diff --git a/src/core/raster/qgsmultibandcolorrenderer.cpp b/src/core/raster/qgsmultibandcolorrenderer.cpp
index 17257c8937b1..8eb08edaa402 100644
--- a/src/core/raster/qgsmultibandcolorrenderer.cpp
+++ b/src/core/raster/qgsmultibandcolorrenderer.cpp
@@ -49,7 +49,7 @@ void QgsMultiBandColorRenderer::draw( QPainter* p, QgsRasterViewPort* viewPort,
     transparencyType = ( QgsRasterDataProvider::DataType )mProvider->dataType( mAlphaBand );
   }
 
-  double oversampling;
+  double oversamplingX, oversamplingY;
   QSet<int> bands;
   bands << mRedBand << mGreenBand << mBlueBand;
   if ( mAlphaBand > 0 )
@@ -63,7 +63,7 @@ void QgsMultiBandColorRenderer::draw( QPainter* p, QgsRasterViewPort* viewPort,
   for ( ; bandIt != bands.constEnd(); ++bandIt )
   {
     bandData.insert( *bandIt, defaultPointer );
-    startRasterRead( *bandIt, viewPort, theQgsMapToPixel, oversampling );
+    startRasterRead( *bandIt, viewPort, theQgsMapToPixel, oversamplingX, oversamplingY );
   }
 
   void* redData;
@@ -135,11 +135,11 @@ void QgsMultiBandColorRenderer::draw( QPainter* p, QgsRasterViewPort* viewPort,
     //draw image
     //top left position in device coords
     QPointF tlPoint = QPointF( viewPort->topLeftPoint.x(), viewPort->topLeftPoint.y() );
-    tlPoint += QPointF( topLeftCol / oversampling, topLeftRow / oversampling );
+    tlPoint += QPointF( topLeftCol / oversamplingX, topLeftRow / oversamplingY );
 
     if ( mResampler ) //resample to output resolution
     {
-      QImage dstImg( nCols / oversampling + 0.5, nRows / oversampling + 0.5, QImage::Format_ARGB32_Premultiplied );
+      QImage dstImg( nCols / oversamplingX, nRows / oversamplingY, QImage::Format_ARGB32_Premultiplied );
       mResampler->resample( img, dstImg );
       p->drawImage( tlPoint, dstImg );
     }
diff --git a/src/core/raster/qgspalettedrasterrenderer.cpp b/src/core/raster/qgspalettedrasterrenderer.cpp
index 7de8c83e98bf..73d2aa40a981 100644
--- a/src/core/raster/qgspalettedrasterrenderer.cpp
+++ b/src/core/raster/qgspalettedrasterrenderer.cpp
@@ -43,18 +43,18 @@ void QgsPalettedRasterRenderer::draw( QPainter* p, QgsRasterViewPort* viewPort,
     return;
   }
 
-  double oversampling;
+  double oversamplingX, oversamplingY;
   QgsRasterDataProvider::DataType transparencyType;
   if ( mAlphaBand > 0 )
   {
     transparencyType = ( QgsRasterDataProvider::DataType )mProvider->dataType( mAlphaBand );
   }
-  startRasterRead( mBandNumber, viewPort, theQgsMapToPixel, oversampling );
+  startRasterRead( mBandNumber, viewPort, theQgsMapToPixel, oversamplingX, oversamplingY );
 
   //Read alpha band if necessary
   if ( mAlphaBand > 0 && mAlphaBand != mBandNumber )
   {
-    startRasterRead( mAlphaBand, viewPort, theQgsMapToPixel, oversampling );
+    startRasterRead( mAlphaBand, viewPort, theQgsMapToPixel, oversamplingX, oversamplingY );
   }
 
   int nCols = 0;
@@ -116,12 +116,12 @@ void QgsPalettedRasterRenderer::draw( QPainter* p, QgsRasterViewPort* viewPort,
 
     //top left position in device coords
     QPointF tlPoint = QPointF( viewPort->topLeftPoint.x(), viewPort->topLeftPoint.y() );
-    tlPoint += QPointF( topLeftCol / oversampling, topLeftRow / oversampling );
+    tlPoint += QPointF( topLeftCol / oversamplingX, topLeftRow / oversamplingY );
 
     //draw image
     if ( mResampler ) //resample to output resolution
     {
-      QImage dstImg( nCols / oversampling, nRows / oversampling, QImage::Format_ARGB32_Premultiplied );
+      QImage dstImg( nCols / oversamplingX, nRows / oversamplingY, QImage::Format_ARGB32_Premultiplied );
       mResampler->resample( img, dstImg );
       p->drawImage( tlPoint, dstImg );
     }
diff --git a/src/core/raster/qgsrasterrenderer.cpp b/src/core/raster/qgsrasterrenderer.cpp
index abccea7c7655..5f0068a3b9a4 100644
--- a/src/core/raster/qgsrasterrenderer.cpp
+++ b/src/core/raster/qgsrasterrenderer.cpp
@@ -21,7 +21,7 @@
 #include "qgsmaptopixel.h"
 
