lunes, 1 de diciembre de 2008

Optimized Spatial Hashing for Collision Detection: Implementation

A couple of weeks ago, I implemented a method to detect collisions between any kind of body -rigid or deformable-. The method is based on the following paper:

M. Teschner, B. Heidelberger, M. Mueller, D. Pomeranets, M. Gross, "Optimized Spatial Hashing for Collision Detection of Deformable Objects," Proc. Vision, Modeling, Visualization VMV'03, Munich, Germany, pp. 47-54, Nov. 19-21, 2003.

Click here to get the paper.

Short conclusion and review after the implementation:

The collision detection method is based on an optimised spatial hashing of the object. I have implemented it for rigid bodies and soft bodies (2D and 3D) so you can use it for both. The method assumes that you can provide the surface vertices of the object and that you can also provide a collision primitive representation of the object. The collision primitives in case of soft bodies can be the surface tetrahedrons, for fluids you can inflate the particles, for cloths you can give a thickness to the triangles or even bounding spheres for triangles.

The method follows two main steps:

STEP 1:
Discretization and hashing of the surface vertices of the object. Each vertex is discretized with respect to a cell (an axis aligned box). The coordinates of the vertex (x,y,z) are divided by the given cell size and rounded down to the next integer. As in the paper the discretized vertex is: (i,j,k) where i = x/l, j = y/l and k = z/l. Then, a hash function maps the discretized vertex to a 1D index h. I used the following code:

inline unsigned int hash(int xi, int yi, int zi) {

unsigned int h = (xi * 73856093)^(yi * 19349663)^(zi * 83492791) ; //same as in paper
return h % sizeOfHashTable
} //sizeOfHashTable=big value
}

STEP 2:
Discretization and hashing of the collision primitives as in step 1. For example, for each surface tetrahedron of the object, I compute the AABB and discretize the minimum and maximum values as we did for the vertices. Hash values are computed for all cells affected by the AABB of the tetrahedron. This is a tricky part and I can explain a bit further if you want. Then, all the vertices found at the according hash table index are tested for intersection. The intersection test can be: vertices inside sphere, tetrahedron or box, or if continuous collision detection, you can use segments versus sphere, tetrahedron or box.

The rest of the paper determines an optimal size of the hash table and optimal hash function.

CONCLUSION:
Good stuff: The method handles collision detection for deformable objects (cloths and volumetric objects) and rigid bodies. The method handles self-collisions since it hashes all collision primitives and all vertices of all objects in the same table.

Bad stuff: It is not edge sensitive which means that it does not detect collision with edges. Object penetration happens when edges are not detected. The method detects if vertices are inside the object but not edges. Authors of the paper claim that this is not a problem.. well, not really, if the objects uses a low resolution with big triangles (like the case of triangles in videogames) the penetrations are really evident and things are very unrealistic.
Besides, it is difficult to compute a proper collision response. If a vertex is inside the object, which is the direction of the repulsion normal ? To solve this, I use previous vertex positions to compute segments and handle this problem.

I am not quite sure about the optimization at the end of section 4.2. It is not clear explained so I am not sure if I got it right (well, it works fine, so I think it is correct). But, is there a big signifcance for this optimization for small hash tables ?

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