[FEATURE] Add new variable @geometry_ring_num for data defined

styling when rendering polygon rings

The variable is available whenever a polygon outline is being
rendered (e.g. as a simple line, marker line, etc). It will
be set to 0 for the exterior ring, and 1, 2, 3... for interior
rings.

python/core/auto_generated/qgsexpressioncontext.sip.in

@@ -791,6 +791,8 @@ Clears all cached values from the context.
     static const QString EXPR_SYMBOL_ANGLE;
     static const QString EXPR_GEOMETRY_PART_COUNT;
     static const QString EXPR_GEOMETRY_PART_NUM;
+
+    static const QString EXPR_GEOMETRY_RING_NUM;
     static const QString EXPR_GEOMETRY_POINT_COUNT;
     static const QString EXPR_GEOMETRY_POINT_NUM;
     static const QString EXPR_CLUSTER_SIZE;

src/core/expression/qgsexpression.cpp

@@ -842,6 +842,7 @@ void QgsExpression::initVariableHelp()
   //symbol variables
   sVariableHelpTexts()->insert( QStringLiteral( "geometry_part_count" ), QCoreApplication::translate( "variable_help", "Number of parts in rendered feature's geometry." ) );
   sVariableHelpTexts()->insert( QStringLiteral( "geometry_part_num" ), QCoreApplication::translate( "variable_help", "Current geometry part number for feature being rendered." ) );
+  sVariableHelpTexts()->insert( QStringLiteral( "geometry_ring_num" ), QCoreApplication::translate( "variable_help", "Current geometry ring number for feature being rendered (for polygon features only). The exterior ring has corresponds to a value of 0." ) );
   sVariableHelpTexts()->insert( QStringLiteral( "geometry_point_count" ), QCoreApplication::translate( "variable_help", "Number of points in the rendered geometry's part. It is only meaningful for line geometries and for symbol layers that set this variable." ) );
   sVariableHelpTexts()->insert( QStringLiteral( "geometry_point_num" ), QCoreApplication::translate( "variable_help", "Current point number in the rendered geometry's part. It is only meaningful for line geometries and for symbol layers that set this variable." ) );
 

src/core/qgsexpressioncontext.cpp

@@ -24,6 +24,7 @@ const QString QgsExpressionContext::EXPR_SYMBOL_COLOR( QStringLiteral( "symbol_c
 const QString QgsExpressionContext::EXPR_SYMBOL_ANGLE( QStringLiteral( "symbol_angle" ) );
 const QString QgsExpressionContext::EXPR_GEOMETRY_PART_COUNT( QStringLiteral( "geometry_part_count" ) );
 const QString QgsExpressionContext::EXPR_GEOMETRY_PART_NUM( QStringLiteral( "geometry_part_num" ) );
+const QString QgsExpressionContext::EXPR_GEOMETRY_RING_NUM( QStringLiteral( "geometry_ring_num" ) );
 const QString QgsExpressionContext::EXPR_GEOMETRY_POINT_COUNT( QStringLiteral( "geometry_point_count" ) );
 const QString QgsExpressionContext::EXPR_GEOMETRY_POINT_NUM( QStringLiteral( "geometry_point_num" ) );
 const QString QgsExpressionContext::EXPR_CLUSTER_SIZE( QStringLiteral( "cluster_size" ) );

src/core/qgsexpressioncontext.h

@@ -727,6 +727,12 @@ class CORE_EXPORT QgsExpressionContext
     static const QString EXPR_GEOMETRY_PART_COUNT;
     //! Inbuilt variable name for geometry part number variable
     static const QString EXPR_GEOMETRY_PART_NUM;
+
+    /**
+     * Inbuilt variable name for geometry ring number variable.
+     * \since QGIS 3.20
+     */
+    static const QString EXPR_GEOMETRY_RING_NUM;
     //! Inbuilt variable name for point count variable
     static const QString EXPR_GEOMETRY_POINT_COUNT;
     //! Inbuilt variable name for point number variable

src/core/symbology/qgslinesymbollayer.cpp

@@ -279,6 +279,9 @@ void QgsSimpleLineSymbolLayer::renderPolygonStroke( const QPolygonF &points, con
     return;
   }
 
