Don't cache labeling if blend modes are used

The flattening of the label results to an image blocks
blend modes applying correctly to underlying layers

src/core/qgsmaprenderercustompainterjob.cpp

@@ -83,8 +83,9 @@ void QgsMapRendererCustomPainterJob::start()
     mLabelingEngineV2->setMapSettings( mSettings );
   }
 
+  bool canUseLabelCache = prepareLabelCache();
   mLayerJobs = prepareJobs( mPainter, mLabelingEngineV2 );
-  mLabelJob = prepareLabelingJob( mPainter, mLabelingEngineV2 );
+  mLabelJob = prepareLabelingJob( mPainter, mLabelingEngineV2, canUseLabelCache );
 
   QgsDebugMsg( "Rendering prepared in (seconds): " + QString( "%1" ).arg( prepareTime.elapsed() / 1000.0 ) );
 

src/core/qgsmaprendererjob.cpp

@@ -35,6 +35,7 @@
 #include "qgscsexception.h"
 #include "qgslabelingengine.h"
 #include "qgsmaplayerlistutils.h"
+#include "qgsvectorlayerlabeling.h"
 
 ///@cond PRIVATE
 
@@ -70,6 +71,39 @@ const QgsMapSettings& QgsMapRendererJob::mapSettings() const
   return mSettings;
 }
 
+bool QgsMapRendererJob::prepareLabelCache() const
+{
+  bool canCache = mCache;
+
+  // calculate which layers will be labeled
+  QSet< QgsMapLayer* > labeledLayers;
+  Q_FOREACH ( const QgsMapLayer* ml, mSettings.layers() )
+  {
+    QgsVectorLayer* vl = const_cast< QgsVectorLayer* >( qobject_cast<const QgsVectorLayer *>( ml ) );
+    if ( vl && QgsPalLabeling::staticWillUseLayer( vl ) )
+      labeledLayers << vl;
+    if ( vl->labeling()->requiresAdvancedEffects( vl ) )
+    {
+      canCache = false;
+      break;
+    }
+  }
+
+  if ( mCache && mCache->hasCacheImage( LABEL_CACHE_ID ) )
+  {
+    // we may need to clear label cache and re-register labeled features - check for that here
+
+    // can we reuse the cached label solution?
+    bool canUseCache = canCache && mCache->dependentLayers( LABEL_CACHE_ID ).toSet() == labeledLayers;
+    if ( !canUseCache )
+    {
+      // no - participating layers have changed
+      mCache->clearCacheImage( LABEL_CACHE_ID );
+    }
+  }
+  return canCache;
+}
+
 
 bool QgsMapRendererJob::reprojectToLayerExtent( const QgsMapLayer *ml, const QgsCoordinateTransform& ct, QgsRectangle &extent, QgsRectangle &r2 )
 {
@@ -190,30 +224,7 @@ LayerRenderJobs QgsMapRendererJob::prepareJobs( QPainter* painter, QgsLabelingEn
     QgsDebugMsg( QString( "CACHE VALID: %1" ).arg( cacheValid ) );
   }
 
-  bool hasCachedLabels = false;
-  if ( cacheValid && mCache->hasCacheImage( LABEL_CACHE_ID ) )
-  {
-    // we may need to clear label cache and re-register labeled features - check for that here
-
-    // calculate which layers will be labeled
-    QSet< QgsMapLayer* > labeledLayers;
-    Q_FOREACH ( const QgsMapLayer* ml, mSettings.layers() )
-    {
-      QgsVectorLayer* vl = const_cast< QgsVectorLayer* >( qobject_cast<const QgsVectorLayer *>( ml ) );
-      if ( vl && QgsPalLabeling::staticWillUseLayer( vl ) )
-        labeledLayers << vl;
-    }
-
-    // can we reuse the cached label solution?
-    bool canUseCache = mCache->dependentLayers( LABEL_CACHE_ID ).toSet() == labeledLayers;
-    if ( !canUseCache )
-    {
-      // no - participating layers have changed
-      mCache->clearCacheImage( LABEL_CACHE_ID );
-    }
-
-    hasCachedLabels = canUseCache;
-  }
+  bool requiresLabelRedraw = !( mCache && mCache->hasCacheImage( LABEL_CACHE_ID ) );
 
   mGeometryCaches.clear();
 
