[FEATURE] Snap geometries algorithm

Makes sure that any two vertices of the vector layer are at least at distance given by the threshold value. The algorithm moves nearby vertices to one location and adds vertices to segments that are passing around other vertices within the threshold. It does not remove any vertices. Also, it does not modify geometries unless needed (it does not snap coordinates to a grid). This algorithm comes handy when doing vector overlay operations such as intersection, union or difference to prevent possible topological errors caused by numerical errors if coordinates are very close to each other. After running the algorithm some previously valid geometries may become invalid and therefore it may be useful to run Fix geometries algorithm afterwards.
qgis · Sep 13, 2018 · 4a06697 · 4a06697
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diff --git a/python/plugins/processing/tests/testdata/custom/snap_geometries.geojson b/python/plugins/processing/tests/testdata/custom/snap_geometries.geojson
@@ -0,0 +1,9 @@
+{
+"type": "FeatureCollection",
+"name": "snap_geometries",
+"crs": { "type": "name", "properties": { "name": "urn:ogc:def:crs:EPSG::3857" } },
+"features": [
+{ "type": "Feature", "properties": { }, "geometry": { "type": "Polygon", "coordinates": [ [ [ 20.123, 10.123 ], [ 20.123, 20.456 ], [ 20.2, 20.456 ], [ 20.2, 20.5 ], [ 30.0, 20.5 ], [ 30.0, 10.123 ], [ 29.8, 10.123 ], [ 21.0, 10.123 ], [ 20.123, 10.123 ] ] ] } },
+{ "type": "Feature", "properties": { }, "geometry": { "type": "Polygon", "coordinates": [ [ [ 31.0, 10.0 ], [ 20.0, 10.0 ], [ 20.0, 5.0 ], [ 31.0, 10.0 ] ] ] } }
+]
+}
diff --git a/python/plugins/processing/tests/testdata/expected/snap_geometries.gml b/python/plugins/processing/tests/testdata/expected/snap_geometries.gml
@@ -0,0 +1,24 @@
+<?xml version="1.0" encoding="utf-8" ?>
+<ogr:FeatureCollection
+     xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+     xsi:schemaLocation="http://ogr.maptools.org/ snap_geometries.xsd"
+     xmlns:ogr="http://ogr.maptools.org/"
+     xmlns:gml="http://www.opengis.net/gml">
+  <gml:boundedBy>
+    <gml:Box>
+      <gml:coord><gml:X>20</gml:X><gml:Y>5</gml:Y></gml:coord>
+      <gml:coord><gml:X>31</gml:X><gml:Y>20.5</gml:Y></gml:coord>
+    </gml:Box>
+  </gml:boundedBy>
+
+  <gml:featureMember>
+    <ogr:snap_geometries fid="snap_geometries.0">
+      <ogr:geometryProperty><gml:Polygon srsName="EPSG:3857"><gml:outerBoundaryIs><gml:LinearRing><gml:coordinates>20.123,10.123 20.123,20.456 20.123,20.456 20.123,20.456 30.0,20.5 30.0,10.123 30.0,10.123 21.0,10.123 20.123,10.123</gml:coordinates></gml:LinearRing></gml:outerBoundaryIs></gml:Polygon></ogr:geometryProperty>
+    </ogr:snap_geometries>
+  </gml:featureMember>
+  <gml:featureMember>
+    <ogr:snap_geometries fid="snap_geometries.1">
+      <ogr:geometryProperty><gml:Polygon srsName="EPSG:3857"><gml:outerBoundaryIs><gml:LinearRing><gml:coordinates>31,10 30.0,10.123 21.0,10.123 20.123,10.123 20,5 31,10</gml:coordinates></gml:LinearRing></gml:outerBoundaryIs></gml:Polygon></ogr:geometryProperty>
+    </ogr:snap_geometries>
+  </gml:featureMember>
+</ogr:FeatureCollection>
diff --git a/python/plugins/processing/tests/testdata/expected/snap_geometries.xsd b/python/plugins/processing/tests/testdata/expected/snap_geometries.xsd
@@ -0,0 +1,23 @@
+<?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="snap_geometries" type="ogr:snap_geometries_Type" substitutionGroup="gml:_Feature"/>
+<xs:complexType name="snap_geometries_Type">
+  <xs:complexContent>
+    <xs:extension base="gml:AbstractFeatureType">
+      <xs:sequence>
+        <xs:element name="geometryProperty" type="gml:PolygonPropertyType" nillable="true" minOccurs="0" maxOccurs="1"/>
+      </xs:sequence>
+    </xs:extension>
+  </xs:complexContent>
+</xs:complexType>
+</xs:schema>
diff --git a/python/plugins/processing/tests/testdata/qgis_algorithm_tests.yaml b/python/plugins/processing/tests/testdata/qgis_algorithm_tests.yaml
@@ -5586,6 +5586,18 @@ tests:
           fields:
             fid: skip
 
+  - algorithm: native:snap
+    name: Test Snap Geometries (to each other)
+    params:
+      INPUT:
+        name: custom/snap_geometries.geojson
+        type: vector
+      THRESHOLD: 0.5
+    results:
+      OUTPUT:
+        name: expected/snap_geometries.gml
+        type: vector
+
   - algorithm: native:taperedbuffer
     name: Tapered buffers (lines)
     params:

diff --git a/src/analysis/CMakeLists.txt b/src/analysis/CMakeLists.txt
@@ -85,6 +85,7 @@ SET(QGIS_ANALYSIS_SRCS
   processing/qgsalgorithmshortestpathpointtopoint.cpp
   processing/qgsalgorithmsimplify.cpp
   processing/qgsalgorithmsmooth.cpp
+  processing/qgsalgorithmsnapgeometries.cpp
   processing/qgsalgorithmsnaptogrid.cpp
   processing/qgsalgorithmsplitwithlines.cpp
   processing/qgsalgorithmstringconcatenation.cpp