+  QgsExpressionContextScope *scope = new QgsExpressionContextScope();
+  QgsExpressionContextScopePopper scopePopper( context.renderContext().expressionContext(), scope );
+
   if ( mDrawInsidePolygon )
     p->save();
 
@@ -307,6 +310,8 @@ void QgsSimpleLineSymbolLayer::renderPolygonStroke( const QPolygonF &points, con
         p->setClipPath( clipPath, Qt::IntersectClip );
       }
 
+      scope->addVariable( QgsExpressionContextScope::StaticVariable( QgsExpressionContext::EXPR_GEOMETRY_RING_NUM, 0, true ) );
+
       renderPolyline( points, context );
     }
     break;
@@ -323,8 +328,14 @@ void QgsSimpleLineSymbolLayer::renderPolygonStroke( const QPolygonF &points, con
       case InteriorRingsOnly:
       {
         mOffset = -mOffset; // invert the offset for rings!
+        int ringIndex = 1;
         for ( const QPolygonF &ring : std::as_const( *rings ) )
+        {
+          scope->addVariable( QgsExpressionContextScope::StaticVariable( QgsExpressionContext::EXPR_GEOMETRY_RING_NUM, ringIndex, true ) );
+
           renderPolyline( ring, context );
+          ringIndex++;
+        }
         mOffset = -mOffset;
       }
       break;
@@ -1310,12 +1321,19 @@ void QgsTemplatedLineSymbolLayerBase::renderPolygonStroke( const QPolygonF &poin
     context.renderContext().setGeometry( curvePolygon->exteriorRing() );
   }
 
+  QgsExpressionContextScope *scope = new QgsExpressionContextScope();
+  QgsExpressionContextScopePopper scopePopper( context.renderContext().expressionContext(), scope );
+
   switch ( mRingFilter )
   {
     case AllRings:
     case ExteriorRingOnly:
+    {
+      scope->addVariable( QgsExpressionContextScope::StaticVariable( QgsExpressionContext::EXPR_GEOMETRY_RING_NUM, 0, true ) );
+
       renderPolyline( points, context );
       break;
+    }
     case InteriorRingsOnly:
       break;
   }
@@ -1334,6 +1352,8 @@ void QgsTemplatedLineSymbolLayerBase::renderPolygonStroke( const QPolygonF &poin
           {
             context.renderContext().setGeometry( curvePolygon->interiorRing( i ) );
           }
+          scope->addVariable( QgsExpressionContextScope::StaticVariable( QgsExpressionContext::EXPR_GEOMETRY_RING_NUM, i + 1, true ) );
+
           renderPolyline( rings->at( i ), context );
         }
         mOffset = -mOffset;

src/core/symbology/qgssymbollayer.cpp

@@ -30,6 +30,7 @@
 #include "qgsmultipoint.h"
 #include "qgslegendpatchshape.h"
 #include "qgsstyle.h"
+#include "qgsexpressioncontextutils.h"
 
 #include <QSize>
 #include <QPainter>
@@ -691,12 +692,18 @@ void QgsLineSymbolLayer::drawPreviewIcon( QgsSymbolRenderContext &context, QSize
 
 void QgsLineSymbolLayer::renderPolygonStroke( const QPolygonF &points, const QVector<QPolygonF> *rings, QgsSymbolRenderContext &context )
 {
+  QgsExpressionContextScope *scope = new QgsExpressionContextScope();
+  QgsExpressionContextScopePopper scopePopper( context.renderContext().expressionContext(), scope );
+
   switch ( mRingFilter )
   {
     case AllRings:
     case ExteriorRingOnly:
+    {
+      scope->addVariable( QgsExpressionContextScope::StaticVariable( QgsExpressionContext::EXPR_GEOMETRY_RING_NUM, 0, true ) );
       renderPolyline( points, context );
       break;
+    }
     case InteriorRingsOnly:
       break;
   }
@@ -708,8 +715,14 @@ void QgsLineSymbolLayer::renderPolygonStroke( const QPolygonF &points, const QVe
       case AllRings:
       case InteriorRingsOnly:
       {
+        int ringIndex = 1;
         for ( const QPolygonF &ring : std::as_const( *rings ) )
+        {
+          scope->addVariable( QgsExpressionContextScope::StaticVariable( QgsExpressionContext::EXPR_GEOMETRY_RING_NUM, ringIndex, true ) );
+
           renderPolyline( ring, context );
+          ringIndex++;
+        }
       }
       break;
       case ExteriorRingOnly:

