Introduce reference line type enum

The type of a reference line will determine how S and T coordinates have
to be calculated. Type POLYLINE is equal to the previous ST definition.
A new type POLYLINE_WITH_T_AXIS is introduced for improved ST handling.

Signed-off-by: Weiss David <david.weiss@in-tech.com>
Signed-off-by: Reinhard Biegel <reinhard.biegel@in-tech.com>

Author: weissdavid

osi_referenceline.proto

@@ -31,6 +31,21 @@ message ReferenceLine
     //
     optional Identifier id = 1;
 
+    optional Type type = 3;
+
+    enum Type
+    {
+        // ReferenceLine is a polyline, where the coordinates of points are
+        // calculated by projection onto the nearest point on the line.
+        //
+        POLYLINE = 0;
+
+        // ReferenceLine is a polyline, where the coordinates of points are
+        // calculated using the t axis definition.
+        //
+        POLYLINE_WITH_T_AXIS = 1;
+    }
+
     // Points comprising the polyline.
     //
     // At least 2 points must be given.
@@ -88,13 +103,22 @@ message ReferenceLine
     //
     // To describe points that are not directly on the polyline, a T coordinate
     // is added. T is the signed 2D distance between the point to describe (P)
-    // and a projected point (P_proj) on the polyline. The T axis (projecting
-    // axis) is the line going through P and the intersection point (I). I is
-    // defined as the intersection of both T axes of two consecutive
-    // ReferenceLinePoints (see example and image below for illustration). If
-    // both T axes of the neighboring ReferenceLinePoint are parallel (so no
-    // intersection point exists), the resulting T axis direction is equal to
-    // the T axis of these ReferenceLinePoints.
+    // and a projected point (P_proj) on the polyline. There are two ways of
+    // defining this point, depending on the ReferenceLine type.
+    //
+    // ## Nearest point (Type POLYLINE)
+    //
+    // The projection point is the nearest point on the polyline (this point might
+    // either be on a line segment or at an edge between two line segments).
+    //
+    // ## T axis definition (Type POLYLINE_WITH_T_AXIS)
+    //
+    // The T axis (projecting axis) is the line going through P and the
+    // intersection point (I). I is defined as the intersection of both
+    // T axes of two consecutive ReferenceLinePoints (see example and
+    // image below for illustration). If both T axes of the neighboring
+    // ReferenceLinePoint are parallel (so no intersection point exists), the
+    // resulting T axis direction is equal to the T axis of these ReferenceLinePoints.
     // The T coordinate of the point in question is then defined as
     // <code>hypot(P.X-P_proj.X,P.Y-P_proj.Y)</code>. The projected point P_proj
     // might either be on a line segment or at an edge between two line segments.
@@ -228,6 +252,8 @@ message ReferenceLine
         //
         // Also see image "S, T coordinates" at #poly_line for reference.
         //
+        // \note This field is only set if the type of the reference line is POLYLINE_WITH_T_AXIS.
+        //
         optional double t_axis_yaw = 3;
     }
 }