Expose normals in libfive_mesh

[lf94]

Seems it's possible to compute normals #45 but they aren't exposed in any way.

[lf94]

From what I understand I need to use https://github.com/libfive/libfive/blob/master/libfive/include/libfive/eval/eval_deriv_array.hpp#L51

[lf94]

Which leads me to a nicer high level function here:

libfive_vec3 libfive_tree_eval_d(libfive_tree t, libfive_vec3 p)
{
    DerivArrayEvaluator e((Tree(t)));
    auto v = e.deriv({p.x, p.y, p.z});
    return {v.x(), v.y(), v.z()};
}

But I'm not sure what exactly when to invoke this. Do I need to traverse the whole SDF, wait until I hit a boundary (0), and then .... something?

Edit: https://jamie-wong.com/2016/07/15/ray-marching-signed-distance-functions/#surface-normals-and-lighting

Edit:

we’ll do an approximation by sampling points around the point on the surface

Aha!

It seems then I need to go into one of the walkers / meshers and add this calculation there when at the boundary?