Evaluate the visible region lazily in QGeoProjectionWebMercator

This change optimizes QGeoProjectionWebMercator::setupCamera, factoring
out the code that calculates the visible region, and replacing it
with a dirty flag.
The computation is deferred to the first time visibleRegion() gets
called.
This is done because the visible region is currently only used to render
some types of map items, meaning that in a multi layer setup, only few
layers might need it.

Change-Id: I5dd75969581513eb60e3cf5d6ba649349bf25378
Reviewed-by: Alex Blasche <alexander.blasche@qt.io>

2 files changed, 19 insertions, 6 deletions

diff --git a/src/location/maps/qgeoprojection.cpp b/src/location/maps/qgeoprojection.cpp
index 5aaf584b..319bf906 100644
--- a/src/location/maps/qgeoprojection.cpp
+++ b/src/location/maps/qgeoprojection.cpp
@@ -93,7 +93,9 @@ QGeoProjectionWebMercator::QGeoProjectionWebMercator()
       m_farPlane(0.0),
       m_halfWidth(0.0),
       m_halfHeight(0.0),
-      m_minimumUnprojectableY(0.0)
+      m_minimumUnprojectableY(0.0),
+      m_verticalEstateToSkip(0.0),
+      m_visibleRegionDirty(false)
 {
 }
 
@@ -299,6 +301,8 @@ bool QGeoProjectionWebMercator::isProjectable(const QDoubleVector2D &wrappedProj
 
 QList<QDoubleVector2D> QGeoProjectionWebMercator::visibleRegion() const
 {
+    if (m_visibleRegionDirty)
+        const_cast<QGeoProjectionWebMercator *>(this)->updateVisibleRegion();
     return m_visibleRegion;
 }
 
@@ -444,16 +448,22 @@ void QGeoProjectionWebMercator::setupCamera()
     const double elevationUpperBound = 90.0 - upperBoundEpsilon;
     const double maxRayElevation = qMin(elevationUpperBound - m_cameraData.tilt(), verticalHalfFOV);
     double maxHalfAperture = 0;
-    double verticalEstateToSkip = 0;
+    m_verticalEstateToSkip = 0;
     if (maxRayElevation < verticalHalfFOV) {
         maxHalfAperture = tan(QLocationUtils::radians(maxRayElevation));
-        verticalEstateToSkip = 1.0 - maxHalfAperture / m_aperture;
+        m_verticalEstateToSkip = 1.0 - maxHalfAperture / m_aperture;
     }
 
-    m_minimumUnprojectableY = verticalEstateToSkip * 0.5 * m_viewportHeight; // verticalEstateToSkip is relative to half aperture
+    m_minimumUnprojectableY = m_verticalEstateToSkip * 0.5 * m_viewportHeight; // m_verticalEstateToSkip is relative to half aperture
+    m_visibleRegionDirty = true;
+}
+
+void QGeoProjectionWebMercator::updateVisibleRegion()
+{
+    m_visibleRegionDirty = false;
 
-    QDoubleVector2D tl = viewportToWrappedMapProjection(QDoubleVector2D(-1, -1 + verticalEstateToSkip ));
-    QDoubleVector2D tr = viewportToWrappedMapProjection(QDoubleVector2D( 1, -1 + verticalEstateToSkip ));
+    QDoubleVector2D tl = viewportToWrappedMapProjection(QDoubleVector2D(-1, -1 + m_verticalEstateToSkip ));
+    QDoubleVector2D tr = viewportToWrappedMapProjection(QDoubleVector2D( 1, -1 + m_verticalEstateToSkip ));
     QDoubleVector2D bl = viewportToWrappedMapProjection(QDoubleVector2D(-1,  1 ));
     QDoubleVector2D br = viewportToWrappedMapProjection(QDoubleVector2D( 1,  1 ));
 
diff --git a/src/location/maps/qgeoprojection_p.h b/src/location/maps/qgeoprojection_p.h
index b17392b1..3a0dec1d 100644
--- a/src/location/maps/qgeoprojection_p.h
+++ b/src/location/maps/qgeoprojection_p.h
@@ -131,6 +131,7 @@ public:
 
 private:
     void setupCamera();
+    void updateVisibleRegion();
 
 public:
     struct Line2D
@@ -187,6 +188,7 @@ private:
     double           m_halfWidth;
     double           m_halfHeight;
     double           m_minimumUnprojectableY;
+    double           m_verticalEstateToSkip;
 
     // For the clipping region
     QDoubleVector3D  m_centerMercator;
@@ -199,6 +201,7 @@ private:
     Line2D           m_nearPlaneMapIntersection;
 
     QList<QDoubleVector2D> m_visibleRegion;
+    bool             m_visibleRegionDirty;
 
     Q_DISABLE_COPY(QGeoProjectionWebMercator)
 };