Useful if the location is not already in disk cache. And makes no
difference if it is already in disk cache.
QTBUG-25786

Change-Id: Ife7c1d103f83695659f4534880268fa5afd8f7c7
Reviewed-by: Alex Wilson <alex.wilson@nokia.com>

1 files changed, 79 insertions, 12 deletions

diff --git a/src/location/maps/qgeocameratiles.cpp b/src/location/maps/qgeocameratiles.cpp
index 9258661b..a63420ed 100644
--- a/src/location/maps/qgeocameratiles.cpp
+++ b/src/location/maps/qgeocameratiles.cpp
@@ -58,6 +58,11 @@
 #include <cmath>
 
 QT_BEGIN_NAMESPACE
+#define ENABLE_PREFETCHING
+// larger values fetch a bigger set when the camera stops
+#define PREFETCH_FRUSTUM_SCALE 2.0
+// #define PREFETCH_NEIGHBOUR_LAYER
+#define PREFETCH_TWO_NEIGHBOUR_LAYERS
 
 struct Frustum
 {
@@ -91,9 +96,9 @@ public:
     int sideLength_;
 
     void updateMetadata();
-    void updateGeometry();
+    void updateGeometry(double viewExpansion = 1.0);
 
-    Frustum frustum() const;
+    Frustum frustum(double fieldOfViewGradient) const;
 
     class LengthSorter
     {
@@ -132,18 +137,78 @@ QGeoCameraTiles::~QGeoCameraTiles()
     delete d_ptr;
 }
 
+void QGeoCameraTiles::findPrefetchTiles()
+{
+#if defined(ENABLE_PREFETCHING)
+    Q_D(QGeoCameraTiles);
+
+    d->tiles_.clear();
+
+    // qDebug() << "prefetch called";
+    int zoom = static_cast<int>(floor(d->camera_.zoomLevel()));
+    d->intZoomLevel_ = zoom;
+    d->sideLength_ = 1 << d->intZoomLevel_;
+    d->updateGeometry(PREFETCH_FRUSTUM_SCALE);
+
+#if defined(PREFETCH_NEIGHBOUR_LAYER)
+    double zoomFraction = d->camera_.zoomLevel() - zoom;
+    int nearestNeighbourLayer = zoomFraction > 0.5 ? zoom + 1 : zoom - 1;
+    if (nearestNeighbourLayer <= d->maxZoom_ && nearestNeighbourLayer >= 0)
+    {
+        double oldZoom = d->camera_.zoomLevel();
+        d->intZoomLevel_ = nearestNeighbourLayer;
+        d->sideLength_ = 1 << d->intZoomLevel_;
+        d->camera_.setZoomLevel(d->intZoomLevel_);
+
+        // Approx heuristic, keeping total # prefetched tiles roughly independent of the
+        // fractional zoom level.
+        double neighbourScale = (1.0 + zoomFraction)/2.0;
+
+        d->updateGeometry(PREFETCH_FRUSTUM_SCALE * neighbourScale);
+        d->camera_.setZoomLevel(oldZoom);
+    }
+#elif defined(PREFETCH_TWO_NEIGHBOUR_LAYERS)
+ //   int size1 = d->tiles_.size();
+
+    // This is a simpler strategy, we just prefetch from layer above and below
+    // for the layer below we only use half the size as this fills the screen
+    double oldZoom = d->camera_.zoomLevel();
+    if (zoom > 0)
+    {
+        d->intZoomLevel_ = zoom-1;
+        d->sideLength_ = 1 << d->intZoomLevel_;
+        d->camera_.setZoomLevel(d->intZoomLevel_);
+        d->updateGeometry(0.5);
+    }
+ //   int size2 = d->tiles_.size();
+    if (zoom < d->maxZoom_)
+    {
+        d->intZoomLevel_ = zoom+1;
+        d->sideLength_ = 1 << d->intZoomLevel_;
+        d->camera_.setZoomLevel(d->intZoomLevel_);
+        d->updateGeometry(1.0);
+    }
+ //   qDebug() << "prefetched main tiles: " << size1 << " higher detail layer: " << d->tiles_.size() - size2 << " low detail layer: " << size2 - size1;
+    d->intZoomLevel_ = zoom;
+    d->sideLength_ = 1 << d->intZoomLevel_;
+    d->camera_.setZoomLevel(oldZoom);
+#endif
+#endif
+}
+
+
 void QGeoCameraTiles::setCamera(const QGeoCameraData &camera)
 {
     Q_D(QGeoCameraTiles);
 
     if (d->camera_ == camera)
         return;
-
     d->camera_ = camera;
 
     d->intZoomLevel_ = static_cast<int>(floor(d->camera_.zoomLevel()));
     d->sideLength_ = 1 << d->intZoomLevel_;
 
+    d->tiles_.clear();
     d->updateGeometry();
 }
 
@@ -155,6 +220,7 @@ void QGeoCameraTiles::setScreenSize(const QSize &size)
         return;
 
     d->screenSize_ = size;
+    d->tiles_.clear();
     d->updateGeometry();
 }
 
@@ -188,6 +254,7 @@ void QGeoCameraTiles::setTileSize(int tileSize)
         return;
 
     d->tileSize_ = tileSize;
+    d->tiles_.clear();
     d->updateGeometry();
 }
 
@@ -205,6 +272,7 @@ void QGeoCameraTiles::setMaximumZoomLevel(int maxZoom)
         return;
 
     d->maxZoom_ = maxZoom;
+    d->tiles_.clear();
     d->updateGeometry();
 }
 
@@ -239,12 +307,11 @@ void QGeoCameraTilesPrivate::updateMetadata()
     tiles_ = newTiles;
 }
 
-void QGeoCameraTilesPrivate::updateGeometry()
+void QGeoCameraTilesPrivate::updateGeometry(double viewExpansion)
 {
-    tiles_.clear();
-
     // Find the frustum from the camera / screen / viewport information
-    Frustum f = frustum();
+    // The larger frustum when stationary is a form of prefetching
+    Frustum f = frustum(viewExpansion);
 
     // Find the polygon where the frustum intersects the plane of the map
     Polygon footprint = frustumFootprint(f);
@@ -263,7 +330,7 @@ void QGeoCameraTilesPrivate::updateGeometry()
     }
 }
 
-Frustum QGeoCameraTilesPrivate::frustum() const
+Frustum QGeoCameraTilesPrivate::frustum(double fieldOfViewGradient) const
 {
     QDoubleVector3D center = sideLength_ * QGeoProjection::coordToMercator(camera_.center());
     center.setZ(0.0);
@@ -294,16 +361,16 @@ Frustum QGeoCameraTilesPrivate::frustum() const
     // this assumes that viewSize = 2*nearPlane x 2*nearPlane
 
     if (aspectRatio > 1.0) {
-        hn = 2 * nearPlane;
+        hn = 2 * fieldOfViewGradient * nearPlane;
         wn = hn * aspectRatio;
 
-        hf = 2 * farPlane;
+        hf = 2 * fieldOfViewGradient * farPlane;
         wf = hf * aspectRatio;
     } else {
-        wn = 2 * nearPlane;
+        wn = 2 * fieldOfViewGradient * nearPlane;
         hn = wn / aspectRatio;
 
-        wf = 2 * farPlane;
+        wf = 2 * fieldOfViewGradient * farPlane;
         hf = wf / aspectRatio;
     }