[FEATURE] New simplify + smoothing expression functions

Exposes simplification and smoothing algorithms to expression
engine, via:

- simplify(): applies Douglas-Peucker geometry simplification
- simplify_vw(): applies Visvalingam-Whyatt geometry simplification
- smooth(): smoothes a geometry

Carto tip: use smooth along with geometry generators to minimise
the typical "GIS" noded look of rendered geometries!

Author: nyalldawson

resources/function_help/json/simplify

@@ -0,0 +1,9 @@
+{
+  "name": "simplify",
+  "type": "function",
+  "description":"Simplifies a geometry by removing nodes using a distance based threshold (ie, the Douglas Peucker algorithm). The algorithm preserves large deviations in geometries and reduces the number of vertices in nearly straight segments.",
+  "arguments": [ {"arg":"geometry","description":"a geometry"},
+                 {"arg":"tolerance","description":"maximum deviation from straight segments for points to be removed"} ],
+  "examples": [ { "expression":"geom_to_wkt(simplify(geometry:=geom_from_wkt('LineString(0 0, 5 0.1, 10 0)'),tolerance:=5))", "returns":"'LineString(0 0, 10 0)'"}]
+}
+

resources/function_help/json/simplify_vw

@@ -0,0 +1,9 @@
+{
+  "name": "simplify_vw",
+  "type": "function",
+  "description":"Simplifies a geometry by removing nodes using an area based threshold (ie, the Visvalingam-Whyatt algorithm). The algorithm removes vertices which create small areas in geometries, eg narrow spikes or nearly straight segments.",
+  "arguments": [ {"arg":"geometry","description":"a geometry"},
+                 {"arg":"tolerance","description":"a measure of the maximum area created by a node for the node to be removed"} ],
+  "examples": [ { "expression":"geom_to_wkt(simplify_vw(geometry:=geom_from_wkt('LineString(0 0, 5 0, 5.01 10, 5.02 0, 10 0)'),tolerance:=5))", "returns":"'LineString(0 0, 10 0)'"}]
+}
+

resources/function_help/json/smooth

@@ -0,0 +1,12 @@
+{
+  "name": "smooth",
+  "type": "function",
+  "description":"Smooths a geometry by adding extra nodes which round off corners in the geometry.",
+  "arguments": [ {"arg":"geometry","description":"a geometry"},
+                 {"arg":"iterations", "optional":true,"description":"number of smoothing iterations to apply. Larger numbers result in smoother but more complex geometries."},
+                 {"arg":"offset", "optional":true,"description":"value between 0 and 0.5 which controls how tightly the smoothed geometry follow the original geometry. Smaller values result in a tighter smoothing, larger values result in looser smoothing."},
+                 {"arg":"min_length", "optional":true,"description":"minimum length of segments to apply smoothing to. This parameter can be used to avoid placing excessive additional nodes in shorter segments of the geometry."},
+                 {"arg":"max_angle", "optional":true,"description":"maximum angle at node for smoothing to be applied (0-180). By lowering the maximum angle intentionally sharp corners in the geometry can be preserved. For instance, a value of 80 degrees will retain right angles in the geometry."} ],
+  "examples": [ { "expression":"geom_to_wkt(smooth(geometry:=geom_from_wkt('LineString(0 0, 5 0, 5 5)'),iterations:=1,offset:=0.2,min_length:=-1,max_angle:=180))", "returns":"'LineString (0 0, 4 0, 5 1, 5 5)'"}]
+}
+

src/core/qgsexpression.cpp

@@ -51,6 +51,7 @@
 #include "qgscurvepolygon.h"
 #include "qgsexpressionprivate.h"
 #include "qgsexpressionsorter.h"
+#include "qgsmaptopixelgeometrysimplifier.h"
 
 #if QT_VERSION < 0x050000
 #include <qtextdocument.h>
@@ -1767,6 +1768,59 @@ static QVariant fcnLineMerge( const QVariantList& values, const QgsExpressionCon
   return QVariant::fromValue( merged );
 }
 