src/gui/symbology/qgslayerpropertieswidget.cpp

@@ -264,6 +264,7 @@ QgsExpressionContext QgsLayerPropertiesWidget::createExpressionContext() const
   expContext << symbolScope;
   expContext.lastScope()->addVariable( QgsExpressionContextScope::StaticVariable( QgsExpressionContext::EXPR_GEOMETRY_PART_COUNT, 1, true ) );
   expContext.lastScope()->addVariable( QgsExpressionContextScope::StaticVariable( QgsExpressionContext::EXPR_GEOMETRY_PART_NUM, 1, true ) );
+  expContext.lastScope()->addVariable( QgsExpressionContextScope::StaticVariable( QgsExpressionContext::EXPR_GEOMETRY_RING_NUM, 1, true ) );
   expContext.lastScope()->addVariable( QgsExpressionContextScope::StaticVariable( QgsExpressionContext::EXPR_GEOMETRY_POINT_COUNT, 1, true ) );
   expContext.lastScope()->addVariable( QgsExpressionContextScope::StaticVariable( QgsExpressionContext::EXPR_GEOMETRY_POINT_NUM, 1, true ) );
   expContext.lastScope()->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "symbol_layer_count" ), 1, true ) );
@@ -283,6 +284,7 @@ QgsExpressionContext QgsLayerPropertiesWidget::createExpressionContext() const
 
   expContext.setHighlightedVariables( QStringList() << QgsExpressionContext::EXPR_ORIGINAL_VALUE << QgsExpressionContext::EXPR_SYMBOL_COLOR
                                       << QgsExpressionContext::EXPR_GEOMETRY_PART_COUNT << QgsExpressionContext::EXPR_GEOMETRY_PART_NUM
+                                      << QgsExpressionContext::EXPR_GEOMETRY_RING_NUM
                                       << QgsExpressionContext::EXPR_GEOMETRY_POINT_COUNT << QgsExpressionContext::EXPR_GEOMETRY_POINT_NUM
                                       << QgsExpressionContext::EXPR_CLUSTER_COLOR << QgsExpressionContext::EXPR_CLUSTER_SIZE
                                       << QStringLiteral( "symbol_layer_count" ) << QStringLiteral( "symbol_layer_index" ) );

src/gui/symbology/qgssymbollayerwidget.cpp

@@ -79,6 +79,7 @@ QgsExpressionContext QgsSymbolLayerWidget::createExpressionContext() const
   expContext << symbolScope;
   expContext.lastScope()->addVariable( QgsExpressionContextScope::StaticVariable( QgsExpressionContext::EXPR_GEOMETRY_PART_COUNT, 1, true ) );
   expContext.lastScope()->addVariable( QgsExpressionContextScope::StaticVariable( QgsExpressionContext::EXPR_GEOMETRY_PART_NUM, 1, true ) );
+  expContext.lastScope()->addVariable( QgsExpressionContextScope::StaticVariable( QgsExpressionContext::EXPR_GEOMETRY_RING_NUM, 1, true ) );
   expContext.lastScope()->addVariable( QgsExpressionContextScope::StaticVariable( QgsExpressionContext::EXPR_GEOMETRY_POINT_COUNT, 1, true ) );
   expContext.lastScope()->addVariable( QgsExpressionContextScope::StaticVariable( QgsExpressionContext::EXPR_GEOMETRY_POINT_NUM, 1, true ) );
   expContext.lastScope()->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "symbol_layer_count" ), 1, true ) );
@@ -99,6 +100,7 @@ QgsExpressionContext QgsSymbolLayerWidget::createExpressionContext() const
   QStringList highlights;
   highlights << QgsExpressionContext::EXPR_ORIGINAL_VALUE << QgsExpressionContext::EXPR_SYMBOL_COLOR
              << QgsExpressionContext::EXPR_GEOMETRY_PART_COUNT << QgsExpressionContext::EXPR_GEOMETRY_PART_NUM
+             << QgsExpressionContext::EXPR_GEOMETRY_RING_NUM
              << QgsExpressionContext::EXPR_GEOMETRY_POINT_COUNT << QgsExpressionContext::EXPR_GEOMETRY_POINT_NUM
              << QgsExpressionContext::EXPR_CLUSTER_COLOR << QgsExpressionContext::EXPR_CLUSTER_SIZE
              << QStringLiteral( "symbol_layer_count" ) << QStringLiteral( "symbol_layer_index" );

