WMTS preview - require a temporary image for updates of the raster

Otherwise we would end up with rendering artifacts due to overpainting

Author: wonder-sk

src/core/qgsmaprenderercustompainterjob.cpp

@@ -333,6 +333,12 @@ bool QgsMapRendererJob::needTemporaryImage( QgsMapLayer* ml )
       return true;
     }
   }
+  else if ( ml->type() == QgsMapLayer::RasterLayer )
+  {
+    // preview of intermediate raster rendering results requires a temporary output image
+    if ( mSettings.testFlag( QgsMapSettings::RenderPartialOutput ) )
+      return true;
+  }
 
   return false;
 }

src/core/qgsmapsettings.h

@@ -164,7 +164,8 @@ class CORE_EXPORT QgsMapSettings
       UseRenderingOptimization = 0x20,  //!< Enable vector simplification and other rendering optimizations
       DrawSelection            = 0x40,  //!< Whether vector selections should be shown in the rendered map
       DrawSymbolBounds         = 0x80,  //!< Draw bounds of symbols (for debugging/testing)
-      RenderMapTile            = 0x100  //!< Draw map such that there are no problems between adjacent tiles
+      RenderMapTile            = 0x100, //!< Draw map such that there are no problems between adjacent tiles
+      RenderPartialOutput      = 0x200, //!< Whether to make extra effort to update map image with partially rendered layers (better for interactive map canvas). Added in QGIS 3.0
       // TODO: ignore scale-based visibility (overview)
     };
     Q_DECLARE_FLAGS( Flags, Flag )

src/core/qgsrendercontext.cpp

@@ -129,6 +129,7 @@ QgsRenderContext QgsRenderContext::fromMapSettings( const QgsMapSettings& mapSet
   ctx.setFlag( DrawSymbolBounds, mapSettings.testFlag( QgsMapSettings::DrawSymbolBounds ) );
   ctx.setFlag( RenderMapTile, mapSettings.testFlag( QgsMapSettings::RenderMapTile ) );
   ctx.setFlag( Antialiasing, mapSettings.testFlag( QgsMapSettings::Antialiasing ) );
+  ctx.setFlag( RenderPartialOutput, mapSettings.testFlag( QgsMapSettings::RenderPartialOutput ) );
   ctx.setRasterScaleFactor( 1.0 );
   ctx.setScaleFactor( mapSettings.outputDpi() / 25.4 ); // = pixels per mm
   ctx.setRendererScale( mapSettings.scale() );

src/core/qgsrendercontext.h

@@ -65,6 +65,7 @@ class CORE_EXPORT QgsRenderContext
       DrawSymbolBounds         = 0x20,  //!< Draw bounds of symbols (for debugging/testing)
       RenderMapTile            = 0x40,  //!< Draw map such that there are no problems between adjacent tiles
       Antialiasing             = 0x80,  //!< Use antialiasing while drawing
+      RenderPartialOutput      = 0x100, //!< Whether to make extra effort to update map image with partially rendered layers (better for interactive map canvas). Added in QGIS 3.0
     };
     Q_DECLARE_FLAGS( Flags, Flag )
 

src/core/raster/qgsrasterdrawer.cpp

@@ -88,10 +88,20 @@ void QgsRasterDrawer::draw( QPainter* p, QgsRasterViewPort* viewPort, const QgsM
       }
     }
 
+    if ( feedback && feedback->render_partial_output )
+    {
+      // there could have been partial preview written before
+      // so overwrite anything with the resulting image.
+      // (we are guaranteed to have a temporary image for this layer, see QgsMapRendererJob::needTemporaryImage)
+      p->setCompositionMode( QPainter::CompositionMode_Source );
+    }
+
     drawImage( p, viewPort, img, topLeftCol, topLeftRow, theQgsMapToPixel );
 
     delete block;
 
+    p->setCompositionMode( QPainter::CompositionMode_SourceOver );  // go back to the default composition mode
+
     // ok this does not matter much anyway as the tile size quite big so most of the time
     // there would be just one tile for the whole display area, but it won't hurt...
     if ( feedback && feedback->isCancelled() )

src/core/raster/qgsrasterinterface.h

@@ -37,12 +37,15 @@
 class CORE_EXPORT QgsRasterBlockFeedback : public QgsFeedback
 {
   public:
-    QgsRasterBlockFeedback( QObject* parent = nullptr ) : QgsFeedback( parent ), preview_only( false ) {}
+    QgsRasterBlockFeedback( QObject* parent = nullptr ) : QgsFeedback( parent ), preview_only( false ), render_partial_output( false ) {}
 
     //! whether the raster provider should return only data that are already available
     //! without waiting for full result
     bool preview_only;
 
+    //! whether our painter is drawing to a temporary image used just by this layer
+    bool render_partial_output;
+
     //! may be emitted by raster data provider to indicate that some partial data are available
     //! and a new preview image may be produced
     virtual void onNewData() {}

src/core/raster/qgsrasterlayerrenderer.cpp

@@ -231,22 +231,29 @@ MyFeedback::MyFeedback( QgsRasterLayerRenderer *r )
     : mR( r )
     , mMinimalPreviewInterval( 250 )
 {
+  render_partial_output = r->mContext.testFlag( QgsRenderContext::RenderPartialOutput );
 }
 
 void MyFeedback::onNewData()
 {
   qDebug( "\nGOT NEW DATA!\n" );
 
+  if ( !render_partial_output )
+    return;  // we were not asked for partial renders and we may not have a temporary image for overwriting...
+
   // update only once upon a time
   // (preview itself takes some time)
   if ( mLastPreview.msecsTo( QTime::currentTime() ) < mMinimalPreviewInterval )
     return;
 
+  // TODO: update only the area that got new data
+
   qDebug( "new raster preview! %d", mLastPreview.msecsTo( QTime::currentTime() ) );
   QTime t;
   t.start();
   QgsRasterBlockFeedback feedback;
   feedback.preview_only = true;
+  feedback.render_partial_output = true;
   QgsRasterIterator iterator( mR->mPipe->last() );
   QgsRasterDrawer drawer( &iterator );
   drawer.draw( mR->mPainter, mR->mRasterViewPort, mR->mMapToPixel, &feedback );

src/gui/qgsmapcanvas.cpp

@@ -143,6 +143,7 @@ QgsMapCanvas::QgsMapCanvas( QWidget * parent )
 
   mSettings.setFlag( QgsMapSettings::DrawEditingInfo );
   mSettings.setFlag( QgsMapSettings::UseRenderingOptimization );
+  mSettings.setFlag( QgsMapSettings::RenderPartialOutput );
 
   //segmentation parameters
   QSettings settings;