Add new angled-point marker placement mode for lines

This adds a new mode called 'angled-point' to the marker-placement modes.
The full list of modes is then:
  point, angled-point, interior, line, vertex-first, vertex-last

Angled point is identical to point, except that when placing a marker on
a line, the marker's angle is taken from the angle of the line segment.

There is another possible use of the "angled-point" concept for polygons,
and that is for placing labels on stand (aka erf) polygons. By computing
a dominant angle for a mostly rectangular polygon, this can produce quite
good results. I'm not sure whether I should implement that now, or if I
could do that later.

include/mapnik/geom_util.hpp

@@ -35,6 +35,12 @@
 #include <vector>
 #include <algorithm>
 
+// boost
+#pragma GCC diagnostic push
+#include <mapnik/warning_ignore.hpp>
+#include <boost/optional.hpp>
+#pragma GCC diagnostic pop
+
 namespace mapnik
 {
 template <typename T>
@@ -298,7 +304,7 @@ bool hit_test_first(PathType & path, double x, double y)
 namespace label {
 
 template <typename PathType>
-bool middle_point(PathType & path, double & x, double & y)
+bool middle_point(PathType & path, double & x, double & y, boost::optional<double&> angle = boost::none)
 {
     double x0 = 0;
     double y0 = 0;
@@ -319,6 +325,10 @@ bool middle_point(PathType & path, double & x, double & y)
             double r = (mid_length - dist)/seg_length;
             x = x0 + (x1 - x0) * r;
             y = y0 + (y1 - y0) * r;
+            if (angle)
+            {
+                *angle = atan2(y1 - y0, x1 - x0);
+            }
             break;
         }
         dist += seg_length;

include/mapnik/markers_placement.hpp

@@ -49,6 +49,10 @@ public:
         case MARKER_POINT_PLACEMENT:
             construct(&point_, locator, detector, params);
             break;
+        case MARKER_ANGLED_POINT_PLACEMENT:
+            construct(&point_, locator, detector, params);
+            point_.use_angle(true);
+            break;
         case MARKER_INTERIOR_PLACEMENT:
             construct(&interior_, locator, detector, params);
             break;
@@ -70,6 +74,7 @@ public:
         {
         default:
         case MARKER_POINT_PLACEMENT:
+        case MARKER_ANGLED_POINT_PLACEMENT:
             destroy(&point_);
             break;
         case MARKER_INTERIOR_PLACEMENT:
@@ -94,6 +99,7 @@ public:
         {
         default:
         case MARKER_POINT_PLACEMENT:
+        case MARKER_ANGLED_POINT_PLACEMENT:
             return point_.get_point(x, y, angle, ignore_placement);
         case MARKER_INTERIOR_PLACEMENT:
             return interior_.get_point(x, y, angle, ignore_placement);

include/mapnik/markers_placements/point.hpp

@@ -38,11 +38,18 @@ public:
         : markers_basic_placement(params),
           locator_(locator),
           detector_(detector),
-          done_(false)
+          done_(false),
+          use_angle_(false)
     {
         locator_.rewind(0);
     }
 
+    // Use angle of line
+    void use_angle(bool enable)
+    {
+        use_angle_ = enable;
+    }
+
     // Start again at first marker. Returns the same list of markers only works when they were NOT added to the detector.
     void rewind()
     {
@@ -58,9 +65,11 @@ public:
             return false;
         }
 
+        angle = 0;
+
         if (this->locator_.type() == geometry::geometry_types::LineString)
         {
-            if (!label::middle_point(this->locator_, x, y))
+            if (!label::middle_point(this->locator_, x, y, use_angle_ ? boost::optional<double&>(angle) : boost::none))
             {
                 this->done_ = true;
                 return false;
@@ -75,8 +84,6 @@ public:
             }
         }
 
-        angle = 0;
-
         if (!this->push_to_detector(x, y, angle, ignore_placement))
         {
             return false;
@@ -90,6 +97,7 @@ protected:
     Locator & locator_;
     Detector & detector_;
     bool done_;
+    bool use_angle_;
 
     // Checks transformed box placement with collision detector.
     // returns false if the box:

include/mapnik/symbolizer_enumerations.hpp

@@ -105,6 +105,7 @@ enum marker_placement_enum : std::uint8_t
     MARKER_LINE_PLACEMENT,
     MARKER_VERTEX_FIRST_PLACEMENT,
     MARKER_VERTEX_LAST_PLACEMENT,
+    MARKER_ANGLED_POINT_PLACEMENT,
     marker_placement_enum_MAX
 };
 

src/symbolizer_enumerations.cpp

@@ -67,6 +67,7 @@ static const char * marker_placement_strings[] = {
     "line",
     "vertex-first",
     "vertex-last",
+    "angled-point",
     ""
 };