Add utility function to slice a QPolygonF linestring

qgis · Jul 24, 2020 · ca5901b · ca5901b
1 parent 2073790
commit ca5901b
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Showing 4 changed files with 164 additions and 1 deletion.
diff --git a/python/core/auto_generated/symbology/qgssymbollayerutils.sip.in b/python/core/auto_generated/symbology/qgssymbollayerutils.sip.in
@@ -721,6 +721,20 @@ Calculate a point on the surface of a QPolygonF
     static bool pointInPolygon( const QPolygonF &points, QPointF point );
 %Docstring
 Calculate whether a point is within of a QPolygonF
+%End
+
+    static QPolygonF polylineSubstring( const QPolygonF &polyline, double startOffset, double endOffset );
+%Docstring
+Returns the substring of a ``polyline`` which starts at ``startOffset`` from the beginning of the line
+and ends at ``endOffset`` from the start of the line.
+
+If ``startOffset`` is less than 0, then the start point will be calculated by subtracting that distance
+from the end of the line. Similarly, if ``endOffset`` is less than zero then the end point will be subtracted
+from the end of the line.
+
+May return an empty linestring if the substring is zero length.
+
+.. versionadded:: 3.16
 %End
 
     static QgsExpression *fieldOrExpressionToExpression( const QString &fieldOrExpression ) /Factory/;

diff --git a/src/core/symbology/qgssymbollayerutils.cpp b/src/core/symbology/qgssymbollayerutils.cpp
@@ -4073,6 +4073,89 @@ bool QgsSymbolLayerUtils::pointInPolygon( const QPolygonF &points, QPointF point
   return inside;
 }
 
+QPolygonF QgsSymbolLayerUtils::polylineSubstring( const QPolygonF &polyline, double startOffset, double endOffset )
+{
+  if ( polyline.size() < 2 )
+    return QPolygonF();
+
+  double totalLength = 0;
+  auto it = polyline.begin();
+  QPointF p1 = *it++;
+  std::vector< double > segmentLengths( polyline.size() - 1 );
+  auto segmentLengthIt = segmentLengths.begin();
+  for ( ; it != polyline.end(); ++it )
+  {
+    QPointF p2 = *it;
+    *segmentLengthIt = std::sqrt( std::pow( p1.x() - p2.x(), 2.0 ) + std::pow( p1.y() - p2.y(), 2.0 ) );
+    totalLength += *segmentLengthIt;
+
+    segmentLengthIt++;
+    p1 = p2;
+  }
+
+  if ( startOffset >= 0 && totalLength <= startOffset )
+    return QPolygonF();
+  if ( endOffset < 0 && totalLength <= -endOffset )
+    return QPolygonF();
+
+  const double startDistance = startOffset < 0 ? totalLength + startOffset : startOffset;
+  const double endDistance = endOffset <= 0 ? totalLength + endOffset : endOffset;
+  QPolygonF substringPoints;
+  substringPoints.reserve( polyline.size() );
+
+  it = polyline.begin();
+  segmentLengthIt = segmentLengths.begin();
+
+  p1 = *it++;
+  bool foundStart = false;
+  if ( qgsDoubleNear( startDistance, 0.0 ) || startDistance < 0 )
+  {
+    substringPoints << p1;
+    foundStart = true;
+  }
+
+  double distanceTraversed = 0;
+  for ( ; it != polyline.end(); ++it )
+  {
+    QPointF p2 = *it;
+    if ( distanceTraversed < startDistance && distanceTraversed + *segmentLengthIt > startDistance )
+    {
+      // start point falls on this segment
+      const double distanceToStart = startDistance - distanceTraversed;
+      double startX, startY;
+      QgsGeometryUtils::pointOnLineWithDistance( p1.x(), p1.y(), p2.x(), p2.y(), distanceToStart, startX, startY );
+      substringPoints << QPointF( startX, startY );
+      foundStart = true;
+    }
+    if ( foundStart && ( distanceTraversed + *segmentLengthIt > endDistance ) )
+    {
+      // end point falls on this segment
+      const double distanceToEnd = endDistance - distanceTraversed;
+      double endX, endY;
+      QgsGeometryUtils::pointOnLineWithDistance( p1.x(), p1.y(), p2.x(), p2.y(), distanceToEnd, endX, endY );
+      if ( substringPoints.last() != QPointF( endX, endY ) )
+        substringPoints << QPointF( endX, endY );
+    }
+    else if ( foundStart )
+    {
+      if ( substringPoints.last() != QPointF( p2.x(), p2.y() ) )
+        substringPoints << QPointF( p2.x(), p2.y() );
+    }
+
+    distanceTraversed += *segmentLengthIt;
+    if ( distanceTraversed > endDistance )
+      break;
+
+    p1 = p2;
+    segmentLengthIt++;
+  }
+
+  if ( ( substringPoints.size() < 2 ) || ( substringPoints.size() == 2 && substringPoints.at( 0 ) == substringPoints.at( 1 ) ) )
+    return QPolygonF();
+
+  return substringPoints;
+}
+
 QgsExpression *QgsSymbolLayerUtils::fieldOrExpressionToExpression( const QString &fieldOrExpression )
 {
   if ( fieldOrExpression.isEmpty() )

diff --git a/src/core/symbology/qgssymbollayerutils.h b/src/core/symbology/qgssymbollayerutils.h
@@ -648,6 +648,20 @@ class CORE_EXPORT QgsSymbolLayerUtils
     //! Calculate whether a point is within of a QPolygonF
     static bool pointInPolygon( const QPolygonF &points, QPointF point );
 
