Use a more appropriate, global snap to grid algorithm when simplifying

geometries for layout exports

This avoids all chance of slivers or overlaps between previously
adjacent features.

python/core/auto_generated/qgsmaptopixelgeometrysimplifier.sip.in

@@ -28,6 +28,7 @@ This class enables simplify the geometries to be rendered in a MapCanvas target
       Distance,
       SnapToGrid,
       Visvalingam,
+      SnappedToGridGlobal,
     };
 
     QgsMapToPixelSimplifier( int simplifyFlags, double tolerance, SimplifyAlgorithm simplifyAlgorithm = Distance );

python/core/auto_generated/qgsvectorsimplifymethod.sip.in

@@ -53,6 +53,7 @@ Gets the simplification hints of the vector layer managed
       Distance,
       SnapToGrid,
       Visvalingam,
+      SnappedToGridGlobal,
     };
 
     void setSimplifyAlgorithm( SimplifyAlgorithm simplifyAlgorithm );

src/core/layout/qgslayoutexporter.cpp

@@ -1471,8 +1471,9 @@ QgsVectorSimplifyMethod QgsLayoutExporter::createExportSimplifyMethod()
   QgsVectorSimplifyMethod simplifyMethod;
   simplifyMethod.setSimplifyHints( QgsVectorSimplifyMethod::GeometrySimplification );
   simplifyMethod.setForceLocalOptimization( true );
-  simplifyMethod.setSimplifyAlgorithm( QgsVectorSimplifyMethod::SnapToGrid );
-  simplifyMethod.setThreshold( 0.5 ); // pixels
+  // we use SnappedToGridGlobal, because it avoids gaps and slivers between previously adjacent polygons
+  simplifyMethod.setSimplifyAlgorithm( QgsVectorSimplifyMethod::SnappedToGridGlobal );
+  simplifyMethod.setThreshold( 0.1f ); // (pixels). We are quite conservative here. This could possibly be bumped all the way up to 1. But let's play it safe.
   return simplifyMethod;
 }
 

src/core/qgsmaptopixelgeometrysimplifier.cpp

@@ -227,6 +227,12 @@ std::unique_ptr< QgsAbstractGeometry > QgsMapToPixelSimplifier::simplifyGeometry
         break;
       }
 
+      case SnappedToGridGlobal:
+      {
+        output.reset( qgsgeometry_cast< QgsCurve * >( srcCurve.snappedToGrid( map2pixelTol, map2pixelTol ) ) );
+        break;
+      }
+
       case Visvalingam:
       {
         map2pixelTol *= map2pixelTol; //-> Use mappixelTol for 'Area' calculations.

src/core/qgsmaptopixelgeometrysimplifier.h

@@ -44,6 +44,7 @@ class CORE_EXPORT QgsMapToPixelSimplifier : public QgsAbstractGeometrySimplifier
       Distance    = 0, //!< The simplification uses the distance between points to remove duplicate points
       SnapToGrid  = 1, //!< The simplification uses a grid (similar to ST_SnapToGrid) to remove duplicate points
       Visvalingam = 2, //!< The simplification gives each point in a line an importance weighting, so that least important points are removed first
+      SnappedToGridGlobal = 3, //!< Snap to a global grid based on the tolerance. Good for consistent results for incoming vertices, regardless of their feature
     };
 
     //! Constructor

src/core/qgsvectorsimplifymethod.h

@@ -56,6 +56,7 @@ class CORE_EXPORT QgsVectorSimplifyMethod
       Distance    = 0, //!< The simplification uses the distance between points to remove duplicate points
       SnapToGrid  = 1, //!< The simplification uses a grid (similar to ST_SnapToGrid) to remove duplicate points
       Visvalingam = 2, //!< The simplification gives each point in a line an importance weighting, so that least important points are removed first
+      SnappedToGridGlobal = 3, //!< Snap to a global grid based on the tolerance. Good for consistent results for incoming vertices, regardless of their feature
     };
     Q_ENUM( SimplifyAlgorithm )