[FEATURE][composer] Allow choice of CRS for map items

This allows the CRS for map items to differ from the canvas/project
CRS. It also allows different map items to have different CRS,
eg an overview map can be set to a different CRS to the main map.

An unfortunate side effect of this change and the ongoing work
to separate compositions from canvas is that datum transforms
are no longer supported in composer. This cannot be fixed until
the datum transform store is rewritten to not depend on canvas
(ie, it's also broken for upcoming multi-canvas work)

python/core/composer/qgscomposermap.sip

@@ -128,6 +128,7 @@ class QgsComposerMap : QgsComposerItem
     // QgsRectangle* currentMapExtent();
 
     QgsCoordinateReferenceSystem crs() const;
+    void setCrs( const QgsCoordinateReferenceSystem& crs );
 
     PreviewMode previewMode() const;
     void setPreviewMode( PreviewMode m );

src/app/composer/qgscomposermapwidget.cpp

@@ -37,6 +37,7 @@
 #include "qgsmapthemecollection.h"
 #include "qgsmapthemes.h"
 #include "qgisgui.h"
+#include "qgscsexception.h"
 
 #include <QMessageBox>
 
@@ -99,6 +100,8 @@ QgsComposerMapWidget::QgsComposerMapWidget( QgsComposerMap* composerMap )
     connect( mOverviewFrameMapComboBox, SIGNAL( itemChanged( QgsComposerItem* ) ), this, SLOT( overviewMapChanged( QgsComposerItem* ) ) );
   }
 
+  connect( mCrsSelector, &QgsProjectionSelectionWidget::crsChanged, this, &QgsComposerMapWidget::mapCrsChanged );
+
   updateGuiElements();
   loadGridEntries();
   loadOverviewEntries();
@@ -260,6 +263,42 @@ void QgsComposerMapWidget::cleanUpOverviewFrameStyleSelector( QgsPanelWidget* co
   mComposerMap->endCommand();
 }
 
+void QgsComposerMapWidget::mapCrsChanged( const QgsCoordinateReferenceSystem& crs )
+{
+  if ( !mComposerMap )
+  {
+    return;
+  }
+
+  if ( mComposerMap->crs() == crs )
+    return;
+
+  // try to reproject to maintain extent
+  QgsCoordinateReferenceSystem oldCrs = mComposerMap->crs();
+
+  bool updateExtent = false;
+  QgsRectangle newExtent;
+  try
+  {
+    QgsCoordinateTransform xForm( oldCrs, crs );
+    QgsRectangle prevExtent = *mComposerMap->currentMapExtent();
+    newExtent = xForm.transformBoundingBox( prevExtent );
+    updateExtent = true;
+  }
+  catch ( QgsCsException & )
+  {
+    //transform failed, don't update extent
+  }
+
+  mComposerMap->beginCommand( tr( "Map CRS changed" ) );
+  mComposerMap->setCrs( crs );
+  if ( updateExtent )
+    mComposerMap->zoomToExtent( newExtent );
+  mComposerMap->endCommand();
+  mComposerMap->cache();
+  mComposerMap->update();
+}
+
 void QgsComposerMapWidget::on_mAtlasCheckBox_toggled( bool checked )
 {
   if ( !mComposerMap )
@@ -471,6 +510,23 @@ void QgsComposerMapWidget::on_mSetToMapCanvasExtentButton_clicked()
 
   QgsRectangle newExtent = QgisApp::instance()->mapCanvas()->mapSettings().visibleExtent();
 
+  //transform?
+  if ( QgisApp::instance()->mapCanvas()->mapSettings().destinationCrs()
+       != mComposerMap->crs() )
+  {
+    try
+    {
+      QgsCoordinateTransform xForm( QgisApp::instance()->mapCanvas()->mapSettings().destinationCrs(),
+                                    mComposerMap->crs() );
+      newExtent = xForm.transformBoundingBox( newExtent );
+    }
+    catch ( QgsCsException & )
+    {
+      //transform failed, better not proceed
+      return;
+    }
+  }
+
   mComposerMap->beginCommand( tr( "Map extent changed" ) );
   mComposerMap->zoomToExtent( newExtent );
   mComposerMap->endCommand();
@@ -487,6 +543,23 @@ void QgsComposerMapWidget::on_mViewExtentInCanvasButton_clicked()
 
