border edge query in VEX

[rich_lord]

I need to find out if an edge is a border edge in VEX. Ive tried reading and researching this for several hours, but am getting nowhere.

 

I have this geo:

 

 

I know the point numbers, but thats about it. How can I tell the the edge defined by points 19 and 9 is a border edge, whereas the edge defined by the points 19 and 8 are not?

 

 

[mestela]

As I understand it there's no direct way in vex, but you can use neighbourcount(). Obviously if you have faces with 3/4/5 verts, it'll break.

 

int nc = neighbourcount(0,@ptnum);
if (nc<4) {
  @P.y += 1;
}

The other way is to use a group to detect unshared edges, but I don't know how well it performs with high polycounts.

 

Waiting now for Anim to show us how to do it cleanly in vex in 3... 2...  

 

 

find_edges.hipnc

[rich_lord]

Thoose methods sadly don't work. They fail in the example above as line 19, 8 reads as a border edge.

 

I have to convert edge groups to points when I'm in VEX as far as I can tell, so grouping by border edge is useless in VEX.

 

I've tried to understand half edges. But it just wont go into my brain! I cant tell if its useful for this.

 

What I'm trying to do if it helps, is to look at a point, and get vectors pointing towards any other connected point, but only along border edges.

Edited April 1, 2016 by rich_lord

[anim]

here is a way with half edges:

current code is for Detail Wrangle, but you can use that function anywhere

int isBorderEdge(int pt0, pt1)
{
    int hedge = pointhedge(0, pt0, pt1); // try finding hedge
    if (hedge == -1 )
    {
        hedge = pointhedge(0, pt1, pt0);  // try reversed hedge
        if (hedge == -1 ) return -1;  // invalid hedge
    }
    return hedge_equivcount(0, hedge) == 1; // 1 if border edge
}

int pt0 = 9;
int pt1 = 19;
int isborder = isBorderEdge(pt0, pt1);
printf("Edge %d-%d is%s edge\n", pt0, pt1, isborder == 1 ? " a border" : isborder == -1 ? " not an" : " not a border");

 

[rich_lord]

Amazing - thanks anim. Works great in my point wrangle!

Edited April 2, 2016 by rich_lord