+    /**
+     * Returns the substring of a \a polyline which starts at \a startOffset from the beginning of the line
+     * and ends at \a endOffset from the start of the line.
+     *
+     * If \a startOffset is less than 0, then the start point will be calculated by subtracting that distance
+     * from the end of the line. Similarly, if \a endOffset is less than zero then the end point will be subtracted
+     * from the end of the line.
+     *
+     * May return an empty linestring if the substring is zero length.
+     *
+     * \since QGIS 3.16
+     */
+    static QPolygonF polylineSubstring( const QPolygonF &polyline, double startOffset, double endOffset );
+
     /**
      * Returns a new valid expression instance for given field or expression string.
      * If the input is not a valid expression, it is assumed that it is a field name and gets properly quoted.

diff --git a/tests/src/python/test_qgssymbollayerutils.py b/tests/src/python/test_qgssymbollayerutils.py
@@ -16,7 +16,7 @@
                        QgsMarkerSymbol,
                        QgsArrowSymbolLayer,
                        QgsUnitTypes)
-from qgis.PyQt.QtGui import QColor
+from qgis.PyQt.QtGui import QColor, QPolygonF
 from qgis.PyQt.QtCore import QSizeF, QPointF
 from qgis.testing import unittest, start_app
 
@@ -246,6 +246,58 @@ def testDecodeSldUom(self):
         decode = QgsSymbolLayerUtils.decodeSldUom("http://www.opengeospatial.org/se/units/pixel")
         self.assertEqual(decode, (QgsUnitTypes.RenderPixels, 1.0))
 
+    def testPolylineSubstring(self):
+        res = QgsSymbolLayerUtils.polylineSubstring(QPolygonF(), 1, 2) # no crash
+        self.assertFalse(res)
+
+        res = QgsSymbolLayerUtils.polylineSubstring(QPolygonF(), -1, 2) # no crash
+        self.assertFalse(res)
+
+        res = QgsSymbolLayerUtils.polylineSubstring(QPolygonF(), 1, -2) # no crash
+        self.assertFalse(res)
+
+        res = QgsSymbolLayerUtils.polylineSubstring(QPolygonF(), -1, -2) # no crash
+        self.assertFalse(res)
+
+        res = QgsSymbolLayerUtils.polylineSubstring(QPolygonF([QPointF(11, 2), QPointF(11, 12), QPointF(111, 12)]), 0, -110)
+        self.assertEqual([p for p in res], [])
+
+        res = QgsSymbolLayerUtils.polylineSubstring(QPolygonF([QPointF(11, 2), QPointF(11, 12), QPointF(111, 12)]), 0, 110)
+        self.assertEqual([p for p in res], [QPointF(11, 2), QPointF(11, 12), QPointF(111, 12)])
+
+        res = QgsSymbolLayerUtils.polylineSubstring(QPolygonF([QPointF(11, 2), QPointF(11, 12), QPointF(111, 12)]), -1, -1000)
+        self.assertFalse([p for p in res])
+
+        res = QgsSymbolLayerUtils.polylineSubstring(QPolygonF([QPointF(11, 2), QPointF(11, 12), QPointF(111, 12)]), 1, -1000)
+        self.assertFalse([p for p in res])
+
+        res = QgsSymbolLayerUtils.polylineSubstring(QPolygonF([QPointF(11, 2), QPointF(11, 12), QPointF(111, 12)]), -1, 1000)
+        self.assertEqual([p for p in res], [QPointF(110.0, 12.0), QPointF(111.0, 12.0)])
+
+        res = QgsSymbolLayerUtils.polylineSubstring(QPolygonF([QPointF(11, 2), QPointF(11, 12), QPointF(111, 12)]), 100000, -10000)
+        self.assertFalse([p for p in res])
+
+        res = QgsSymbolLayerUtils.polylineSubstring(QPolygonF([QPointF(11, 2), QPointF(11, 12), QPointF(111, 12)]), 1, -109)
+        self.assertEqual([p for p in res], [])
+
+        res = QgsSymbolLayerUtils.polylineSubstring(QPolygonF([QPointF(11, 2), QPointF(11, 12), QPointF(111, 12)]), 1, 109)
+        self.assertEqual([p for p in res], [QPointF(11.0, 3.0), QPointF(11.0, 12.0), QPointF(110.0, 12.0)])
+
+        res = QgsSymbolLayerUtils.polylineSubstring(QPolygonF([QPointF(11, 2), QPointF(11, 12), QPointF(111, 12)]), -109, 109)
+        self.assertEqual([p for p in res], [QPointF(11.0, 3.0), QPointF(11.0, 12.0), QPointF(110.0, 12.0)])
+
+        res = QgsSymbolLayerUtils.polylineSubstring(QPolygonF([QPointF(11, 2), QPointF(11, 12), QPointF(111, 12)]), 1, -1000)
+        self.assertEqual([p for p in res], [])
+
+        res = QgsSymbolLayerUtils.polylineSubstring(QPolygonF([QPointF(11, 2), QPointF(11, 12), QPointF(111, 12)]), 1, 10)
+        self.assertEqual([p for p in res], [QPointF(11, 3), QPointF(11, 12)])
+
+        res = QgsSymbolLayerUtils.polylineSubstring(QPolygonF([QPointF(11, 2), QPointF(11, 12), QPointF(111, 12)]), 1, 0)
+        self.assertEqual([p for p in res], [QPointF(11, 3), QPointF(11, 12), QPointF(111, 12)])
+
+        res = QgsSymbolLayerUtils.polylineSubstring(QPolygonF([QPointF(11, 2), QPointF(11, 12), QPointF(111, 12)]), 1, -90)
+        self.assertEqual([p for p in res], [QPointF(11, 3), QPointF(11, 12), QPointF(21, 12)])
+
 
 if __name__ == '__main__':
     unittest.main()