+static QVariant fcnSimplify( const QVariantList& values, const QgsExpressionContext*, QgsExpression* parent )
+{
+  QgsGeometry geom = getGeometry( values.at( 0 ), parent );
+
+  if ( geom.isEmpty() )
+    return QVariant();
+
+  double tolerance = getDoubleValue( values.at( 1 ), parent );
+
+  QgsGeometry simplified = geom.simplify( tolerance );
+  if ( simplified.isEmpty() )
+    return QVariant();
+
+  return simplified;
+}
+
+static QVariant fcnSimplifyVW( const QVariantList& values, const QgsExpressionContext*, QgsExpression* parent )
+{
+  QgsGeometry geom = getGeometry( values.at( 0 ), parent );
+
+  if ( geom.isEmpty() )
+    return QVariant();
+
+  double tolerance = getDoubleValue( values.at( 1 ), parent );
+
+  QgsMapToPixelSimplifier simplifier( QgsMapToPixelSimplifier::SimplifyGeometry, tolerance, QgsMapToPixelSimplifier::Visvalingam );
+
+  QgsGeometry simplified = simplifier.simplify( geom );
+  if ( simplified.isEmpty() )
+    return QVariant();
+
+  return simplified;
+}
+
+static QVariant fcnSmooth( const QVariantList& values, const QgsExpressionContext*, QgsExpression* parent )
+{
+  QgsGeometry geom = getGeometry( values.at( 0 ), parent );
+
+  if ( geom.isEmpty() )
+    return QVariant();
+
+  int iterations = qMin( getIntValue( values.at( 1 ), parent ), 10 );
+  double offset = qBound( 0.0, getDoubleValue( values.at( 2 ), parent ), 0.5 );
+  double minLength = getDoubleValue( values.at( 3 ), parent );
+  double maxAngle = qBound( 0.0, getDoubleValue( values.at( 4 ), parent ), 180.0 );
+
+  QgsGeometry smoothed = geom.smooth( iterations, offset, minLength, maxAngle );
+  if ( smoothed.isEmpty() )
+    return QVariant();
+
+  return smoothed;
+}
+
 static QVariant fcnMakePoint( const QVariantList& values, const QgsExpressionContext*, QgsExpression* parent )
 {
   if ( values.count() < 2 || values.count() > 4 )
@@ -3166,6 +3220,7 @@ const QStringList& QgsExpression::BuiltinFunctions()
     << "disjoint" << "intersects" << "touches" << "crosses" << "contains"
     << "relate"
     << "overlaps" << "within" << "buffer" << "offset_curve" << "single_sided_buffer"
+    << "simplify" << "simplify_vw" << "smooth"
     << "centroid" << "bounds" << "reverse" << "exterior_ring"
     << "boundary" << "line_merge"
     << "bounds_width" << "bounds_height" << "is_closed" << "convex_hull" << "difference"
@@ -3377,6 +3432,12 @@ const QList<QgsExpression::Function*>& QgsExpression::Functions()
     << new StaticFunction( "boundary", ParameterList() << Parameter( "geometry" ), fcnBoundary, "GeometryGroup" )
     << new StaticFunction( "line_merge", ParameterList() << Parameter( "geometry" ), fcnLineMerge, "GeometryGroup" )
     << new StaticFunction( "bounds", 1, fcnBounds, "GeometryGroup" )
+    << new StaticFunction( "simplify", ParameterList() << Parameter( "geometry" ) << Parameter( "tolerance" ), fcnSimplify, "GeometryGroup" )
+    << new StaticFunction( "simplify_vw", ParameterList() << Parameter( "geometry" ) << Parameter( "tolerance" ), fcnSimplifyVW, "GeometryGroup" )
+    << new StaticFunction( "smooth", ParameterList() << Parameter( "geometry" ) << Parameter( "iterations", true, 1 )
+                           << Parameter( "offset", true, 0.25 )
+                           << Parameter( "min_length", true, -1 )
+                           << Parameter( "max_angle", true, 180 ), fcnSmooth, "GeometryGroup" )
     << new StaticFunction( "num_points", 1, fcnGeomNumPoints, "GeometryGroup" )
     << new StaticFunction( "num_interior_rings", 1, fcnGeomNumInteriorRings, "GeometryGroup" )
     << new StaticFunction( "num_rings", 1, fcnGeomNumRings, "GeometryGroup" )

tests/src/core/testqgsexpression.cpp

@@ -798,6 +798,18 @@ class TestQgsExpression: public QObject
       QTest::newRow( "distance_to_vertex null" ) << "distance_to_vertex(NULL, 0)" << false << QVariant();
       QTest::newRow( "distance_to_vertex point" ) << "distance_to_vertex(geom_from_wkt('POINT(1 2)'),0)" << false << QVariant( 0.0 );
       QTest::newRow( "distance_to_vertex line" ) << "distance_to_vertex(geometry:=geom_from_wkt('LineString(0 0, 10 0, 10 10)'),vertex:=1)" << false << QVariant( 10.0 );
+      QTest::newRow( "simplify not geom" ) << "simplify('g',5)" << true << QVariant();
+      QTest::newRow( "simplify null" ) << "simplify(NULL,5)" << false << QVariant();
+      QTest::newRow( "simplify point" ) << "geom_to_wkt(simplify(geom_from_wkt('POINT(1 2)'),10))" << false << QVariant( "Point (1 2)" );
+      QTest::newRow( "simplify line" ) << "geom_to_wkt(simplify(geometry:=geom_from_wkt('LineString(0 0, 5 0, 10 0)'),tolerance:=5))" << false << QVariant( "LineString (0 0, 10 0)" );
+      QTest::newRow( "simplify_vw not geom" ) << "simplify_vw('g',5)" << true << QVariant();
+      QTest::newRow( "simplify_vw null" ) << "simplify_vw(NULL,5)" << false << QVariant();
+      QTest::newRow( "simplify_vw point" ) << "geom_to_wkt(simplify_vw(geom_from_wkt('POINT(1 2)'),10))" << false << QVariant( "Point (1 2)" );
+      QTest::newRow( "simplify_vw line" ) << "geom_to_wkt(simplify_vw(geometry:=geom_from_wkt('LineString(0 0, 5 0, 5.01 10, 5.02 0, 10 0)'),tolerance:=5))" << false << QVariant( "LineString (0 0, 10 0)" );
+      QTest::newRow( "smooth not geom" ) << "smooth('g',5)" << true << QVariant();
+      QTest::newRow( "smooth null" ) << "smooth(NULL,5)" << false << QVariant();
+      QTest::newRow( "smooth point" ) << "geom_to_wkt(smooth(geom_from_wkt('POINT(1 2)'),10))" << false << QVariant( "Point (1 2)" );
+      QTest::newRow( "smooth line" ) << "geom_to_wkt(smooth(geometry:=geom_from_wkt('LineString(0 0, 5 0, 5 5)'),iterations:=1,offset:=0.2,min_length:=-1,max_angle:=180))" << false << QVariant( "LineString (0 0, 4 0, 5 1, 5 5)" );
 
       // string functions
       QTest::newRow( "lower" ) << "lower('HeLLo')" << false << QVariant( "hello" );