   if ( !currentMapExtent.isEmpty() )
   {
+    //transform?
+    if ( QgisApp::instance()->mapCanvas()->mapSettings().destinationCrs()
+         != mComposerMap->crs() )
+    {
+      try
+      {
+        QgsCoordinateTransform xForm( mComposerMap->crs(),
+                                      QgisApp::instance()->mapCanvas()->mapSettings().destinationCrs() );
+        currentMapExtent = xForm.transformBoundingBox( currentMapExtent );
+      }
+      catch ( QgsCsException & )
+      {
+        //transform failed, better not proceed
+        return;
+      }
+    }
+
     QgisApp::instance()->mapCanvas()->setExtent( currentMapExtent );
     QgisApp::instance()->mapCanvas()->refresh();
   }
@@ -532,6 +605,8 @@ void QgsComposerMapWidget::updateGuiElements()
 
   blockAllSignals( true );
 
+  whileBlocking( mCrsSelector )->setCrs( mComposerMap->crs() );
+
   //width, height, scale
   double scale = mComposerMap->scale();
 

src/app/composer/qgscomposermapwidget.h

@@ -121,6 +121,8 @@ class QgsComposerMapWidget: public QgsComposerItemBaseWidget, private Ui::QgsCom
     void updateOverviewFrameStyleFromWidget();
     void cleanUpOverviewFrameStyleSelector( QgsPanelWidget* container );
 
+    void mapCrsChanged( const QgsCoordinateReferenceSystem& crs );
+
   private:
     QgsComposerMap* mComposerMap;
 

src/core/composer/qgscomposermap.cpp

@@ -124,6 +124,9 @@ QgsComposerMap::QgsComposerMap( QgsComposition *composition )
 
 void QgsComposerMap::init()
 {
+  if ( mComposition && mComposition->project() )
+    mCrs = mComposition->project()->crs();
+
   mGridStack = new QgsComposerMapGridStack( this );
   mOverviewStack = new QgsComposerMapOverviewStack( this );
   connectUpdateSlot();
@@ -205,10 +208,12 @@ QgsMapSettings QgsComposerMap::mapSettings( const QgsRectangle& extent, QSizeF s
   QgsExpressionContext expressionContext = createExpressionContext();
 
   QgsMapSettings jobMapSettings;
+  jobMapSettings.setDestinationCrs( mCrs );
+  jobMapSettings.setCrsTransformEnabled( true );
   jobMapSettings.setExtent( extent );
   jobMapSettings.setOutputSize( size.toSize() );
   jobMapSettings.setOutputDpi( dpi );
-  jobMapSettings.setMapUnits( crs().mapUnits() );
+  jobMapSettings.setMapUnits( mCrs.mapUnits() );
   jobMapSettings.setBackgroundColor( Qt::transparent );
   jobMapSettings.setOutputImageFormat( ms.outputImageFormat() );
   jobMapSettings.setRotation( mEvaluatedMapRotation );
@@ -233,8 +238,6 @@ QgsMapSettings QgsComposerMap::mapSettings( const QgsRectangle& extent, QSizeF s
   }
   jobMapSettings.setLayers( layers );
   jobMapSettings.setLayerStyleOverrides( layerStyleOverridesToRender( expressionContext ) );
-  jobMapSettings.setDestinationCrs( crs() );
-  jobMapSettings.setCrsTransformEnabled( ms.hasCrsTransformEnabled() );
   jobMapSettings.setFlags( ms.flags() );
   jobMapSettings.setFlag( QgsMapSettings::DrawSelection, false );
 
@@ -253,7 +256,7 @@ QgsMapSettings QgsComposerMap::mapSettings( const QgsRectangle& extent, QSizeF s
   jobMapSettings.setFlag( QgsMapSettings::DrawEditingInfo, false );
   jobMapSettings.setFlag( QgsMapSettings::UseAdvancedEffects, mComposition->useAdvancedEffects() ); // respect the composition's useAdvancedEffects flag
 