 QgsRasterRenderer::QgsRasterRenderer( QgsRasterDataProvider* provider, QgsRasterResampler* resampler ): mProvider( provider ), mResampler( resampler ),
-  mOpacity( 1.0 ), mRasterTransparency( 0 ), mAlphaBand( -1 ), mInvertColor( false )
+    mOpacity( 1.0 ), mRasterTransparency( 0 ), mAlphaBand( -1 ), mInvertColor( false )
 {
 }
 
@@ -35,9 +35,8 @@ QgsRasterRenderer::~QgsRasterRenderer()
   }
 }
 
-void QgsRasterRenderer::startRasterRead( int bandNumber, QgsRasterViewPort* viewPort, const QgsMapToPixel* mapToPixel, double& oversampling )
+void QgsRasterRenderer::startRasterRead( int bandNumber, QgsRasterViewPort* viewPort, const QgsMapToPixel* mapToPixel, double& oversamplingX, double& oversamplingY )
 {
-  oversampling = 1.0; //default (e.g. for nearest neighbour)
   if ( !viewPort || !mapToPixel || !mProvider )
   {
     return;
@@ -47,6 +46,7 @@ void QgsRasterRenderer::startRasterRead( int bandNumber, QgsRasterViewPort* view
   removePartInfo( bandNumber );
 
   //calculate oversampling factor
+  double oversampling = 1.0; //approximate global oversampling factor
   if ( mResampler )
   {
     QgsRectangle providerExtent = mProvider->extent();
@@ -63,11 +63,16 @@ void QgsRasterRenderer::startRasterRead( int bandNumber, QgsRasterViewPort* view
   RasterPartInfo pInfo;
   pInfo.nCols = viewPort->drawableAreaXDim * oversampling;
   pInfo.nRows = viewPort->drawableAreaYDim * oversampling;
+
+  //effective oversampling factors are different to global one because of rounding
+  oversamplingX = ( double )pInfo.nCols / viewPort->drawableAreaXDim;
+  oversamplingY = ( double )pInfo.nRows / viewPort->drawableAreaYDim;
+
   int totalMemoryUsage = pInfo.nCols * pInfo.nRows * mProvider->dataTypeSize( bandNumber );
-  int parts = totalMemoryUsage / 100000000 /*100000*/ + 1;
+  int parts = totalMemoryUsage / 100000 + 1;
   pInfo.nPartsPerDimension = sqrt( parts );
-  pInfo.nColsPerPart = pInfo.nCols / pInfo.nPartsPerDimension;
-  pInfo.nRowsPerPart = pInfo.nRows / pInfo.nPartsPerDimension;
+  pInfo.nColsPerPart = floor( pInfo.nCols / pInfo.nPartsPerDimension / oversamplingX ) * oversamplingX;
+  pInfo.nRowsPerPart = floor( pInfo.nRows / pInfo.nPartsPerDimension / oversamplingY ) * oversamplingY;
   pInfo.currentCol = 0;
   pInfo.currentRow = 0;
   pInfo.data = 0;
diff --git a/src/core/raster/qgsrasterrenderer.h b/src/core/raster/qgsrasterrenderer.h
index 15289013195c..3f8a679e8721 100644
--- a/src/core/raster/qgsrasterrenderer.h
+++ b/src/core/raster/qgsrasterrenderer.h
@@ -57,13 +57,13 @@ class QgsRasterRenderer
     void setAlphaBand( int band ) { mAlphaBand = band; }
     int alphaBand() const { return mAlphaBand; }
 
-    void setInvertColor( bool invert ){ mInvertColor = invert; }
+    void setInvertColor( bool invert ) { mInvertColor = invert; }
     bool invertColor() const { return mInvertColor; }
 
   protected:
     inline double readValue( void *data, QgsRasterDataProvider::DataType type, int index );
 
-    void startRasterRead( int bandNumber, QgsRasterViewPort* viewPort, const QgsMapToPixel* mapToPixel, double& oversampling );
+    void startRasterRead( int bandNumber, QgsRasterViewPort* viewPort, const QgsMapToPixel* mapToPixel, double& oversamplingX, double& oversamplingY );
     bool readNextRasterPart( int bandNumber, QgsRasterViewPort* viewPort, int& nCols, int& nRows, void** rasterData, int& topLeftCol, int& topLeftRow );
     void stopRasterRead( int bandNumber );