Ray-triangle intersection

Question

I saw that Fast Minimum Storage Ray/Triangle Intersection by Moller and Trumbore is frequently recommended.

The thing is, I don\'t mind pre-computing and storing any amo

Answer 1

I have done a lot of benchmarks, and I can confidently say that the fastest (published) method is the one invented by Havel and Herout and presented in their paper Yet Faster Ray-Triangle Intersection (Using SSE4). Even without using SSE it is about twice as fast as Möller and Trumbore's algorithm.

My C implementation of Havel-Herout:

    typedef struct {
        vec3 n0; float d0;
        vec3 n1; float d1;
        vec3 n2; float d2;
    } isect_hh_data;

    void
    isect_hh_pre(vec3 v0, vec3 v1, vec3 v2, isect_hh_data *D) {
        vec3 e1 = v3_sub(v1, v0);
        vec3 e2 = v3_sub(v2, v0);
        D->n0 = v3_cross(e1, e2);
        D->d0 = v3_dot(D->n0, v0);

        float inv_denom = 1 / v3_dot(D->n0, D->n0);

        D->n1 = v3_scale(v3_cross(e2, D->n0), inv_denom);
        D->d1 = -v3_dot(D->n1, v0);

        D->n2 = v3_scale(v3_cross(D->n0, e1), inv_denom);
        D->d2 = -v3_dot(D->n2, v0);
    }

    inline bool
    isect_hh(vec3 o, vec3 d, float *t, vec2 *uv, isect_hh_data *D) {
        float det = v3_dot(D->n0, d);
        float dett = D->d0 - v3_dot(o, D->n0);
        vec3 wr = v3_add(v3_scale(o, det), v3_scale(d, dett));
        uv->x = v3_dot(wr, D->n1) + det * D->d1;
        uv->y = v3_dot(wr, D->n2) + det * D->d2;
        float tmpdet0 = det - uv->x - uv->y;
        int pdet0 = ((int_or_float)tmpdet0).i;
        int pdetu = ((int_or_float)uv->x).i;
        int pdetv = ((int_or_float)uv->y).i;
        pdet0 = pdet0 ^ pdetu;
        pdet0 = pdet0 | (pdetu ^ pdetv);
        if (pdet0 & 0x80000000)
            return false;
        float rdet = 1 / det;
        uv->x *= rdet;
        uv->y *= rdet;
        *t = dett * rdet;
        return *t >= ISECT_NEAR && *t <= ISECT_FAR;
    }

Answer 2

As others have mentioned, the most effective way to speed things up is to use an acceleration structure to reduce the number of ray-triangle intersections needed. That said, you still want your ray-triangle intersections to be fast. If you're happy to precompute stuff, you can try the following:

Convert your ray lines and your triangle edges to Plücker coordinates. This allows you to determine if your ray line passes through a triangle at 6 multiply/add's per edge. You will still need to compare your ray start and end points with the triangle plane (at 4 multiply/add's per point) to make sure it actually hits the triangle.

Worst case runtime expense is 26 multiply/add's total. Also, note that you only need to compute the ray/edge sign once per ray/edge combination, so if you're evaluating a mesh, you may be able to use each edge evaluation twice.

Also, these numbers assume everything is being done in homogeneous coordinates. You may be able to reduce the number of multiplications some by normalizing things ahead of time.

Answer 3

Found this article by Dan Sunday:

Based on a count of the operations done up to the first rejection test, this algorithm is a bit less efficient than the MT (Möller & Trumbore) algorithm, [...]. However, the MT algorithm uses two cross products whereas our algorithm uses only one, and the one we use computes the normal vector of the triangle's plane, which is needed to compute the line parameter rI. But, when the normal vectors have been precomputed and stored for all triangles in a scene (which is often the case), our algorithm would not have to compute this cross product at all. But, in this case, the MT algorithm would still compute two cross products, and be less efficient than our algorithm.

http://geomalgorithms.com/a06-_intersect-2.html

Answer 4

One suggestion could be to implement the octree (http://en.wikipedia.org/wiki/Octree) algorithm to partition your 3D Space into very fine blocks. The finer the partitioning the more memory required, but the better accuracy the tree gets.

You still need to check ray/triangle intersections, but the idea is that the tree can tell you when you can skip the ray/triangle intersection, because the ray is guaranteed not to hit the triangle.

However, if you start moving your triangle around, you need to update the Octree, and then I'm not sure it's going to save you anything.