-  jobMapSettings.datumTransformStore() = ms.datumTransformStore();
+  jobMapSettings.datumTransformStore().setDestinationCrs( mCrs );
 
   return jobMapSettings;
 }
@@ -873,14 +876,12 @@ QgsRectangle* QgsComposerMap::currentMapExtent()
 
 QgsCoordinateReferenceSystem QgsComposerMap::crs() const
 {
-  if ( mComposition )
-  {
-    return mComposition->mapSettings().destinationCrs();
-  }
-  else
-  {
-    return QgsCoordinateReferenceSystem();
-  }
+  return mCrs;
+}
+
+void QgsComposerMap::setCrs( const QgsCoordinateReferenceSystem& crs )
+{
+  mCrs = crs;
 }
 
 const QgsRectangle* QgsComposerMap::currentMapExtent() const
@@ -1273,6 +1274,10 @@ bool QgsComposerMap::writeXml( QDomElement& elem, QDomDocument & doc ) const
   extentElem.setAttribute( QStringLiteral( "ymax" ), qgsDoubleToString( mExtent.yMaximum() ) );
   composerMapElem.appendChild( extentElem );
 
+  QDomElement crsElem = doc.createElement( QStringLiteral( "crs" ) );
+  mCrs.writeXml( crsElem, doc );
+  composerMapElem.appendChild( crsElem );
+
   // follow map theme
   composerMapElem.setAttribute( QStringLiteral( "followPreset" ), mFollowVisibilityPreset ? "true" : "false" );
   composerMapElem.setAttribute( QStringLiteral( "followPresetName" ), mFollowVisibilityPresetName );
@@ -1374,6 +1379,13 @@ bool QgsComposerMap::readXml( const QDomElement& itemElem, const QDomDocument& d
     setNewExtent( QgsRectangle( xmin, ymin, xmax, ymax ) );
   }
 
+  QDomNodeList crsNodeList = itemElem.elementsByTagName( QStringLiteral( "crs" ) );
+  if ( !crsNodeList.isEmpty() )
+  {
+    QDomElement crsElem = crsNodeList.at( 0 ).toElement();
+    mCrs.readXml( crsElem );
+  }
+
   //map rotation
   if ( !qgsDoubleNear( itemElem.attribute( QStringLiteral( "mapRotation" ), QStringLiteral( "0" ) ).toDouble(), 0.0 ) )
   {

src/core/composer/qgscomposermap.h

@@ -170,9 +170,17 @@ class CORE_EXPORT QgsComposerMap : public QgsComposerItem
     /**
      * Returns the map's coordinate reference system.
      * @note added in QGIS 3.0
+     * @see setCrs()
      */
     QgsCoordinateReferenceSystem crs() const;
 
+    /**
+     * Sets the map's coordinate reference system.
+     * @see crs()
+     * @note added in QGIS 3.0
+     */
+    void setCrs( const QgsCoordinateReferenceSystem& crs );
+
     PreviewMode previewMode() const {return mPreviewMode;}
     void setPreviewMode( PreviewMode m );
 
@@ -454,6 +462,9 @@ class CORE_EXPORT QgsComposerMap : public QgsComposerItem
     // so that full rectangle in paper is used.
     QgsRectangle mExtent;
 
+    //! Map CRS
+    QgsCoordinateReferenceSystem mCrs;
+
     // Current temporary map region in map units. This is overwritten when atlas feature changes. It's also
     // used when the user changes the map extent and an atlas preview is enabled. This allows the user
     // to manually tweak each atlas preview page without affecting the actual original map extent.

src/core/composer/qgscomposition.cpp

@@ -2832,6 +2832,12 @@ bool QgsComposition::exportAsPDF( const QString& file )
 void QgsComposition::georeferenceOutput( const QString& file, QgsComposerMap* map,
     const QRectF& exportRegion, double dpi ) const
 {
+  if ( !map )
+    map = referenceMap();
+
+  if ( !map )
+    return; // no reference map
+
   if ( dpi < 0 )
     dpi = printResolution();