@@ -258,9 +269,9 @@ LayerRenderJobs QgsMapRendererJob::prepareJobs( QPainter* painter, QgsLabelingEn
     if ( mCache && ml->type() == QgsMapLayer::VectorLayer )
     {
       QgsVectorLayer* vl = qobject_cast<QgsVectorLayer *>( ml );
-      bool requiresLabelRedraw = false;
-      requiresLabelRedraw = ( labelingEngine2 && QgsPalLabeling::staticWillUseLayer( vl ) ) && !hasCachedLabels;
-      if ( vl->isEditable() || requiresLabelRedraw )
+      bool requiresLabeling = false;
+      requiresLabeling = ( labelingEngine2 && QgsPalLabeling::staticWillUseLayer( vl ) ) && requiresLabelRedraw;
+      if ( vl->isEditable() || requiresLabeling )
       {
         mCache->clearCacheImage( ml->id() );
       }
@@ -345,7 +356,7 @@ LayerRenderJobs QgsMapRendererJob::prepareJobs( QPainter* painter, QgsLabelingEn
   return layerJobs;
 }
 
-LabelRenderJob QgsMapRendererJob::prepareLabelingJob( QPainter* painter, QgsLabelingEngine* labelingEngine2 )
+LabelRenderJob QgsMapRendererJob::prepareLabelingJob( QPainter* painter, QgsLabelingEngine* labelingEngine2, bool canUseLabelCache )
 {
   LabelRenderJob job;
   job.context = QgsRenderContext::fromMapSettings( mSettings );
@@ -354,8 +365,8 @@ LabelRenderJob QgsMapRendererJob::prepareLabelingJob( QPainter* painter, QgsLabe
   job.context.setExtent( mSettings.visibleExtent() );
 
   // if we can use the cache, let's do it and avoid rendering!
-  bool canUseCache = mCache && mCache->hasCacheImage( LABEL_CACHE_ID );
-  if ( canUseCache )
+  bool hasCache = canUseLabelCache && mCache && mCache->hasCacheImage( LABEL_CACHE_ID );
+  if ( hasCache )
   {
     job.cached = true;
     job.complete = true;
@@ -364,7 +375,7 @@ LabelRenderJob QgsMapRendererJob::prepareLabelingJob( QPainter* painter, QgsLabe
   }
   else
   {
-    if ( mCache || !painter )
+    if ( canUseLabelCache && ( mCache || !painter ) )
     {
       // Flattened image for drawing labels
       QImage * mypFlattenedImage = nullptr;

src/core/qgsmaprendererjob.h

@@ -70,6 +70,8 @@ struct LabelRenderJob
   QImage* img = nullptr;
   //! If true, img already contains cached image from previous rendering
   bool cached = false;
+  //! Will be true if labeling is eligible for caching
+  bool canUseCache = false;
   //! If true then label render is complete
   bool complete = false;
   //! Time it took to render the labels in ms (it is -1 if not rendered or still rendering)
@@ -218,6 +220,13 @@ class CORE_EXPORT QgsMapRendererJob : public QObject
 
     int mRenderingTime = 0;
 
+    /**
+     * Prepares the cache for storing the result of labeling. Returns false if
+     * the render cannot use cached labels and should not cache the result.
+     * @note not available in Python bindings
+     */
+    bool prepareLabelCache() const;
+
     //! @note not available in python bindings
     LayerRenderJobs prepareJobs( QPainter* painter, QgsLabelingEngine* labelingEngine2 );
 
@@ -226,7 +235,7 @@ class CORE_EXPORT QgsMapRendererJob : public QObject
      * @note not available in python bindings
      * @note added in QGIS 3.0
      */
-    LabelRenderJob prepareLabelingJob( QPainter* painter, QgsLabelingEngine* labelingEngine2 );
+    LabelRenderJob prepareLabelingJob( QPainter* painter, QgsLabelingEngine* labelingEngine2, bool canUseLabelCache = true );
 
     //! @note not available in python bindings
     static QImage composeImage( const QgsMapSettings& settings, const LayerRenderJobs& jobs, const LabelRenderJob& labelJob );

src/core/qgsmaprendererparalleljob.cpp

@@ -63,8 +63,9 @@ void QgsMapRendererParallelJob::start()
     mLabelingEngineV2->setMapSettings( mSettings );
   }
 
+  bool canUseLabelCache = prepareLabelCache();
   mLayerJobs = prepareJobs( nullptr, mLabelingEngineV2 );
-  mLabelJob = prepareLabelingJob( nullptr, mLabelingEngineV2 );
+  mLabelJob = prepareLabelingJob( nullptr, mLabelingEngineV2, canUseLabelCache );
 
