vector flows along edge of primitive

[CinnamonMetal]

How do you add a vector for a point that follows the edge of a primitive ? Suppose a point is on an edge, now I want a vector to point along the edge.  If I use set() I'm simply setting a vector for an axis, which I don't want.

 

Edited April 10, 2018 by CinnamonMetal

[Parboil]

 

You can use the neighbours() function.

int pt = 0;
vector pt_pos = point(0, "P", pt);
int neighbours[] = neighbours(0, pt);
vector neighbour_pos = point(0, "P", neighbours[0]);
vector along_edge = normalize(neighbour_pos - pt_pos);

 

[Atom]

And a quick way to visualize that is to assign the result to the normals. (v@N).

[CinnamonMetal]

int vp3 = vertexpoint(0,@vrtList[1]);
int neighbours = (0,vp3);
v@alongEdge = vp1 - vp3;

The attribute vrtList is an array containing the primitive vertices.

When displaying the attribute alongEdge the vector doesn't flow along the vertices, in this case; vertices three to vertices one. Instead the vector is directed out from the third vertices, as it's not following the edge of the primitive ?

I hope this illustration helps.  The red points are vertices; the yellow arrow is the vector that flows along the edge of the primitive which is what I want to achieve between vertices. 

Edited May 2, 2018 by CinnamonMetal