Further tweaks to line labeling, add tests

Sponsored by Andreas Neumann

Author: nyalldawson

src/core/pal/feature.cpp

@@ -683,6 +683,7 @@ int FeaturePart::createCandidatesAlongLineNearStraightSegments( QList<LabelPosit
   straightSegmentLengths << currentStraightSegmentLength;
   straightSegmentAngles << QgsGeometryUtils::normalizedAngle( atan2( y[numberNodes-1] - segmentStartY, x[numberNodes-1] - segmentStartX ) );
   longestSegmentLength = qMax( longestSegmentLength, currentStraightSegmentLength );
+  double middleOfLine = totalLineLength / 2.0;
 
   if ( totalLineLength < labelWidth )
   {
@@ -729,6 +730,10 @@ int FeaturePart::createCandidatesAlongLineNearStraightSegments( QList<LabelPosit
 
       candidateLength = sqrt(( candidateEndX - candidateStartX ) * ( candidateEndX - candidateStartX ) + ( candidateEndY - candidateStartY ) * ( candidateEndY - candidateStartY ) );
 
+
+      // LOTS OF DIFFERENT COSTS TO BALANCE HERE - feel free to tweak these, but please add a unit test
+      // which covers the situation you are adjusting for (eg "given equal length lines, choose the more horizontal line")
+
       cost = candidateLength / labelWidth;
       if ( cost > 0.98 )
         cost = 0.0001;
@@ -738,10 +743,15 @@ int FeaturePart::createCandidatesAlongLineNearStraightSegments( QList<LabelPosit
         cost = ( 1 - cost ) / 100; // ranges from 0.0001 to 0.01 (however a cost 0.005 is already a lot!)
       }
 
-      // penalize positions which are further from the line's midpoint
+      // penalize positions which are further from the straight segments's midpoint
       double labelCenter = currentDistanceAlongLine + labelWidth / 2.0;
       double costCenter = 2 * qAbs( labelCenter - distanceToCenterOfSegment ) / ( distanceToEndOfSegment - distanceToStartOfSegment ); // 0 -> 1
       cost += costCenter * 0.0005;  // < 0, 0.0005 >
+
+      // penalize positions which are further from absolute center of whole linestring
+      double costLineCenter = 2 * qAbs( labelCenter - middleOfLine ) / totalLineLength; // 0 -> 1
+      cost += costLineCenter * 0.0005;  // < 0, 0.0005 >
+
       cost += segmentCost * 0.0005; // prefer labels on longer straight segments
       cost += segmentAngleCost * 0.0001; // prefer more horizontal segments, but this is less important than length considerations
 

tests/src/python/test_qgspallabeling_placement.py

@@ -276,6 +276,7 @@ def test_polygon_placement_perimeter(self):
         self.layer = TestQgsPalLabeling.loadFeatureLayer('polygon_perimeter')
         self._TestMapSettings = self.cloneMapSettings(self._MapSettings)
         self.lyr.placement = QgsPalLayerSettings.Line
+        self.lyr.placementFlags = QgsPalLayerSettings.AboveLine
         self.checkTest()
         self.removeMapLayer(self.layer)
         self.layer = None
@@ -385,6 +386,51 @@ def test_prefer_line_below_instead_of_online(self):
         self.removeMapLayer(self.layer)
         self.layer = None
 
+    def test_prefer_longer_lines_over_shorter(self):
+        # Test that labeling a line using parallel labels will tend to place the labels over the longer straight parts of
+        # the line
+        self.layer = TestQgsPalLabeling.loadFeatureLayer('line_placement_1')
+        self._TestMapSettings = self.cloneMapSettings(self._MapSettings)
+        self.lyr.placement = QgsPalLayerSettings.Line
+        self.checkTest()
+        self.removeMapLayer(self.layer)
+        self.layer = None
+
+    def test_prefer_more_horizontal_lines(self):
+        # Test that labeling a line using parallel labels will tend to place the labels over more horizontal sections
+        self.layer = TestQgsPalLabeling.loadFeatureLayer('line_placement_2')
+        self._TestMapSettings = self.cloneMapSettings(self._MapSettings)
+        self.lyr.placement = QgsPalLayerSettings.Line
+        self.checkTest()
+        self.removeMapLayer(self.layer)
+        self.layer = None
+
+    def test_label_line_over_small_angles(self):
+        # Test that labeling a line using parallel labels will place labels near center of straightish line
+        self.layer = TestQgsPalLabeling.loadFeatureLayer('line_placement_3')
+        self._TestMapSettings = self.cloneMapSettings(self._MapSettings)
+        self.lyr.placement = QgsPalLayerSettings.Line
+        self.checkTest()
+        self.removeMapLayer(self.layer)
+        self.layer = None
+
+    def test_label_line_toward_center(self):
+        # Test that labeling a line using parallel labels will try to place labels as close to center of line as possible
+        self.layer = TestQgsPalLabeling.loadFeatureLayer('line_placement_4')
+        self._TestMapSettings = self.cloneMapSettings(self._MapSettings)
+        self.lyr.placement = QgsPalLayerSettings.Line
+        self.checkTest()
+        self.removeMapLayer(self.layer)
+        self.layer = None
+
+    def test_label_line_avoid_jaggy(self):
+        # Test that labeling a line using parallel labels won't place labels over jaggy bits of line
+        self.layer = TestQgsPalLabeling.loadFeatureLayer('line_placement_5')
+        self._TestMapSettings = self.cloneMapSettings(self._MapSettings)
+        self.lyr.placement = QgsPalLayerSettings.Line
+        self.checkTest()
+        self.removeMapLayer(self.layer)
+        self.layer = None
 
 if __name__ == '__main__':
     # NOTE: unless PAL_SUITE env var is set all test class methods will be run