[processing] Add test & adjust handling of negative distance value

for multi ring (constant) algorithm

Author: nirvn

python/plugins/processing/tests/testdata/custom/circular_strings.gpkg

@@ -0,0 +1,58 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<xs:schema targetNamespace="http://ogr.maptools.org/" xmlns:ogr="http://ogr.maptools.org/" xmlns:xs="http://www.w3.org/2001/XMLSchema" xmlns:gml="http://www.opengis.net/gml" elementFormDefault="qualified" version="1.0">
+<xs:import namespace="http://www.opengis.net/gml" schemaLocation="http://schemas.opengis.net/gml/2.1.2/feature.xsd"/>
+<xs:element name="FeatureCollection" type="ogr:FeatureCollectionType" substitutionGroup="gml:_FeatureCollection"/>
+<xs:complexType name="FeatureCollectionType">
+  <xs:complexContent>
+    <xs:extension base="gml:AbstractFeatureCollectionType">
+      <xs:attribute name="lockId" type="xs:string" use="optional"/>
+      <xs:attribute name="scope" type="xs:string" use="optional"/>
+    </xs:extension>
+  </xs:complexContent>
+</xs:complexType>
+<xs:element name="multiring_negative_buffer" type="ogr:multiring_negative_buffer_Type" substitutionGroup="gml:_Feature"/>
+<xs:complexType name="multiring_negative_buffer_Type">
+  <xs:complexContent>
+    <xs:extension base="gml:AbstractFeatureType">
+      <xs:sequence>
+        <xs:element name="geometryProperty" type="gml:MultiPolygonPropertyType" nillable="true" minOccurs="0" maxOccurs="1"/>
+        <xs:element name="name" nillable="true" minOccurs="0" maxOccurs="1">
+          <xs:simpleType>
+            <xs:restriction base="xs:string">
+              <xs:maxLength value="5"/>
+            </xs:restriction>
+          </xs:simpleType>
+        </xs:element>
+        <xs:element name="intval" nillable="true" minOccurs="0" maxOccurs="1">
+          <xs:simpleType>
+            <xs:restriction base="xs:integer">
+              <xs:totalDigits value="10"/>
+            </xs:restriction>
+          </xs:simpleType>
+        </xs:element>
+        <xs:element name="floatval" nillable="true" minOccurs="0" maxOccurs="1">
+          <xs:simpleType>
+            <xs:restriction base="xs:decimal">
+            </xs:restriction>
+          </xs:simpleType>
+        </xs:element>
+        <xs:element name="ringId" nillable="true" minOccurs="0" maxOccurs="1">
+          <xs:simpleType>
+            <xs:restriction base="xs:integer">
+              <xs:totalDigits value="10"/>
+            </xs:restriction>
+          </xs:simpleType>
+        </xs:element>
+        <xs:element name="distance" nillable="true" minOccurs="0" maxOccurs="1">
+          <xs:simpleType>
+            <xs:restriction base="xs:decimal">
+              <xs:totalDigits value="21"/>
+              <xs:fractionDigits value="6"/>
+            </xs:restriction>
+          </xs:simpleType>
+        </xs:element>
+      </xs:sequence>
+    </xs:extension>
+  </xs:complexContent>
+</xs:complexType>
+</xs:schema>

python/plugins/processing/tests/testdata/custom/pol.gpkg

@@ -5329,6 +5329,19 @@ tests:
         name: expected/multiring_buffer.gml
         type: vector
 
+  - algorithm: native:multiringconstantbuffer
+    name: Multi-ring negative buffer with polygons
+    params:
+      DISTANCE: -0.05
+      INPUT:
+        name: polys.gml
+        type: vector
+      RINGS: 3
+    results:
+      OUTPUT:
+        name: expected/multiring_negative_buffer.gml
+        type: vector
+
   - algorithm: native:segmentizebymaxangle
     name: Segmentize by angle
     params:

python/plugins/processing/tests/testdata/expected/merged_pol.gpkg

@@ -133,6 +133,11 @@ QgsFeatureList QgsMultiRingConstantBufferAlgorithm::processFeature( const QgsFea
 
   QgsFeatureList outputs;
 
+  // Set previous geometry to a zero-distance buffer of the original geometry,
+  // this is needed for negative distance values
+  previousGeometry = feature.geometry().buffer( 0.0, 40 );
+  previousGeometry.convertToMultiType();
+
   for ( int i = 1; i <= rings; ++i )
   {
     QgsFeature out;
@@ -145,7 +150,11 @@ QgsFeatureList QgsMultiRingConstantBufferAlgorithm::processFeature( const QgsFea
       continue;
     }
 
-    if ( i == 1 )
+    if ( distance < 0.0 )
+    {
+      out.setGeometry( previousGeometry.symDifference( outputGeometry ) );
+    }
+    else if ( i == 1 )
     {
       out.setGeometry( outputGeometry );
     }
@@ -159,6 +168,18 @@ QgsFeatureList QgsMultiRingConstantBufferAlgorithm::processFeature( const QgsFea
     out.setAttributes( attrs );
     outputs.append( out );
   }
+
+  // For negative distance values, the last generated buffer geometry needs to be added
+  if ( distance < 0.0 )
+  {
+    QgsFeature out;
+    out.setGeometry( previousGeometry );
+    QgsAttributes attrs = feature.attributes();
+    attrs << 0 << 0.0;
+    out.setAttributes( attrs );
+    outputs.append( out );
+  }
+
   return outputs;
 }