src/core/raster/qgsrasterblock.cpp

@@ -410,6 +410,24 @@ bool QgsRasterBlock::convert( QgsRasterBlock::DataType destDataType )
   return true;
 }
 
+void QgsRasterBlock::applyNodataValues( const QList<Range>& rangeList )
+{
+  if ( rangeList.isEmpty() )
+  {
+    return;
+  }
+
+  size_t size = mWidth * mHeight;
+  for ( size_t i = 0; i < size; ++i )
+  {
+    double val = value( i );
+    if ( valueInRange( val, rangeList ) )
+    {
+      setValue( i, mNoDataValue );
+    }
+  }
+}
+
 QImage QgsRasterBlock::image() const
 {
   if ( mImage )

src/core/raster/qgsrasterblock.h

@@ -304,6 +304,8 @@ class CORE_EXPORT QgsRasterBlock
 
     inline static void writeValue( void *data, QgsRasterBlock::DataType type, size_t index, double value );
 
+    void applyNodataValues( const QList<Range>& rangeList );
+
   private:
 
     static QImage::Format imageFormat( QgsRasterBlock::DataType theDataType );

src/core/raster/qgsrasterdataprovider.cpp

@@ -194,22 +194,7 @@ QgsRasterBlock * QgsRasterDataProvider::block( int theBandNo, QgsRectangle  cons
 
   // apply user no data values
   // TODO: there are other readBlock methods where no data are not applied
-  QList<QgsRasterBlock::Range> myNoDataRangeList = userNoDataValue( theBandNo );
-  if ( !myNoDataRangeList.isEmpty() )
-  {
-    double myNoDataValue = noDataValue( theBandNo );
-    size_t size = theWidth * theHeight;
-    for ( size_t i = 0; i < size; i++ )
-    {
-      double value = block->value( i );
-
-      if ( QgsRasterBlock::valueInRange( value, myNoDataRangeList ) )
-      {
-        block->setValue( i, myNoDataValue );
-      }
-    }
-  }
-
+  block->applyNodataValues( userNoDataValue( theBandNo ) );
   return block;
 }
 

src/providers/gdal/qgsgdalprovider.cpp

@@ -363,6 +363,19 @@ QImage* QgsGdalProvider::draw( QgsRectangle  const & viewExtent, int pixelWidth,
   return image;
 }
 
+QgsRasterBlock* QgsGdalProvider::block( int theBandNo, const QgsRectangle &theExtent, int theWidth, int theHeight )
+{
+  QgsRasterBlock *block = new QgsRasterBlock( dataType( theBandNo ), theWidth, theHeight, noDataValue( theBandNo ) );
+
+  if ( block->isEmpty() )
+  {
+    return block;
+  }
+
+  readBlock( theBandNo, theExtent, theWidth, theHeight, block->data() );
+  block->applyNodataValues( userNoDataValue( theBandNo ) );
+  return block;
+}
 
 void QgsGdalProvider::readBlock( int theBandNo, int xBlock, int yBlock, void *block )
 {

src/providers/gdal/qgsgdalprovider.h

@@ -166,6 +166,8 @@ class QgsGdalProvider : public QgsRasterDataProvider, QgsGdalProviderBase
     int xSize() const;
     int ySize() const;
 
+    /**Reimplemented from QgsRasterDataProvider to bypass second resampling (more efficient for local file based sources)*/
+    QgsRasterBlock *block( int theBandNo, const QgsRectangle &theExtent, int theWidth, int theHeight );
 
     void readBlock( int bandNo, int xBlock, int yBlock, void *data );
     void readBlock( int bandNo, QgsRectangle  const & viewExtent, int width, int height, void *data );