Fix layers with layer wide opacity "flashing" by rendering

intermediate states without opacity

(cherry-picked from 19f6b62)

src/core/qgsmaprenderercustompainterjob.cpp

@@ -273,7 +273,9 @@ void QgsMapRendererCustomPainterJob::doRender()
     if ( job.img )
     {
       // If we flattened this layer for alternate blend modes, composite it now
+      mPainter->setOpacity( job.opacity );
       mPainter->drawImage( 0, 0, *job.img );
+      mPainter->setOpacity( 1.0 );
     }
 
   }

src/core/qgsmaprendererjob.cpp

@@ -245,6 +245,11 @@ LayerRenderJobs QgsMapRendererJob::prepareJobs( QPainter* painter, QgsPalLabelin
     job.cached = false;
     job.img = nullptr;
     job.blendMode = ml->blendMode();
+    job.opacity = 1.0;
+    if ( QgsVectorLayer* vl = qobject_cast<QgsVectorLayer *>( ml ) )
+    {
+      job.opacity = 1.0 - vl->layerTransparency() / 100.0;
+    }
     job.layerId = ml->id();
     job.renderingTime = -1;
 
@@ -361,8 +366,10 @@ QImage QgsMapRendererJob::composeImage( const QgsMapSettings& settings, const La
     const LayerRenderJob& job = *it;
 
     painter.setCompositionMode( job.blendMode );
+    painter.setOpacity( job.opacity );
 
     Q_ASSERT( job.img );
+
     painter.drawImage( 0, 0, *job.img );
   }
 

src/core/qgsmaprendererjob.h

@@ -46,6 +46,7 @@ struct LayerRenderJob
   QImage* img; // may be null if it is not necessary to draw to separate image (e.g. sequential rendering)
   QgsMapLayerRenderer* renderer; // must be deleted
   QPainter::CompositionMode blendMode;
+  double opacity;
   bool cached; // if true, img already contains cached image from previous rendering
   QString layerId;
   int renderingTime; //!< time it took to render the layer in ms (it is -1 if not rendered or still rendering)

src/core/qgsvectorlayerrenderer.cpp

@@ -53,7 +53,6 @@ QgsVectorLayerRenderer::QgsVectorLayerRenderer( QgsVectorLayer* layer, QgsRender
     , mDiagrams( false )
     , mLabelProvider( nullptr )
     , mDiagramProvider( nullptr )
-    , mLayerTransparency( 0 )
 {
   mSource = new QgsVectorLayerFeatureSource( layer );
 
@@ -64,7 +63,6 @@ QgsVectorLayerRenderer::QgsVectorLayerRenderer( QgsVectorLayer* layer, QgsRender
 
   mGeometryType = layer->geometryType();
 
-  mLayerTransparency = layer->layerTransparency();
   mFeatureBlendMode = layer->featureBlendMode();
 
   mSimplifyMethod = layer->simplifyMethod();
@@ -262,18 +260,6 @@ bool QgsVectorLayerRenderer::render()
     mRendererV2->paintEffect()->end( mContext );
   }
 
-  //apply layer transparency for vector layers
-  if ( mContext.useAdvancedEffects() && mLayerTransparency != 0 )
-  {
-    // a layer transparency has been set, so update the alpha for the flattened layer
-    // by combining it with the layer transparency
-    QColor transparentFillColor = QColor( 0, 0, 0, 255 - ( 255 * mLayerTransparency / 100 ) );
-    // use destination in composition mode to merge source's alpha with destination
-    mContext.painter()->setCompositionMode( QPainter::CompositionMode_DestinationIn );
-    mContext.painter()->fillRect( 0, 0, mContext.painter()->device()->width(),
-                                  mContext.painter()->device()->height(), transparentFillColor );
-  }
-
   return true;
 }
 

src/core/qgsvectorlayerrenderer.h

@@ -136,7 +136,6 @@ class QgsVectorLayerRenderer : public QgsMapLayerRenderer
     //! may be null. no need to delete: if exists it is owned by labeling engine
     QgsVectorLayerDiagramProvider* mDiagramProvider;
 
-    int mLayerTransparency;
     QPainter::CompositionMode mFeatureBlendMode;
 
     QgsVectorSimplifyMethod mSimplifyMethod;