gistfile1.txt

// select the face direction in the tangent plane to `this_face`
// most closely aligned with `v`

#include <limits>

glm::ivec3 get_closest_tangent(int this_face, glm::vec3 v) {

        glm::ivec3 best;
        float bestdot = std::numeric_limits<float>::lowest();
        for (int face = 0; face < 6; face++) {
                // exclude normal direction and its inverse
                if (face == this_face || face == (this_face ^ 1))
                        continue;

                auto vec = surface_index_to_normal(face);
                auto val = glm::dot(glm::vec3(vec), v);
                if (val > bestdot) {
                        best = vec;
                        bestdot = val;
                }
        }

        return best;
}