What is a fast algorithm for determining whether or not a point is inside a 3D mesh? For simplicity you can assume the mesh is all triangles and has no holes.
What I know so far is that one popular way of determining whether or not a ray has crossed a mesh is to count the number of ray/triangle intersections. It has to be fast because I am using it for a haptic medical simulation. So I cannot test all of the triangles for ray intersection. I need some kind of hashing or tree data structure to store the triangles in to help determine which triangle are relevant.
Also, I know that if I have any arbitrary 2D projection of the vertices, a simple point/triangle intersection test is all necessary. However, I’d still need to know which triangles are relevant and, in addition, which triangles lie in front of a the point and only test those triangles.
I solved my own problem. Basically, I take an arbitrary 2D projection (throw out one of the coordinates), and hash the AABBs (Axis Aligned Bounding Boxes) of the triangles to a 2D array. (A set of 3D cubes as mentioned by titus is overkill, as it only gives you a constant factor speedup.) Use the 2D array and the 2D projection of the point you are testing to get a small set of triangles, which you do a 3D ray/triangle intersection test on (see Intersections of Rays, Segments, Planes and Triangles in 3D) and count the number of triangles the ray intersection where the z-coordinate (the coordinate thrown out) is greater than the z-coordinate of the point. An even number of intersections means it is outside the mesh. An odd number of intersections means it is inside the mesh. This method is not only fast, but very easy to implement (which is exactly what I was looking for).