src/gui/symbology/qgssymbolslistwidget.cpp

@@ -467,6 +467,7 @@ QgsExpressionContext QgsSymbolsListWidget::createExpressionContext() const
 
   expContext.setHighlightedVariables( QStringList() << QgsExpressionContext::EXPR_ORIGINAL_VALUE << QgsExpressionContext::EXPR_SYMBOL_COLOR
                                       << QgsExpressionContext::EXPR_GEOMETRY_PART_COUNT << QgsExpressionContext::EXPR_GEOMETRY_PART_NUM
+                                      << QgsExpressionContext::EXPR_GEOMETRY_RING_NUM
                                       << QgsExpressionContext::EXPR_GEOMETRY_POINT_COUNT << QgsExpressionContext::EXPR_GEOMETRY_POINT_NUM
                                       << QgsExpressionContext::EXPR_CLUSTER_COLOR << QgsExpressionContext::EXPR_CLUSTER_SIZE
                                       << QStringLiteral( "symbol_layer_count" ) << QStringLiteral( "symbol_layer_index" ) );

tests/src/python/test_qgsarrowsymbollayer.py

@@ -26,7 +26,7 @@
 import os
 
 from qgis.PyQt.QtCore import QSize, QDir
-from qgis.PyQt.QtGui import QColor
+from qgis.PyQt.QtGui import QColor, QPainter, QImage
 
 from qgis.core import (
     QgsVectorLayer,
@@ -40,7 +40,11 @@
     QgsProperty,
     QgsSymbolLayer,
     QgsMapSettings,
-    QgsSymbol
+    QgsSymbol,
+    QgsGeometry,
+    QgsFeature,
+    QgsRenderContext,
+    QgsRenderChecker
 )
 
 from qgis.testing import start_app, unittest
@@ -167,6 +171,20 @@ def testColors(self):
         self.assertEqual(sym_layer.subSymbol().color(), QColor(250, 150, 200))
         self.assertEqual(sym_layer.color(), QColor(250, 150, 200))
 
+    def testRingNumberVariable(self):
+        # test test geometry_ring_num variable
+        s3 = QgsFillSymbol()
+        s3.deleteSymbolLayer(0)
+        s3.appendSymbolLayer(
+            QgsArrowSymbolLayer())
+        s3.symbolLayer(0).setIsCurved(False)
+        s3.symbolLayer(0).subSymbol()[0].setDataDefinedProperty(QgsSymbolLayer.PropertyFillColor,
+                                                                QgsProperty.fromExpression('case when @geometry_ring_num=0 then \'green\' when @geometry_ring_num=1 then \'blue\' when @geometry_ring_num=2 then \'red\' end'))
+
+        g = QgsGeometry.fromWkt('Polygon((0 0, 10 0, 10 10, 0 10, 0 0),(1 1, 1 2, 2 2, 2 1, 1 1),(8 8, 9 8, 9 9, 8 9, 8 8))')
+        rendered_image = self.renderGeometry(s3, g)
+        assert self.imageCheck('arrow_ring_num', 'arrow_ring_num', rendered_image)
+
     def testOpacityWithDataDefinedColor(self):
         line_shp = os.path.join(TEST_DATA_DIR, 'lines.shp')
         line_layer = QgsVectorLayer(line_shp, 'Lines', 'ogr')
@@ -239,6 +257,53 @@ def testDataDefinedOpacity(self):
         self.report += renderchecker.report()
         self.assertTrue(res)
 
