[labeling] Fix some valid curved label placements are rejected

because we fail to correctly determine character angles, and
instead reject the placement as violating the max character in/out
angles

This happens predominantly in maps with geographic CRS, but is
also seen in large scale projected maps. The calculation for
line circle intersections used to determine curved label character
placement becomes numerically unstable when the map extent width/height
is small.

src/core/pal/feature.cpp

@@ -1405,10 +1405,12 @@ static std::unique_ptr< LabelPosition > _createCurvedCandidate( LabelPosition *l
 std::size_t FeaturePart::createCurvedCandidatesAlongLine( std::vector< std::unique_ptr< LabelPosition > > &lPos, PointSet *mapShape, bool allowOverrun, Pal *pal )
 {
   LabelInfo *li = mLF->curvedLabelInfo();
-
   // label info must be present
+  if ( !li )
+    return 0;
+
   const int characterCount = li->count();
-  if ( !li || characterCount == 0 )
+  if ( characterCount == 0 )
     return 0;
 
   // TODO - we may need an explicit penalty for overhanging labels. Currently, they are penalized just because they

src/core/pal/feature.h

@@ -56,6 +56,13 @@ namespace pal
   {
     public:
 
+      /**
+       * Constructor for LabelInfo
+       * \param characterHeight height of characters
+       * \param characterWidths vector of character widths
+       * \param maxinangle maximum acceptable in angle (in degrees)
+       * \param maxoutangle maximum acceptable out angle (in degrees)
+       */
       LabelInfo( double characterHeight, std::vector< double > characterWidths, double maxinangle = 20.0, double maxoutangle = -20.0 )
         : maxCharAngleInsideRadians( maxinangle * M_PI / 180 )
           // outside angle should be negative

src/core/pal/geomfunction.cpp

@@ -378,6 +378,21 @@ void GeomFunction::findLineCircleIntersection( double cx, double cy, double radi
     double x1, double y1, double x2, double y2,
     double &xRes, double &yRes )
 {
+  double multiplier = 1;
+  if ( radius < 10 )
+  {
+    // these calculations get unstable for small coordinates differences, e.g. as a result of map labeling in a geographic
+    // CRS
+    multiplier = 10000;
+    x1 *= multiplier;
+    y1 *= multiplier;
+    x2 *= multiplier;
+    y2 *= multiplier;
+    cx *= multiplier;
+    cy *= multiplier;
+    radius *= multiplier;
+  }
+
   double dx = x2 - x1;
   double dy = y2 - y1;
 
@@ -406,4 +421,10 @@ void GeomFunction::findLineCircleIntersection( double cx, double cy, double radi
     xRes = x1 + t * dx;
     yRes = y1 + t * dy;
   }
+
+  if ( multiplier != 1 )
+  {
+    xRes /= multiplier;
+    yRes /= multiplier;
+  }
 }