Try to do automatic choice of the reference layer in GUI (using smallest cell area)

python/analysis/raster/qgsalignraster.sip

@@ -157,5 +157,8 @@ class QgsAlignRaster
     //! write contents of the object to standard error stream - for debugging
     void dump() const;
 
+    //! Return index of the layer which has smallest cell size (returns -1 on error)
+    int suggestedReferenceLayer() const;
+
 };
 

src/analysis/raster/qgsalignraster.cpp

@@ -23,6 +23,7 @@
 #include <QPair>
 #include <QString>
 
+#include "qgscoordinatereferencesystem.h"
 #include "qgsrectangle.h"
 
 
@@ -386,6 +387,36 @@ void QgsAlignRaster::dump() const
   qDebug( "extent %s", e.toString().toAscii().constData() );
 }
 
+int QgsAlignRaster::suggestedReferenceLayer() const
+{
+  int bestIndex = -1;
+  double bestCellArea = qInf();
+  QSizeF cs;
+  int i = 0;
+
+  // using WGS84 as a destination CRS... but maybe some projected CRS
+  // would be a better a choice to more accurately compute areas?
+  // (Why earth is not flat???)
+  QgsCoordinateReferenceSystem destCRS( "EPSG:4326" );
+  QString destWkt = destCRS.toWkt();
+
+  foreach ( const Item& raster, mRasters )
+  {
+    if ( !suggestedWarpOutput( RasterInfo( raster.inputFilename ), destWkt, &cs ) )
+      return false;
+
+    double cellArea = cs.width() * cs.height();
+    if ( cellArea < bestCellArea )
+    {
+      bestCellArea = cellArea;
+      bestIndex = i;
+    }
+    ++i;
+  }
+
+  return bestIndex;
+}
+
 
 bool QgsAlignRaster::createAndWarp( const Item& raster )
 {

src/analysis/raster/qgsalignraster.h

@@ -205,6 +205,9 @@ class ANALYSIS_EXPORT QgsAlignRaster
     //! write contents of the object to standard error stream - for debugging
     void dump() const;
 
+    //! Return index of the layer which has smallest cell size (returns -1 on error)
+    int suggestedReferenceLayer() const;
+
   protected:
 
     //! Internal function for processing of one raster (1. create output, 2. do the alignment)

src/app/qgsalignrasterdialog.cpp

@@ -112,7 +112,10 @@ void QgsAlignRasterDialog::populateLayersView()
 {
   mCboReferenceLayer->clear();
 
+  int refLayerIndex = mAlign->suggestedReferenceLayer();
+
   QStandardItemModel* model = new QStandardItemModel();
+  int i = 0;
   foreach ( QgsAlignRaster::Item item, mAlign->rasters() )
   {
     QString layerName = _rasterLayerName( item.inputFilename );
@@ -120,13 +123,20 @@ void QgsAlignRasterDialog::populateLayersView()
     QStandardItem* si = new QStandardItem( QgsLayerItem::iconRaster(), layerName );
     model->appendRow( si );
 
+    if ( i == refLayerIndex )
+      layerName += tr( " [best reference]" );
+
     mCboReferenceLayer->addItem( layerName );
+    ++i;
   }
 
   mViewLayers->setModel( model );
 
   buttonBox->button( QDialogButtonBox::Ok )->setEnabled( model->rowCount() > 0 );
 
+  if ( refLayerIndex >= 0 )
+    mCboReferenceLayer->setCurrentIndex( refLayerIndex );
+
   updateAlignedRasterInfo();
 }
 

tests/src/analysis/testqgsalignraster.cpp

@@ -252,6 +252,20 @@ class TestAlignRaster : public QObject
       QVERIFY( !res );
     }
 
+    void testSuggestedReferenceLayer()
+    {
+      QgsAlignRaster align;
+
+      QCOMPARE( align.suggestedReferenceLayer(), -1 );
+
+      QgsAlignRaster::List rasters;
+      rasters << QgsAlignRaster::Item( SRC_FILE, QString() );
+      align.setRasters( rasters );
+
+      QCOMPARE( align.suggestedReferenceLayer(), 0 );
+
+    }
+
 };
 
 QTEST_MAIN( TestAlignRaster )