Fix DPI scaling issue with paint effects and composer

python/core/effects/qgspainteffect.sip

@@ -230,6 +230,13 @@ class QgsPaintEffect
      */
     virtual QRectF boundingRect( const QRectF& rect, const QgsRenderContext& context ) const;
 
+    /** Applies a workaround to a QPainter to avoid an issue with incorrect scaling
+     * when drawing QPictures. This may need to be called by derived classes prior
+     * to rendering results onto a painter.
+     * @param painter destination painter
+     */
+    void fixQPictureDpi( QPainter* painter ) const;
+
 };
 
 /** \ingroup core

src/core/effects/qgseffectstack.cpp

@@ -100,7 +100,11 @@ void QgsEffectStack::draw( QgsRenderContext &context )
     QPicture *resultPic = new QPicture();
     QPainter p( resultPic );
     context.setPainter( &p );
+    //effect stack has it's own handling of the QPicture DPI issue, so
+    //we disable QgsPaintEffect's internal workaround
+    effect->requiresQPainterDpiFix = false;
     effect->render( *pic, context );
+    effect->requiresQPainterDpiFix = true;
     p.end();
 
     results << resultPic;
@@ -113,7 +117,6 @@ void QgsEffectStack::draw( QgsRenderContext &context )
   sourcePic = 0;
 
   context.setPainter( destPainter );
-
   //then, we render all the results in the opposite order
   for ( int i = 0; i < mEffectList.count(); ++i )
   {
@@ -125,7 +128,11 @@ void QgsEffectStack::draw( QgsRenderContext &context )
     QPicture* pic = results.takeLast();
     if ( mEffectList[i]->drawMode() != QgsPaintEffect::Modifier )
     {
+      context.painter()->save();
+      fixQPictureDpi( context.painter() );
       context.painter()->drawPicture( 0, 0, *pic );
+      context.painter()->restore();
+
     }
     delete pic;
   }

src/core/effects/qgspainteffect.cpp

@@ -23,19 +23,10 @@
 Q_GUI_EXPORT extern int qt_defaultDpiX();
 Q_GUI_EXPORT extern int qt_defaultDpiY();
 
-static void _fixQPictureDPI( QPainter* p )
-{
-  // QPicture makes an assumption that we drawing to it with system DPI.
-  // Then when being drawn, it scales the painter. The following call
-  // negates the effect. There is no way of setting QPicture's DPI.
-  // See QTBUG-20361
-  p->scale(( double )qt_defaultDpiX() / p->device()->logicalDpiX(),
-           ( double )qt_defaultDpiY() / p->device()->logicalDpiY() );
-}
-
 QgsPaintEffect::QgsPaintEffect()
     : mEnabled( true )
     , mDrawMode( ModifyAndRender )
+    , requiresQPainterDpiFix( true )
     , mPicture( 0 )
     , mSourceImage( 0 )
     , mOwnsImage( false )
@@ -49,6 +40,7 @@ QgsPaintEffect::QgsPaintEffect()
 QgsPaintEffect::QgsPaintEffect( const QgsPaintEffect &other )
     : mEnabled( other.enabled() )
     , mDrawMode( other.drawMode() )
+    , requiresQPainterDpiFix( true )
     , mPicture( 0 )
     , mSourceImage( 0 )
     , mOwnsImage( false )
@@ -180,10 +172,17 @@ void QgsPaintEffect::end( QgsRenderContext &context )
 
 void QgsPaintEffect::drawSource( QPainter &painter )
 {
-  painter.save();
-  _fixQPictureDPI( &painter );
-  painter.drawPicture( 0, 0, *mPicture );
-  painter.restore();
+  if ( requiresQPainterDpiFix )
+  {
+    painter.save();
+    fixQPictureDpi( &painter );
+    painter.drawPicture( 0, 0, *mPicture );
+    painter.restore();
+  }
+  else
+  {
+    painter.drawPicture( 0, 0, *mPicture );
+  }
 }
 
 QImage* QgsPaintEffect::sourceAsImage( QgsRenderContext &context )
@@ -222,6 +221,16 @@ QRectF QgsPaintEffect::boundingRect( const QRectF &rect, const QgsRenderContext
   return rect;
 }
 
+void QgsPaintEffect::fixQPictureDpi( QPainter *painter ) const
+{
+  // QPicture makes an assumption that we drawing to it with system DPI.
+  // Then when being drawn, it scales the painter. The following call
+  // negates the effect. There is no way of setting QPicture's DPI.
+  // See QTBUG-20361
+  painter->scale(( double )qt_defaultDpiX() / painter->device()->logicalDpiX(),
+                 ( double )qt_defaultDpiY() / painter->device()->logicalDpiY() );
+}
+
 QRectF QgsPaintEffect::imageBoundingRect( const QgsRenderContext &context ) const
 {
   return boundingRect( mPicture->boundingRect(), context );

src/core/effects/qgspainteffect.h

@@ -161,6 +161,7 @@ class CORE_EXPORT QgsPaintEffect
 
     bool mEnabled;
     DrawMode mDrawMode;
+    bool requiresQPainterDpiFix;
 
     /** Handles drawing of the effect's result on to the specified render context.
      * Derived classes must reimplement this method to apply any transformations to
@@ -216,6 +217,13 @@ class CORE_EXPORT QgsPaintEffect
      */
     virtual QRectF boundingRect( const QRectF& rect, const QgsRenderContext& context ) const;
 
+    /** Applies a workaround to a QPainter to avoid an issue with incorrect scaling
+     * when drawing QPictures. This may need to be called by derived classes prior
+     * to rendering results onto a painter.
+     * @param painter destination painter
+     */
+    void fixQPictureDpi( QPainter* painter ) const;
+
   private:
 
     const QPicture* mPicture;
@@ -228,6 +236,8 @@ class CORE_EXPORT QgsPaintEffect
 
     QRectF imageBoundingRect( const QgsRenderContext& context ) const;
 
+    friend class QgsEffectStack;
+
 };
 
 /** \ingroup core