[labeling] Fix candidate generation for horizontal line labels

The previous candidate generation code just used the exact same logic as parallel line labels but forced the angle to be horizontal. This was completely wrong, because the parallel line candidate generation is totally designed around parallel labels. Instead, use specific logic for horizontal line candidate generation, resulting in optimal candidate generation for horizontal labels.
qgis · Feb 19, 2020 · 69c4013 · 69c4013
1 parent 4b22623
commit 69c4013
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diff --git a/src/core/pal/feature.cpp b/src/core/pal/feature.cpp
@@ -696,6 +696,64 @@ std::size_t FeaturePart::createCandidatesAlongLine( std::vector< std::unique_ptr
   return candidates;
 }
 
+std::size_t FeaturePart::createHorizontalCandidatesAlongLine( std::vector<std::unique_ptr<LabelPosition> > &lPos, PointSet *mapShape, Pal *pal )
+{
+  const double labelWidth = getLabelWidth();
+  const double labelHeight = getLabelHeight();
+
+  PointSet *line = mapShape;
+  int nbPoints = line->nbPoints;
+  std::vector< double > &x = line->x;
+  std::vector< double > &y = line->y;
+
+  std::vector< double > segmentLengths( nbPoints - 1 ); // segments lengths distance bw pt[i] && pt[i+1]
+  std::vector< double >distanceToSegment( nbPoints ); // absolute distance bw pt[0] and pt[i] along the line
+
+  double totalLineLength = 0.0; // line length
+  for ( int i = 0; i < line->nbPoints - 1; i++ )
+  {
+    if ( i == 0 )
+      distanceToSegment[i] = 0;
+    else
+      distanceToSegment[i] = distanceToSegment[i - 1] + segmentLengths[i - 1];
+
+    segmentLengths[i] = GeomFunction::dist_euc2d( x[i], y[i], x[i + 1], y[i + 1] );
+    totalLineLength += segmentLengths[i];
+  }
+  distanceToSegment[line->nbPoints - 1] = totalLineLength;
+
+  const std::size_t candidateTargetCount = maximumLineCandidates();
+  const double lineStepDistance = totalLineLength / ( candidateTargetCount + 1 ); // distance to move along line with each candidate
+  double currentDistanceAlongLine = lineStepDistance;
+
+  double candidateCenterX, candidateCenterY;
+  int i = 0;
+  while ( currentDistanceAlongLine < totalLineLength )
+  {
+    if ( pal->isCanceled() )
+    {
+      return lPos.size();
+    }
+
+    line->getPointByDistance( segmentLengths.data(), distanceToSegment.data(), currentDistanceAlongLine, &candidateCenterX, &candidateCenterY );
+
+    // penalize positions which are further from the line's midpoint
+    double cost = std::fabs( totalLineLength / 2 - currentDistanceAlongLine ) / totalLineLength; // <0, 0.5>
+    cost /= 1000;  // < 0, 0.0005 >
+
+    lPos.emplace_back( qgis::make_unique< LabelPosition >( i, candidateCenterX - labelWidth / 2, candidateCenterY - labelHeight / 2, labelWidth, labelHeight, 0, cost, this ) );
+
+    currentDistanceAlongLine += lineStepDistance;
+
+    i++;
+
+    if ( lineStepDistance < 0 )
+      break;
+  }
+
+  return lPos.size();
+}
+
 std::size_t FeaturePart::createCandidatesAlongLineNearStraightSegments( std::vector< std::unique_ptr< LabelPosition > > &lPos, PointSet *mapShape, Pal *pal )
 {
   double labelWidth = getLabelWidth();
@@ -1718,8 +1776,11 @@ std::vector< std::unique_ptr< LabelPosition > > FeaturePart::createCandidates( P
         else
           createCandidatesAroundPoint( x[0], y[0], lPos, angle );
         break;
+
       case GEOS_LINESTRING:
-        if ( mLF->layer()->isCurved() )
+        if ( mLF->layer()->arrangement() == QgsPalLayerSettings::Horizontal )
+          createHorizontalCandidatesAlongLine( lPos, this, pal );
+        else if ( mLF->layer()->isCurved() )
           createCurvedCandidatesAlongLine( lPos, this, true, pal );
         else
           createCandidatesAlongLine( lPos, this, true, pal );

diff --git a/src/core/pal/feature.h b/src/core/pal/feature.h
@@ -194,6 +194,15 @@ namespace pal
        */
       std::size_t createCandidatesAlongLine( std::vector<std::unique_ptr<LabelPosition> > &lPos, PointSet *mapShape, bool allowOverrun, Pal *pal );
 
+      /**
+       * Generate horizontal candidates for line feature.
+       * \param lPos pointer to an array of candidates, will be filled by generated candidates
+       * \param mapShape a pointer to the line
+       * \param pal point to pal settings object, for cancellation support
+       * \returns the number of generated candidates
+       */
+      std::size_t createHorizontalCandidatesAlongLine( std::vector<std::unique_ptr<LabelPosition> > &lPos, PointSet *mapShape, Pal *pal );
+
       /**
        * Generate candidates for line feature, by trying to place candidates towards the middle of the longest
        * straightish segments of the line. Segments closer to horizontal are preferred over vertical segments.