public static double ComputeUnscaledFormFactor(
this Polygon3d polygon,
V3d p, V3d n,
double eps = 1e-6)
{
var vc = polygon.PointCount;
V3d[] cpa = new V3d[vc + 1];
var cc = 0;
var pb = polygon[0] - p;
var hb = Vec.Dot(pb, n); bool hbp = hb > eps, hbn = hb < -eps;
if (hb >= -eps)
{
cpa[cc++] = pb;
}
var p0 = pb; var h0 = hb; var h0p = hbp; var h0n = hbn;
for (int vi = 1; vi < vc; vi++)
{
var p1 = polygon[vi] - p; var h1 = Vec.Dot(p1, n);
bool h1p = h1 > eps, h1n = h1 < -eps;
if (h0p && h1n || h0n && h1p)
{
cpa[cc++] = p0 + (p1 - p0) * (h0 / (h0 - h1));
}
if (h1 >= -eps)
{
cpa[cc++] = p1;
}
p0 = p1; h0 = h1; h0p = h1p; h0n = h1n;
}
if (h0p && hbn || h0n && hbp)
{
cpa[cc++] = p0 + (pb - p0) * (h0 / (h0 - hb));
}
var cpr = cpa.Map(cc, v => v.Length);
var cv = Vec.Cross(cpa[0], cpa[cc - 1]);
double ff = Vec.Dot(n, cv)
* Fun.AcosClamped(Vec.Dot(cpa[0], cpa[cc - 1])
/ (cpr[0] * cpr[cc - 1]))
/ cv.Length;
for (int ci = 0; ci < cc - 1; ci++)
{
cv = Vec.Cross(cpa[ci + 1], cpa[ci]);
ff += Vec.Dot(n, cv)
* Fun.AcosClamped(Vec.Dot(cpa[ci + 1], cpa[ci])
/ (cpr[ci + 1] * cpr[ci]))
/ cv.Length;
}
return(ff);
}