# A Beautiful Ray/Mesh Intersection Algorithm

2014-05-30 Tavian Barnes Comments

In my last post, I talked about a beautiful method for computing ray/triangle intersections. In this post, I will extend it to computing intersections with triangle fans. Since meshes are often stored in a corner table, which is simply an array of triangle fans, this gives an efficient algorithm for ray tracing triangle meshes.

The aforementioned algorithm computes ray/triangle intersections with 1 division, 20 multiplications, and up to 18 additions. It also required storing an affine transformation matrix, which takes up 4/3 as much space as just storing the vertices. But if we have many triangles which all share a common vertex, we can exploit that structure to save time and memory.

Say our triangle fan is composed of triangles $ABC$, $ACD$, $ADE$, etc. As before, we compute

$P_{ABC} = \begin{bmatrix} \overrightarrow{AB} & \overrightarrow{AC} & \overrightarrow{AB} \times \overrightarrow{AC} & \overrightarrow{A} \\ 0 & 0 & 0 & 1 \end{bmatrix}^{-1}.$

Computing the change of basis from here to the next triangle is even easier. We want to find new coordinates $\langle u', v', w' \rangle$ such that

$\begin{array}{ccccccccc} \vec{p} &=& u\hphantom{'}\,\overrightarrow{AB} &+& v\hphantom{'}\,\overrightarrow{AC} &+& w\hphantom{'}\,\overrightarrow{AB}\times\overrightarrow{AC} &+& \overrightarrow{A} \\ {} &=& u'\,\overrightarrow{AC} &+& v'\,\overrightarrow{AD} &+& w'\,\overrightarrow{AC}\times\overrightarrow{AD} &+& \overrightarrow{A}. \end{array}$

Two things are apparent: the first is that there is no further translation to perform because both coordinate systems have their origin at $\overrightarrow{A}$. The second is that only $u'$ depends on $v$. This means the matrix $P^{ABC}_{ACD}$ that takes us to the new basis has the special form

$P^{ABC}_{ACD} = P_{ACD}\,P^{-1}_{ABC} = \begin{bmatrix} a & 1 & d & 0 \\ b & 0 & e & 0 \\ c & 0 & f & 0 \\ 0 & 0 & 0 & 1 \end{bmatrix},$

so we only need to store two of its columns, and transforming the ray into the new space can be done much faster than with a full matrix multiplication. The following transformation is applied to both the ray origin and direction:

\begin{aligned} u' &= a\,u + v + d\,w \\ v' &= b\,u + e\,w \\ w' &= c\,u + f\,w. \end{aligned}

In the end, for a triangle fan with $n$ vertices, the ray intersection can be computed with

\begin{aligned} n & \text{ divisions,} \\ 6 + 14\,n & \text{ multiplications, and} \\ 7 + 11\,n & \text{ additions}. \end{aligned}

The multiplications and additions are also easily vectorisable. The storage requirement is $6\,(n + 1)$ floating-point values, which is equivalent to storing all the vertices and precomputed normals.

The implementation of this algorithm in my ray tracer Dimension is available here.

Mo 2014-05-30

Pretty cool, I wonder if this can be extended to triangle strips as well. That one might be a lot more challenging though.

 Also I wonder if these dollar signs will break your page with a MathJax injection. I imagine it's more intelligent than that.

Mo 2014-05-30

Nope. :)

Tavian Barnes 2014-05-30

Haha yeah the MathJax is all done with a WordPress plugin, I write $m^at_h$ and it prettifies it. I'm hoping it doesn't scan the page and replace $m^at_h$ with MathJax.

And you're right, it looks hard to do this well with triangle strips because they don't all share a common vertex. The above still works except the translation comes back which wastes time and space.