fix mesh centroid when coordinates are very big (#40757)

(cherry picked from commit ee6d797)

src/core/mesh/qgstriangularmesh.cpp

@@ -58,19 +58,27 @@ static void ENP_centroid( const QPolygonF &pX, double &cx, double &cy )
 
   double signedArea = 0.0;
 
+  const QPointF &pt0 = pX.first();
+  QPolygonF localPolygon( pX.count() );
+  for ( int i = 0; i < pX.count(); ++i )
+    localPolygon[i] = pX.at( i ) - pt0;
+
   // For all vertices except last
   int i = 0;
-  for ( ; i < pX.size() - 1; ++i )
+  for ( ; i < localPolygon.size() - 1; ++i )
   {
-    ENP_centroid_step( pX, cx, cy, signedArea, i, i + 1 );
+    ENP_centroid_step( localPolygon, cx, cy, signedArea, i, i + 1 );
   }
   // Do last vertex separately to avoid performing an expensive
   // modulus operation in each iteration.
-  ENP_centroid_step( pX, cx, cy, signedArea, i, 0 );
+  ENP_centroid_step( localPolygon, cx, cy, signedArea, i, 0 );
 
   signedArea *= 0.5;
   cx /= ( 6.0 * signedArea );
   cy /= ( 6.0 * signedArea );
+
+  cx = cx + pt0.x();
+  cy = cy + pt0.y();
 }
 
 void QgsTriangularMesh::triangulate( const QgsMeshFace &face, int nativeIndex )

tests/src/core/testqgstriangularmesh.cpp

@@ -23,6 +23,7 @@
 #include "qgstriangularmesh.h"
 #include "qgsapplication.h"
 #include "qgsproject.h"
+#include "qgis.h"
 
 /**
  * \ingroup UnitTests
@@ -43,6 +44,8 @@ class TestQgsTriangularMesh : public QObject
 
     void test_triangulate();
 
+    void test_centroids();
+
   private:
     void populateMeshVertices( QgsTriangularMesh &mesh );
 
@@ -66,9 +69,6 @@ void TestQgsTriangularMesh::initTestCase()
   QgsApplication::init();
   QgsApplication::initQgis();
   QgsApplication::showSettings();
-
-
-
 }
 
 void TestQgsTriangularMesh::cleanupTestCase()
@@ -125,5 +125,47 @@ void TestQgsTriangularMesh::test_triangulate()
   }
 }
 
+void TestQgsTriangularMesh::test_centroids()
+{
+  QgsTriangularMesh triangularMesh;
+
+  QgsMesh nativeMesh;
+  nativeMesh.vertices << QgsMeshVertex( 0, 10, 0 ) << QgsMeshVertex( 10, 10, 0 ) << QgsMeshVertex( 10, 0, 0 ) << QgsMeshVertex( 0, 0, 0 )
+                      << QgsMeshVertex( 20, 0, 0 ) << QgsMeshVertex( 30, 10, 0 ) << QgsMeshVertex( 20, 10, 0 );
+
+  nativeMesh.faces << QgsMeshFace( {0, 1, 2, 3} ) << QgsMeshFace( {1, 2, 4, 5} );
+
+  triangularMesh.update( &nativeMesh, QgsCoordinateTransform() );
+
+  QVector<QgsMeshVertex> centroids = triangularMesh.faceCentroids();
+
+  QCOMPARE( 2, centroids.count() );
+
+  QVERIFY( qgsDoubleNear( centroids.at( 0 ).x(), 5, 0.00001 ) );
+  QVERIFY( qgsDoubleNear( centroids.at( 0 ).y(), 5, 0.00001 ) );
+  QVERIFY( qgsDoubleNear( centroids.at( 1 ).x(), 17.777777777, 0.00001 ) );
+  QVERIFY( qgsDoubleNear( centroids.at( 1 ).y(), 5.5555555555, 0.00001 ) );
+
+  // with big coordinates
+  nativeMesh.clear();
+  triangularMesh = QgsTriangularMesh();
+
+  nativeMesh.vertices << QgsMeshVertex( 900000000, 300000010, 0 ) << QgsMeshVertex( 900000010, 300000010, 0 ) << QgsMeshVertex( 900000010, 300000000, 0 ) << QgsMeshVertex( 900000000, 300000000, 0 )
+                      << QgsMeshVertex( 900000020, 300000000, 0 ) << QgsMeshVertex( 900000030, 300000010, 0 ) << QgsMeshVertex( 900000020, 300000010, 0 );
+
+  nativeMesh.faces << QgsMeshFace( {0, 1, 2, 3} ) << QgsMeshFace( {1, 2, 4, 5} );
+  triangularMesh.update( &nativeMesh, QgsCoordinateTransform() );
+
+  centroids = triangularMesh.faceCentroids();
+
+  QCOMPARE( 2, centroids.count() );
+
+  QVERIFY( qgsDoubleNear( centroids.at( 0 ).x(), 900000005, 0.00001 ) );
+  QVERIFY( qgsDoubleNear( centroids.at( 0 ).y(), 300000005, 0.00001 ) );
+  QVERIFY( qgsDoubleNear( centroids.at( 1 ).x(), 900000017.777777, 0.00001 ) );
+  QVERIFY( qgsDoubleNear( centroids.at( 1 ).y(), 300000005.555555, 0.00001 ) );
+
+}
+
 QGSTEST_MAIN( TestQgsTriangularMesh )
 #include "testqgstriangularmesh.moc"