+    def renderGeometry(self, symbol, geom):
+        f = QgsFeature()
+        f.setGeometry(geom)
+
+        image = QImage(200, 200, QImage.Format_RGB32)
+
+        painter = QPainter()
+        ms = QgsMapSettings()
+        extent = geom.get().boundingBox()
+        # buffer extent by 10%
+        if extent.width() > 0:
+            extent = extent.buffered((extent.height() + extent.width()) / 20.0)
+        else:
+            extent = extent.buffered(10)
+
+        ms.setExtent(extent)
+        ms.setOutputSize(image.size())
+        context = QgsRenderContext.fromMapSettings(ms)
+        context.setPainter(painter)
+        context.setScaleFactor(96 / 25.4)  # 96 DPI
+
+        painter.begin(image)
+        try:
+            image.fill(QColor(0, 0, 0))
+            symbol.startRender(context)
+            symbol.renderFeature(f, context)
+            symbol.stopRender(context)
+        finally:
+            painter.end()
+
+        return image
+
+    def imageCheck(self, name, reference_image, image):
+        self.report += "<h2>Render {}</h2>\n".format(name)
+        temp_dir = QDir.tempPath() + '/'
+        file_name = temp_dir + 'symbol_' + name + ".png"
+        image.save(file_name, "PNG")
+        checker = QgsRenderChecker()
+        checker.setControlPathPrefix("symbol_arrow")
+        checker.setControlName("expected_" + reference_image)
+        checker.setRenderedImage(file_name)
+        checker.setColorTolerance(2)
+        result = checker.compareImages(name, 20)
+        self.report += checker.report()
+        print((self.report))
+        return result
+
 
 if __name__ == '__main__':
     unittest.main()

tests/src/python/test_qgsmarkerlinesymbollayer.py

@@ -151,6 +151,20 @@ def testRingFilter(self):
         rendered_image = self.renderGeometry(s3, g)
         assert self.imageCheck('markerline_interioronly', 'markerline_interioronly', rendered_image)
 
+    def testRingNumberVariable(self):
+        # test test geometry_ring_num variable
+        s3 = QgsFillSymbol()
+        s3.deleteSymbolLayer(0)
+        s3.appendSymbolLayer(
+            QgsMarkerLineSymbolLayer())
+        s3.symbolLayer(0).subSymbol()[0].setDataDefinedProperty(QgsSymbolLayer.PropertyFillColor,
+                                                                QgsProperty.fromExpression('case when @geometry_ring_num=0 then \'green\' when @geometry_ring_num=1 then \'blue\' when @geometry_ring_num=2 then \'red\' end'))
+        s3.symbolLayer(0).setAverageAngleLength(0)
+
+        g = QgsGeometry.fromWkt('Polygon((0 0, 10 0, 10 10, 0 10, 0 0),(1 1, 1 2, 2 2, 2 1, 1 1),(8 8, 9 8, 9 9, 8 9, 8 8))')
+        rendered_image = self.renderGeometry(s3, g)
+        assert self.imageCheck('markerline_ring_num', 'markerline_ring_num', rendered_image)
+
     def testPartNum(self):
         # test geometry_part_num variable
         s = QgsLineSymbol()

tests/src/python/test_qgssimplelinesymbollayer.py

@@ -271,6 +271,19 @@ def testDashCornerTweakDashRender(self):
         rendered_image = self.renderGeometry(s, g)
         assert self.imageCheck('simpleline_dashcornertweak', 'simpleline_dashcornertweak', rendered_image)
 
+    def testRingNumberVariable(self):
+        # test test geometry_ring_num variable
+        s3 = QgsFillSymbol()
+        s3.deleteSymbolLayer(0)
+        s3.appendSymbolLayer(
+            QgsSimpleLineSymbolLayer(color=QColor(255, 0, 0), width=2))
+        s3.symbolLayer(0).setDataDefinedProperty(QgsSymbolLayer.PropertyStrokeColor,
+                                                 QgsProperty.fromExpression('case when @geometry_ring_num=0 then \'green\' when @geometry_ring_num=1 then \'blue\' when @geometry_ring_num=2 then \'red\' end'))
+
+        g = QgsGeometry.fromWkt('Polygon((0 0, 10 0, 10 10, 0 10, 0 0),(1 1, 1 2, 2 2, 2 1, 1 1),(8 8, 9 8, 9 9, 8 9, 8 8))')
+        rendered_image = self.renderGeometry(s3, g)
+        assert self.imageCheck('simpleline_ring_num', 'simpleline_ring_num', rendered_image)
+
     def testRingFilter(self):
         # test filtering rings during rendering