Draw background on painter before rendering map for composer map items (fix #7461)

src/app/composer/qgscomposeritemwidget.cpp

@@ -102,7 +102,8 @@ void QgsComposerItemWidget::on_mBackgroundColorButton_colorChanged( const QColor
 //  QColor newColor( newBackgroundColor );
   mItem->beginCommand( tr( "Background color changed" ) );
 //  newColor.setAlpha( 255 - ( mTransparencySpinBox->value() * 2.55 ) );
-  mItem->setBrush( QBrush( QColor( newBackgroundColor ), Qt::SolidPattern ) );
+  mItem->setBackgroundColor( newBackgroundColor );
+
   //if the item is a composer map, we need to regenerate the map image
   //because it usually is cached
   QgsComposerMap* cm = dynamic_cast<QgsComposerMap *>( mItem );
@@ -248,6 +249,15 @@ void QgsComposerItemWidget::on_mBackgroundGroupBox_toggled( bool state )
 
   mItem->beginCommand( tr( "Item background toggled" ) );
   mItem->setBackgroundEnabled( state );
+
+  //if the item is a composer map, we need to regenerate the map image
+  //because it usually is cached
+  QgsComposerMap* cm = dynamic_cast<QgsComposerMap *>( mItem );
+  if ( cm )
+  {
+    cm->cache();
+  }
+
   mItem->update();
   mItem->endCommand();
 }

src/core/composer/qgscomposeritem.cpp

@@ -51,6 +51,7 @@ QgsComposerItem::QgsComposerItem( QgsComposition* composition, bool manageZValue
     , mVAlignSnapItem( 0 )
     , mFrame( false )
     , mBackground( true )
+    , mBackgroundColor( QColor( 255, 255, 255, 255 ) )
     , mItemPositionLocked( false )
     , mLastValidViewScaleFactor( -1 )
     , mRotation( 0 )
@@ -72,6 +73,7 @@ QgsComposerItem::QgsComposerItem( qreal x, qreal y, qreal width, qreal height, Q
     , mVAlignSnapItem( 0 )
     , mFrame( false )
     , mBackground( true )
+    , mBackgroundColor( QColor( 255, 255, 255, 255 ) )
     , mItemPositionLocked( false )
     , mLastValidViewScaleFactor( -1 )
     , mRotation( 0 )
@@ -326,7 +328,7 @@ bool QgsComposerItem::_readXML( const QDomElement& itemElem, const QDomDocument&
     if ( redOk && greenOk && blueOk && alphaOk )
     {
       QColor brushColor( bgRed, bgGreen, bgBlue, bgAlpha );
-      setBrush( QBrush( brushColor ) );
+      setBackgroundColor( brushColor );
     }
   }
 
@@ -884,6 +886,12 @@ void QgsComposerItem::drawBackground( QPainter* p )
   }
 }
 
+void QgsComposerItem::setBackgroundColor( const QColor& backgroundColor )
+{
+  mBackgroundColor = backgroundColor;
+  setBrush( QBrush( mBackgroundColor, Qt::SolidPattern ) );
+}
+
 void QgsComposerItem::setBlendMode( QPainter::CompositionMode blendMode )
 {
   mBlendMode = blendMode;

src/core/composer/qgscomposeritem.h

@@ -197,6 +197,19 @@ class CORE_EXPORT QgsComposerItem: public QObject, public QGraphicsRectItem
      */
     void setBackgroundEnabled( bool drawBackground ) {mBackground = drawBackground;}
 
+    /** Gets the background color for this item
+     * @returns background color
+     * @note introduced in 2.0
+     */
+    QColor backgroundColor() const { return mBackgroundColor; }
+
+    /** Sets the background color for this item
+     * @param backgroundColor new background color
+     * @returns nothing
+     * @note introduced in 2.0
+     */
+    void setBackgroundColor( const QColor& backgroundColor );
+
     /** Returns the item's composition blending mode */
     QPainter::CompositionMode blendMode() const {return mBlendMode;}
 
@@ -309,6 +322,8 @@ class CORE_EXPORT QgsComposerItem: public QObject, public QGraphicsRectItem
     bool mFrame;
     /**True if item background needs to be painted*/
     bool mBackground;
+    /**Background color*/
+    QColor mBackgroundColor;
 
     /**True if item position  and size cannot be changed with mouse move
     @note: this member was added in version 1.2*/

src/core/composer/qgscomposermap.cpp

@@ -228,7 +228,18 @@ void QgsComposerMap::cache( void )
   double forcedWidthScaleFactor = w / requestExtent.width() / mapUnitsToMM();
 
   mCacheImage = QImage( w, h,  QImage::Format_ARGB32 );
-  mCacheImage.fill( QColor( 255, 255, 255, 0 ).rgba() ); // the background is drawn by composerItem, but we still need to start with that empty fill to avoid artifacts
+
+  if ( hasBackground() )
+  {
+    //Initially fill image with specified background color. This ensures that layers with blend modes will
+    //preview correctly
+    mCacheImage.fill( backgroundColor().rgba() );
+  }
+  else
+  {
+    //no background, but start with empty fill to avoid artifacts
+    mCacheImage.fill( QColor( 255, 255, 255, 0 ).rgba() );
+  }
 
   double mapUnitsPerPixel = mExtent.width() / w;
 
@@ -257,10 +268,10 @@ void QgsComposerMap::paint( QPainter* painter, const QStyleOptionGraphicsItem* i
   painter->save();
   painter->setClipRect( thisPaintRect );
 
-  drawBackground( painter );
-
   if ( mComposition->plotStyle() == QgsComposition::Preview && mPreviewMode == Rectangle )
   {
+    // Fill with background color
+    drawBackground( painter );
     QFont messageFont( "", 12 );
     painter->setFont( messageFont );
     painter->setPen( QColor( 0, 0, 0 ) );
@@ -272,6 +283,8 @@ void QgsComposerMap::paint( QPainter* painter, const QStyleOptionGraphicsItem* i
     //Qt 4.4.0 and 4.4.1 have problems with recursive paintings
     //QgsComposerMap::cache() and QgsComposerMap::update() need to be called by
     //client functions
+    
+    //Background color is already included in cached image, so no need to draw
 
     QgsRectangle requestRectangle;
     requestedExtent( requestRectangle );
@@ -323,6 +336,9 @@ void QgsComposerMap::paint( QPainter* painter, const QStyleOptionGraphicsItem* i
       return;
     }
 
+    // Fill with background color
+    drawBackground( painter );
+
     QgsRectangle requestRectangle;
     requestedExtent( requestRectangle );