   QgsDebugMsg( QString( "QThreadPool max thread count is %1" ).arg( QThreadPool::globalInstance()->maxThreadCount() ) );
 

src/core/qgsrulebasedlabeling.cpp

@@ -119,6 +119,20 @@ void QgsRuleBasedLabeling::Rule::updateElseRules()
   }
 }
 
+bool QgsRuleBasedLabeling::Rule::requiresAdvancedEffects() const
+{
+  if ( mSettings && mSettings->format().containsAdvancedEffects() )
+    return true;
+
+  Q_FOREACH ( Rule* rule, mChildren )
+  {
+    if ( rule->requiresAdvancedEffects() )
+      return true;
+  }
+
+  return false;
+}
+
 void QgsRuleBasedLabeling::Rule::subProviderIds( QStringList& list ) const
 {
   Q_FOREACH ( const Rule* rule, mChildren )
@@ -437,3 +451,8 @@ QgsPalLayerSettings QgsRuleBasedLabeling::settings( QgsVectorLayer* layer, const
 
   return QgsPalLayerSettings();
 }
+
+bool QgsRuleBasedLabeling::requiresAdvancedEffects( QgsVectorLayer* ) const
+{
+  return mRootRule->requiresAdvancedEffects();
+}

src/core/qgsrulebasedlabeling.h

@@ -260,6 +260,12 @@ class CORE_EXPORT QgsRuleBasedLabeling : public QgsAbstractVectorLayerLabeling
         //! register individual features
         RegisterResult registerFeature( QgsFeature& feature, QgsRenderContext& context, RuleToProviderMap& subProviders, QgsGeometry* obstacleGeometry = nullptr );
 
+        /**
+         * Returns true if this rule or any of its children requires advanced composition effects
+         * to render.
+         */
+        bool requiresAdvancedEffects() const;
+
       protected:
 
         /**
@@ -326,6 +332,7 @@ class CORE_EXPORT QgsRuleBasedLabeling : public QgsAbstractVectorLayerLabeling
     virtual QgsVectorLayerLabelProvider *provider( QgsVectorLayer* layer ) const override;
     virtual QStringList subProviders() const override;
     virtual QgsPalLayerSettings settings( QgsVectorLayer* layer, const QString& providerId = QString() ) const override;
+    bool requiresAdvancedEffects( QgsVectorLayer* layer ) const override;
 
   protected:
     Rule* mRootRule;

src/core/qgsvectorlayerlabeling.cpp

@@ -60,3 +60,8 @@ QgsPalLayerSettings QgsVectorLayerSimpleLabeling::settings( QgsVectorLayer* laye
   else
     return QgsPalLayerSettings();
 }
+
+bool QgsVectorLayerSimpleLabeling::requiresAdvancedEffects( QgsVectorLayer* layer ) const
+{
+  return settings( layer ).format().containsAdvancedEffects();
+}

src/core/qgsvectorlayerlabeling.h

@@ -56,6 +56,14 @@ class CORE_EXPORT QgsAbstractVectorLayerLabeling
     //! they are identified by their ID (e.g. in case of rule-based labeling, provider ID == rule key)
     virtual QgsPalLayerSettings settings( QgsVectorLayer* layer, const QString& providerId = QString() ) const = 0;
 
+    /**
+     * Returns true if drawing labels requires advanced effects like composition
+     * modes, which could prevent it being used as an isolated cached image
+     * or exported to a vector format.
+     * @note added in QGIS 3.0
+     */
+    virtual bool requiresAdvancedEffects( QgsVectorLayer* layer ) const = 0;
+
     // static stuff
 
     //! Try to create instance of an implementation based on the XML data
@@ -79,6 +87,7 @@ class CORE_EXPORT QgsVectorLayerSimpleLabeling : public QgsAbstractVectorLayerLa
     virtual QgsVectorLayerLabelProvider* provider( QgsVectorLayer* layer ) const override;
     virtual QDomElement save( QDomDocument& doc ) const override;
     virtual QgsPalLayerSettings settings( QgsVectorLayer* layer, const QString& providerId = QString() ) const override;
+    bool requiresAdvancedEffects( QgsVectorLayer* layer ) const override;
 };
 
 #endif // QGSVECTORLAYERLABELING_H