[pal][api] Add label candidate generation mechanism to generate candi…

…dates outside of polygon features Based on a tweaked version of the logic presented by Rylov & Reimer from "A practical algorithm for the external annotation of area features" (2016), this placement mode generates a set of label candidates which sit at a set distance outside of a polygon's exterior ring. Designed for candidate generation for small polygons wrt label size, or in other cases where it's not possible or desirable to fit the label inside the polygon features itself
qgis · May 3, 2020 · 94324f9 · 94324f9
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diff --git a/src/core/pal/feature.cpp b/src/core/pal/feature.cpp
@@ -41,6 +41,7 @@
 #include "costcalculator.h"
 #include "qgsgeometryutils.h"
 #include "qgslabeling.h"
+#include "qgspolygon.h"
 #include <QLinkedList>
 #include <cmath>
 #include <cfloat>
@@ -1797,6 +1798,202 @@ std::size_t FeaturePart::createCandidatesForPolygon( std::vector< std::unique_pt
   return nbp;
 }
 
+std::size_t FeaturePart::createCandidatesOutsidePolygon( std::vector<std::unique_ptr<LabelPosition> > &lPos, Pal *pal )
+{
+  // calculate distance between horizontal lines
+  const std::size_t maxPolygonCandidates = mLF->layer()->maximumPolygonLabelCandidates();
+  std::size_t candidatesCreated = 0;
+
+  double labelWidth = getLabelWidth();
+  double labelHeight = getLabelHeight();
+  double distanceToLabel = getLabelDistance();
+  const QgsMargins &visualMargin = mLF->visualMargin();
+
+  /*
+   * From Rylov & Reimer (2016) "A practical algorithm for the external annotation of area features":
+   *
+   * The list of rules adapted to the
+   * needs of externally labelling areal features is as follows:
+   * R1. Labels should be placed horizontally.
+   * R2. Label should be placed entirely outside at some
+   * distance from the area feature.
+   * R3. Name should not cross the boundary of its area
+   * feature.
+   * R4. The name should be placed in way that takes into
+   * account the shape of the feature by achieving a
+   * balance between the feature and its name, emphasizing their relationship.
+   * R5. The lettering to the right and slightly above the
+   * symbol is prioritized.
+   *
+   * In the following subsections we utilize four of the five rules
+   * for two subtasks of label placement, namely, for candidate
+   * positions generation (R1, R2, and R3) and for measuring their
+   * ‘goodness’ (R4). The rule R5 is applicable only in the case when
+   * the area of a polygonal feature is small and the feature can be
+   * treated and labelled as a point-feature
+   */
+
+  /*
+   * QGIS approach (cite Dawson (2020) if you want ;) )
+   *
+   * We differ from the horizontal sweep line approach described by Rylov & Reimer and instead
+   * rely on just generating a set of points at regular intervals along the boundary of the polygon (exterior ring).
+   *
+   * In practice, this generates similar results as Rylov & Reimer, but has the additional benefits that:
+   * 1. It avoids the need to calculate intersections between the sweep line and the polygon
+   * 2. For horizontal or near horizontal segments, Rylov & Reimer propose generating evenly spaced points along
+   * these segments-- i.e. the same approach as we do for the whole polygon
+   * 3. It's easier to determine in advance exactly how many candidate positions we'll be generating, and accordingly
+   * we can easily pick the distance between points along the exterior ring so that the number of positions generated
+   * matches our target number (targetPolygonCandidates)
+   */
+
+  // TO consider -- for very small polygons (wrt label size), treat them just like a point feature?
+
+  double cx, cy;
+  getCentroid( cx, cy, false );
+
+  GEOSContextHandle_t ctxt = QgsGeos::getGEOSHandler();
+  geos::unique_ptr buffer( GEOSBuffer_r( ctxt, geos(), mLF->distLabel(), 1 ) );
+  std::unique_ptr< QgsAbstractGeometry> gg( QgsGeos::fromGeos( buffer.get() ) );
+
+  const QgsPolygon *poly = qgsgeometry_cast< const QgsPolygon * >( gg.get() );
+  if ( !poly )
+    return candidatesCreated;
+
+  const QgsLineString *ring = qgsgeometry_cast< const QgsLineString *>( poly->exteriorRing() );
+  if ( !ring )
+    return candidatesCreated;
+
+  // we cheat here -- we don't use the polygon area when calculating the number of candidates, and rather use the perimeter (because that's more relevant,
+  // i.e a loooooong skinny polygon with small area should still generate a large number of candidates)
+  const double ringLength = ring->length();
+  const double circleArea = std::pow( ringLength, 2 ) / ( 4 * M_PI );
+  const std::size_t candidatesForArea = static_cast< std::size_t>( std::ceil( mLF->layer()->mPal->maximumPolygonCandidatesPerMapUnitSquared() * circleArea ) );
+  const std::size_t targetPolygonCandidates = std::max( static_cast< std::size_t >( 4 ), maxPolygonCandidates > 0 ? std::min( maxPolygonCandidates,  candidatesForArea ) : candidatesForArea );
+
+  // assume each position generates one candidate
+  const double delta = ringLength / targetPolygonCandidates;
+  geos::unique_ptr geosPoint;
+
+  const double maxDistCentroidToLabelX = std::max( xmax - cx, cx - xmin ) + distanceToLabel;
+  const double maxDistCentroidToLabelY = std::max( ymax - cy, cy - ymin ) + distanceToLabel;
