[svg] Add scale function and examples of it

doc/imports.qbk

@@ -151,6 +151,7 @@
 [import src/examples/geometries/register/multi_polygon.cpp]
 
 [import src/examples/io/svg.cpp]
+[import src/examples/io/svg_mapper_scale.cpp]
 [import src/examples/io/wkt.cpp]
 [import src/examples/io/read_wkt.cpp]
 

doc/reference/io/svg.qbk

@@ -12,5 +12,9 @@
 [svg_mapper]
 [svg_mapper_output]
 
+[heading Example with arrows and groups]
+[svg_mapper_scale]
+[svg_mapper_scale_output]
+
 [heading See also]
 * [link geometry.reference.io.svg.svg The SVG manipulator]

doc/src/examples/io/Jamfile

@@ -15,5 +15,6 @@ project boost-geometry-doc-example-io
     ;
 
 exe svg : svg.cpp ;
+exe svg_mapper_scale : svg_mapper_scale.cpp ;
 exe wkt : wkt.cpp ;
 exe read_wkt : read_wkt.cpp ;

doc/src/examples/io/svg.cpp

@@ -37,7 +37,7 @@ int main()
     std::ofstream svg("my_map.svg");
     boost::geometry::svg_mapper<point_type> mapper(svg, 400, 400);
 
-    // Add geometries such that all these geometries fit on the map
+    // Add geometries such that all these geometries fit exactly on the map
     mapper.add(a);
     mapper.add(b);
     mapper.add(c);

doc/src/examples/io/svg_mapper_scale.cpp

@@ -0,0 +1,75 @@
+// Boost.Geometry
+// QuickBook Example
+
+// Copyright (c) 2020 Barend Gehrels, Amsterdam, the Netherlands.
+
+// Use, modification and distribution is subject to the Boost Software License,
+// Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
+// http://www.boost.org/LICENSE_1_0.txt)
+
+//[svg_mapper_scale
+//` Shows the usage of svg_mapper with the scale function
+
+#include <iostream>
+#include <fstream>
+
+#include <boost/geometry.hpp>
+#include <boost/geometry/geometries/point_xy.hpp>
+
+int main()
+{
+    // Specify the basic type
+    using point_type = boost::geometry::model::d2::point_xy<double>;
+
+    // Declare linestrings and set their values
+    boost::geometry::model::linestring<point_type> a, b, c;
+    a.push_back({1, 0});
+    a.push_back({3, 3});
+
+    b.push_back({5, 0});
+    b.push_back({3, 2});
+
+    c.push_back({4, 5});
+    c.push_back({3, 4});
+
+    // Declare a stream and an SVG mapper
+    std::ofstream svg("my_map.svg");
+    boost::geometry::svg_mapper<point_type> mapper(svg, 400, 400);
+
+    // Add geometries such that all these geometries fit exactly on the map
+    mapper.add(a);
+    mapper.add(b);
+    mapper.add(c);
+
+    // Zoom out 5% to have a bit margin around the geometries
+    // This also forces writing the SVG header.
+    mapper.scale(0.95);
+
+    // Write a marker definition.
+    svg << "<defs>";
+    svg << "<marker id=\"arrowhead\" markerWidth=\"5\" markerHeight=\"3.5\""
+           " refX=\"0\" refY=\"1.75\" orient=\"auto\">"
+           " <polygon points=\"0 0, 5 1.75, 0 3.5\"/></marker>";
+    svg << "</defs>";
+
+    // Group the first two geometries
+    svg << "<g>";
+    mapper.map(a, "opacity:0.5;stroke-width:1;stroke:gray;marker-end:url(#arrowhead)");
+    mapper.map(b, "opacity:0.5;stroke-width:3;stroke:gray;marker-end:url(#arrowhead)");
+    svg << "</g>";
+
+    mapper.map(c, "opacity:0.5;stroke-width:5;stroke:red;marker-end:url(#arrowhead)");
+
+    return 0;
+}
+
+//]
+
+
+//[svg_mapper_scale_output
+/*`
+Output:
+
+[$img/io/svg_mapper_scale.png]
+*/
+//]

include/boost/geometry/io/svg/svg_mapper.hpp

@@ -368,6 +368,35 @@ public :
         }
     }
 
+    /*!
+    \brief Optional step, zoom in or zoom out, and forces writing the header. After writing the
+      transformation, geometries should not be added, and mapping can begin. After writing the
+      header, other SVG elements can be streamed (for example defs) which can later be used
+      in styles.
+      Call this function after add and before map or text.
+    \param scale A value larger than 1.0 zooms in, a value smaller than 1.0 zooms out.
+      A scale of 1.0 doesn't change any scaling, but still writes the SVG header.
+    */
+    void scale(double scale = 1.0)
+    {
+        if (scale != 1.0 && scale > 0)
+        {
+            // Zoom out (scale < 1) or zoom in (scale > 1).
+            // Typically, users might specify 0.95, to have a small margin around their geometries.
+            auto& b = m_bounding_box;
+            auto const w = geometry::get<1, 0>(b) - geometry::get<0, 0>(b);
+            auto const h = geometry::get<1, 1>(b) - geometry::get<0, 1>(b);
+
+            auto const& m = std::max(w, h) * (1.0 - scale);
+            geometry::set<0, 0>(b, geometry::get<0, 0>(b) - m);
+            geometry::set<0, 1>(b, geometry::get<0, 1>(b) - m);
+            geometry::set<1, 0>(b, geometry::get<1, 0>(b) + m);
+            geometry::set<1, 1>(b, geometry::get<1, 1>(b) + m);
+        }
+
+        init_matrix();
+    }
+
     /*!
     \brief Maps a geometry into the SVG map using the specified style
     \tparam Geometry \tparam_geometry

test/io/svg/svg.cpp

@@ -131,6 +131,8 @@ void test_all()
         mapper.add(m_po);
         mapper.add(var);
 
+        mapper.scale(0.95);
+
         mapper.map(pt, style);
         mapper.map(b, style);
         mapper.map(s, style);

test/to_svg.hpp

@@ -259,11 +259,11 @@ inline void geom_to_svg(std::string const& wkt1, std::string const& wkt2, std::s
 }
 
 struct to_svg_assign_policy
-    : boost::geometry::detail::overlay::assign_null_policy
 {
     static bool const include_no_turn = false;
     static bool const include_degenerate = false;
     static bool const include_opposite = false;
+    static bool const include_start_turn = false;
 };
 
 template <typename G>