SVG++ provides option to automatically calculate marker symbols orientation on lines (path, line, polyline or polygon element).

Markers Policy turns on/off and configures marker position calculations. Marker Events Policy defines how marker positions are passed to the user code.

Markers Policy Concept

struct makers_policy_concept
  typedef /* ... */ directionality_policy;

  static const bool calculate_markers = /* true or false */;
  static const bool always_calculate_auto_orient = /* true or false */;
calculate_markers = true
Enables marker position calculations. If calculate_markers = false, then other class members aren’t used.

Marker properties define which vertices contain markers. Depending on the value of orient attribute of marker element, marker orientation may be fixed or calculated by line geometry (auto value).

always_calculate_auto_orient = true
In this case marker orientation is calculated for each vertex (orentation required in orient=”auto” case).
always_calculate_auto_orient = false
In this case, before processing each line element, marker_get_config method of Marker Events Policy is called, to request from user code which verteces markers are required (user code should know this from marker properties). Marker positions are returned by Marker Events Policy only for vertices, for which user code requested marker calculations.
Class that defines how marker orientation is calculated and passed. By default marker orientation is double value, containing angle in radians.

File svgpp/policy/markers.hpp contains some predefined Markers Policies: policy::markers::calculate_always, policy::markers::calculate and policy::markers::raw. policy::markers::raw used by default disables automatic marker calculation.

Named class template parameter for Markers Policy is markers_policy.

Marker Events Policy Concept

namespace svgpp
  enum marker_vertex { marker_start, marker_mid, marker_end };
  enum marker_config { marker_none, marker_orient_fixed, marker_orient_auto };
struct marker_events_policy_concept
  typedef /* ... */ context_type;

  static void marker_get_config(context_type & context,
    marker_config & start, marker_config & mid, marker_config & end);

  static void marker(context_type & context, marker_vertex v,
    number_type x, number_type y, directionality_type directionality, unsigned marker_index);
  static void marker(context_type & context, marker_vertex v,
    number_type x, number_type y, tag::orient_fixed directionality, unsigned marker_index);

marker_get_config method is called if makers_policy::always_calculate_auto_orient = false. User code must set for each vertex type (start, mid and end), whether it wants to calculate orientation and position (marker_orient_auto), position only (marker_orient_fixed) or neither of them (marker_none).

marker method is called for each vertex calculated with this arguments:

x and y - marker position

v - vertex type (marker_start, marker_mid or marker_end)

directionality - marker orientation (by default double value in radians).
If marker_get_config returned marker_orient_fixed for this type of vertex, then tag tag::orient_fixed is passed instead of orientation value.
Marker orientations aren’t always calculated and passed in the order, because orientation of marker on first vertex of subpath may be calculated only after subpath is finished. marker_index is the 0-based index of the marker in drawing order.

Named class template parameter for Marker Events Policy is marker_events_policy.

Default Marker Events Policy (policy::marker_events::forward_to_method) fowards calls to its static methods to context object methods.


class Context
  void marker(svgpp::marker_vertex v, double x, double y, double directionality, unsigned marker_index)
    if (markers_.size() <= marker_index)
      markers_.resize(marker_index + 1);
    auto & m = markers_[marker_index];
    m.v = v;
    m.x = x;
    m.y = y;
    m.directionality = directionality;

  struct MarkerPosition
    svgpp::marker_vertex v;
    double x, y, directionality;

  std::vector<MarkerPosition> markers_;

void load()
    /* ... */
  >::load_document(xml_root_element, context);