+  const double estimateOfMaxPossibleDistanceCentroidToLabel = std::sqrt( maxDistCentroidToLabelX * maxDistCentroidToLabelX + maxDistCentroidToLabelY * maxDistCentroidToLabelY );
+
+  // Satisfy R1: Labels should be placed horizontally.
+  const double labelAngle = 0;
+
+  std::size_t i = lPos.size();
+  auto addCandidate = [&]( double x, double y, QgsPalLayerSettings::PredefinedPointPosition position )
+  {
+    double labelX = 0;
+    double labelY = 0;
+    LabelPosition::Quadrant quadrant = LabelPosition::QuadrantAboveLeft;
+
+    // Satisfy R2: Label should be placed entirely outside at some distance from the area feature.
+    createCandidateAtOrderedPositionOverPoint( labelX, labelY, quadrant, x, y, labelWidth, labelHeight, position, distanceToLabel, visualMargin, 0, 0 );
+
+    std::unique_ptr< LabelPosition > candidate = qgis::make_unique< LabelPosition >( i, labelX, labelY, labelWidth, labelHeight, labelAngle, 0, this, false, quadrant );
+    if ( candidate->intersects( preparedGeom() ) )
+    {
+      // satisfy R3. Name should not cross the boundary of its area feature.
+      return;
+    }
+
+    // cost candidates by their distance to the feature's centroid (following Rylov & Reimer)
+
+    // Satisfy R4. The name should be placed in way that takes into
+    // account the shape of the feature by achieving a
+    // balance between the feature and its name, emphasizing their relationship.
+
+
+    // here we deviate a little from R&R, and instead of just calculating the centroid distance
+    // to centroid of label, we calculate the distance from the centroid to the nearest point on the label
+
+    const double centroidDistance = candidate->getDistanceToPoint( cx, cy );
+    const double centroidCost = centroidDistance / estimateOfMaxPossibleDistanceCentroidToLabel;
+    candidate->setCost( centroidCost );
+
+    lPos.emplace_back( std::move( candidate ) );
+    candidatesCreated++;
+    ++i;
+  };
+
+  ring->visitPointsByRegularDistance( delta, [&]( double x, double y, double, double,
+                                      double startSegmentX, double startSegmentY, double, double,
+                                      double endSegmentX, double endSegmentY, double, double )
+  {
+    // get normal angle for segment
+    float angle = atan2( static_cast< float >( endSegmentY - startSegmentY ), static_cast< float >( endSegmentX - startSegmentX ) ) * 180 / M_PI;
+    if ( angle < 0 )
+      angle += 360;
+
+    // adapted fom Rylov & Reimer figure 9
+    if ( angle >= 0 && angle <= 5 )
+    {
+      addCandidate( x, y, QgsPalLayerSettings::TopMiddle );
+      addCandidate( x, y, QgsPalLayerSettings::TopLeft );
+    }
+    else if ( angle <= 85 )
+    {
+      addCandidate( x, y, QgsPalLayerSettings::TopLeft );
+    }
+    else if ( angle <= 90 )
+    {
+      addCandidate( x, y, QgsPalLayerSettings::TopLeft );
+      addCandidate( x, y, QgsPalLayerSettings::MiddleLeft );
+    }
+
+    else if ( angle <= 95 )
+    {
+      addCandidate( x, y, QgsPalLayerSettings::MiddleLeft );
+      addCandidate( x, y, QgsPalLayerSettings::BottomLeft );
+    }
+    else if ( angle <= 175 )
+    {
+      addCandidate( x, y, QgsPalLayerSettings::BottomLeft );
+    }
+    else if ( angle <= 180 )
+    {
+      addCandidate( x, y, QgsPalLayerSettings::BottomLeft );
+      addCandidate( x, y, QgsPalLayerSettings::BottomMiddle );
+    }
+
+    else if ( angle <= 185 )
+    {
+      addCandidate( x, y, QgsPalLayerSettings::BottomMiddle );
+      addCandidate( x, y, QgsPalLayerSettings::BottomRight );
+    }
+    else if ( angle <= 265 )
+    {
+      addCandidate( x, y, QgsPalLayerSettings::BottomRight );
+    }
+    else if ( angle <= 270 )
+    {
+      addCandidate( x, y, QgsPalLayerSettings::BottomRight );
+      addCandidate( x, y, QgsPalLayerSettings::MiddleRight );
+    }
+    else if ( angle <= 275 )
+    {
+      addCandidate( x, y, QgsPalLayerSettings::MiddleRight );
+      addCandidate( x, y, QgsPalLayerSettings::TopRight );
+    }
+    else if ( angle <= 355 )
+    {
+      addCandidate( x, y, QgsPalLayerSettings::TopRight );
+    }
+    else
+    {
+      addCandidate( x, y, QgsPalLayerSettings::TopRight );
+      addCandidate( x, y, QgsPalLayerSettings::TopMiddle );
+    }
+
+    return !pal->isCanceled();
+  } );
+
+  return candidatesCreated;
+}
+
 std::vector< std::unique_ptr< LabelPosition > > FeaturePart::createCandidates( Pal *pal )
 {
   std::vector< std::unique_ptr< LabelPosition > > lPos;

diff --git a/src/core/pal/feature.h b/src/core/pal/feature.h
@@ -267,6 +267,14 @@ namespace pal
        */
       std::size_t createCandidatesForPolygon( std::vector<std::unique_ptr<LabelPosition> > &lPos, PointSet *mapShape, Pal *pal );
 
+      /**
+       * Generate candidates outside of polygon features.
+       * \param lPos pointer to an array of candidates, will be filled by generated candidates
+       * \param pal point to pal settings object, for cancellation support
+       * \returns the number of generated candidates
+       */
+      std::size_t createCandidatesOutsidePolygon( std::vector<std::unique_ptr<LabelPosition> > &lPos, Pal *pal );
+
       /**
        * Tests whether this feature part belongs to the same QgsLabelFeature as another
        * feature part.