Add QgsCoordinateTransform::conversionFactor

directly copied from QgsMapSettings
qgis · Sep 10, 2018 · 89526a0 · 89526a0
1 parent 85ff90f
commit 89526a0
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Showing 4 changed files with 34 additions and 8 deletions.
diff --git a/python/core/auto_generated/qgscoordinatetransform.sip.in b/python/core/auto_generated/qgscoordinatetransform.sip.in
@@ -356,6 +356,17 @@ This should be called whenever the srs database has
 been modified in order to ensure that outdated CRS transforms are not created.
 
 .. versionadded:: 3.0
+%End
+
+    double conversionFactor( const QgsRectangle &referenceExtent ) const;
+%Docstring
+Computes an *estimated* conversion factor between source and destination units:
+
+sourceUnits * conversionFactor = destinationUnits
+
+:param referenceExtent: A reference extent based on which to perform the computation
+
+.. versionadded:: 3.4
 %End
 
 };

diff --git a/src/core/qgscoordinatetransform.cpp b/src/core/qgscoordinatetransform.cpp
@@ -798,3 +798,14 @@ void QgsCoordinateTransform::invalidateCache()
   sTransforms.clear();
   sCacheLock.unlock();
 }
+
+double QgsCoordinateTransform::conversionFactor( const QgsRectangle &ReferenceExtent ) const
+{
+  QgsPointXY source1( ReferenceExtent.xMinimum(), ReferenceExtent.yMinimum() );
+  QgsPointXY source2( ReferenceExtent.xMaximum(), ReferenceExtent.yMaximum() );
+  double distSourceUnits = std::sqrt( source1.sqrDist( source2 ) );
+  QgsPointXY dest1 = transform( source1 );
+  QgsPointXY dest2 = transform( source2 );
+  double distDestUnits = std::sqrt( dest1.sqrDist( dest2 ) );
+  return distDestUnits / distSourceUnits;
+}
diff --git a/src/core/qgscoordinatetransform.h b/src/core/qgscoordinatetransform.h
@@ -405,6 +405,17 @@ class CORE_EXPORT QgsCoordinateTransform
      */
     static void invalidateCache();
 
+    /**
+     * Computes an *estimated* conversion factor between source and destination units:
+     *
+     *   sourceUnits * conversionFactor = destinationUnits
+     *
+     * \param referenceExtent A reference extent based on which to perform the computation
+     *
+     * \since QGIS 3.4
+     */
+    double conversionFactor( const QgsRectangle &referenceExtent ) const;
+
   private:
 
     mutable QExplicitlySharedDataPointer<QgsCoordinateTransformPrivate> d;

diff --git a/src/core/qgsmapsettings.cpp b/src/core/qgsmapsettings.cpp
@@ -396,14 +396,7 @@ QgsCoordinateTransform QgsMapSettings::layerTransform( const QgsMapLayer *layer
 
 double QgsMapSettings::layerToMapUnits( const QgsMapLayer *layer, const QgsRectangle &referenceExtent ) const
 {
-  QgsRectangle extent = referenceExtent.isEmpty() ? layer->extent() : referenceExtent;
-  QgsPointXY l1( extent.xMinimum(), extent.yMinimum() );
-  QgsPointXY l2( extent.xMaximum(), extent.yMaximum() );
-  double distLayerUnits = std::sqrt( l1.sqrDist( l2 ) );
-  QgsPointXY m1 = layerToMapCoordinates( layer, l1 );
-  QgsPointXY m2 = layerToMapCoordinates( layer, l2 );
-  double distMapUnits = std::sqrt( m1.sqrDist( m2 ) );
-  return distMapUnits / distLayerUnits;
+  return layerTransform( layer ).conversionFactor( referenceExtent );
 }