How does priminteriorweights work

[AntoineSfx]

input 0 is a poly

input 1 is a point (preferably inside the poly but it should work  if it's outside, even though it's not really an interesting case)

I have a ray SOP in input 2 that computes the hitprimUV and hitprim, the coordinates of the point inside the poly from input0

I'm successfully reconstructing the  projected point  in the variable sampled with this wrangle:

 

vector2 v= point(2,"hitprimuv",0);

vector sampled;
int res  = prim_attribute(0, sampled, "P", 0, v);  // result written in sampled, it's passed by reference

int smp = addpoint(0,sampled);
setpointgroup(0,"sampled",smp,1,"set");

int x;
int verts[];
float weights[];

int pi = priminteriorweights(0,0,v,  verts, weights);

i[]@verts = verts;
f[]@w     = weights;

 

 

This KINDA works: for a triangle and a square, I'm able to reconstruct the projected point with those 2 detail wrangles in succession:

#dw1

 

int verts[] = prim(0,"verts",0);
float w[] = prim(0,"w",0);
foreach(int idx; verts) {
setpointattrib(0,"w",idx,w[idx],"add");
}

#dw2

vector I  = set(0,0,0);
foreach(int idx; expandpointgroup(0,"")) {
vector v = point(0,"P",idx);
float  w = point(0,"w",idx);
I += w*v;
}

int np  = addpoint(0,I);
setpointgroup(0,"interior",np,1,"set");

 

which is the weighted sum of points with their barycentric coordinates

It is actually always equal to 1, which is good;

But, starting from 5 points and more, the verts array starts receiving more elements than the number of points, and the two reconstructed points no longer match.