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How to calculate edge normal like PolyExtrude


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Hello there!

How would you calculate the vector depicted as the red arrows in the image.

PolyExtrude node uses it when given an unshared edge group.

I've tried a polyframe's tangentu and tangent crossed in vex with the Normal. Without luck.

It doesn't seem to be just a continuation of each points inner edges (the not unshared ones.... :/ ). But can't wrap my head around where to go with it.



Edited by bastian
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Add a pt. wrangle, you will need to compute 2 vectors:

First vector is a cross between v@N and a vector {0,1,0}.
Second vector is a cross between first vector and v@N.
The second vector should be the one you are looking for.


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Prim wrangle (tangent)

int vtc[];
vtc = primvertices(0, @primnum);

int n = len(vtc);
int jm1, j, jp1;
int pm1, p, pp1;
vector A,B;
for (int i = 0; i < n; i++)
    jm1 = i - 1;
    j = i;
    jp1 = i + 1;
    if (chi('mode') == 0)
        jm1 %= (n-1);
        jp1 %= (n-1);
    } else
        jm1 = max(0, jm1);
        jp1 = min(n-1, jp1);
    pm1 = vertexpoint(0, vtc[jm1]);
    p = vertexpoint(0, vtc[j]);
    pp1 = vertexpoint(0, vtc[jp1]);
    A = point(0, 'P', p) - point(0, 'P', pm1);
    B = point(0, 'P', pp1) - point(0, 'P', p);
    setpointattrib(0, 'tangentu', p, normalize(lerp(A, B, 0.5))); 

Point wrang-set normal by tangent

vector up = chv('up');

v@N = normalize(cross(v@tangentu, up));

if (chi('reverse'))
    v@N *= -1;


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Damn, had a hunch Odforce was the place to ask this question.

I'm afk but just gotten done testing @islamooss suggestion on a graphing calculator app I found.

@Librarianserve this up! It's a little more than I have patience testing on the app but I'll test it first thing in the morning!

Thank you!


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So I tried your suggestions and both don't work on corners and place where two edges meet, or on edges "facing upwards".
The picture in the post doesn't quite show how inconsistent the mesh is. My mistake.

I've done another pretty "hacky" way of just doing a extra PolyExtrude, wrangle a vector to the extruded edge.
But can imagine it breaking easily with a denser mesh.

I can definitely work with the results here, but would love a consistent method as it isn't the first time I've come across this problem.
Thanks again!




Edited by bastian
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