Continuous collision detection between two moving tetrahedra

17
votes

My question is fairly simple. I have two tetrahedra, each with a current position, a linear speed in space, an angular velocity and a center of mass (center of rotation, actually).

Having this data, I am trying to find a (fast) algorithm which would precisely determine (1) whether they would collide at some point in time, and if it is the case, (2) after how much time they collided and (3) the point of collision.

Most people would solve this by doing triangle-triangle collision detection, but this would waste a few CPU cycles on redundant operations such as checking the same edge of one tetrahedron against the same edge of the other tetrahedron upon checking up different triangles. This only means I'll optimize things a bit. Nothing to worry about.

The problem is that I am not aware of any public CCD (continuous collision detection) triangle-triangle algorithm which takes self-rotation in account.

Therefore, I need an algorithm which would be inputted the following data:

  • vertex data for three triangles
  • position and center of rotation/mass
  • linear velocity and angular velocity

And would output the following:

  • Whether there is a collision
  • After how much time the collision occurred
  • In which point in space the collision occurred

Thanks in advance for your help.

7
physicscollision-detection
+1 just because the title is SO cool -- you couldn't have made it better if you were TRYING!-). (Sorry I don't have enough expertise in three-dimensional computational geometry to actually help here). – Alex Martelli
Yes it is a cool title. I don't remember enough maths to solve this but I believe you'll want to be solving a differential parametric equation modelling the current position (x,y,z) = (f(t),g(t),h(t)) for each object. You can optimize it by first finding if they're ever close enough for their to be a collision based on minimal spheres for each object. If they aren't, they don't collide. If they are, then you can do the complex calculations. – cletus
OP here. I would likely use other techniques to filter which pairs of objects actually need to be tested for collision (some call it broadphase). If no-one comes up with any real equations or a better way to do it, I'll just put everything in one distance equation (in respect to time) and write an algorithm that tries to find a solution to it. To be honest with you, I don't really feel like doing that especially if it has already been done. – x26

7 Answers

6
votes

The commonly used discrete collision detection would check the triangles of each shape for collision, over successive discrete points in time. While straightforward to compute, it could miss a fast moving object hitting another one, due to the collision happening between discrete points in time tested.

Continuous collision detection would first compute the volumes traced by each triangle over an infinity of time. For a triangle moving at constant speed and without rotation, this volume could look like a triangular prism. CCD would then check for collision between the volumes, and finally trace back if and at what time the triangles actually shared the same space.

When angular velocity is introduced, the volume traced by each triangle no longer looks like a prism. It might look more like the shape of a screw, like a strand of DNA, or some other non-trivial shapes you might get by rotating a triangle around some arbitrary axis while dragging it linearly. Computing the shape of such volume is no easy feat.

One approach might first compute the sphere that contains an entire tetrahedron when it is rotating at the given angular velocity vector, if it was not moving linearly. You can compute a rotation circle for each vertex, and derive the sphere from that. Given a sphere, we can now approximate the extruded CCD volume as a cylinder with the radius of the sphere and progressing along the linear velocity vector. Finding collisions of such cylinders gets us a first approximation for an area to search for collisions in.

A second, complementary approach might attempt to approximate the actual volume traced by each triangle by breaking it down into small, almost-prismatic sub-volumes. It would take the triangle positions at two increments of time, and add surfaces generated by tracing the triangle vertices at those moments. It's an approximation because it connects a straight line rather than an actual curve. For the approximation to avoid gross errors, the duration between each successive moments needs to be short enough such that the triangle only completes a small fraction of a rotation. The duration can be derived from the angular velocity.

The second approach creates many more polygons! You can use the first approach to limit the search volume, and then use the second to get higher precision.

If you're solving this for a game engine, you might find the precision of above sufficient (I would still shudder at the computational cost). If, rather, you're writing a CAD program or working on your thesis, you might find it less than satisfying. In the latter case, you might want to refine the second approach, perhaps by a better geometric description of the volume occupied by a turning, moving triangle -- when limited to a small turn angle.

1
votes

I have spent quite a lot of time wondering about geometry problems like this one, and it seems like accurate solutions, despite their simple statements, are way too complicated to be practical, even for analogous 2D cases.

But intuitively I see that such solutions do exist when you consider linear translation velocities and linear angular velocities. Don't think you'll find the answer on the web or in any book because what we're talking about here are special, yet complex, cases. An iterative solution is probably what you want anyway -- the rest of the world is satisfied with those, so why shouldn't you be?

1
votes

If you were trying to collide non-rotating tetrahedra, I'd suggest a taking the Minkowski sum and performing a ray check, but that won't work with rotation.

The best I can come up with is to perform swept-sphere collision using their bounding spheres to give you a range of times to check using bisection or what-have-you.

1
votes

Here's an outline of a closed-form mathematical approach. Each element of this will be easy to express individually, and the final combination of these would be a closed form expression if one could ever write it out:

1) The equation of motion for each point of the tetrahedra is fairly simple in it's own coordinate system. The motion of the center of mass (CM) will just move smoothly along a straight line and the corner points will rotate around an axis through the CM, assumed to be the z-axis here, so the equation for each corner point (parameterized by time, t) is p = vt + x + r(sin(wt+s)i + cos(wt + s)j ), where v is the vector velocity of the center of mass; r is the radius of the projection onto the x-y plane; i, j, and k are the x, y and z unit vectors; and x and s account for the starting position and phase of rotation at t=0.

2) Note that each object has it's own coordinate system to easily represent the motion, but to compare them you'll need to rotate each into a common coordinate system, which may as well be the coordinate system of the screen. (Note though that the different coordinate systems are fixed in space and not traveling with the tetrahedra.) So determine the rotation matrices and apply them to each trajectory (i.e. the points and CM of each of the tetrahedra).

3) Now you have an equation for each trajectory all within the same coordinate system and you need to find the times of the intersections. This can be found by testing whether any of the line segments from the points to the CM of a tetrahedron intersects the any of the triangles of another. This also has a closed-form expression, as can be found here.

Layering these steps will make for terribly ugly equations, but it wouldn't be hard to solve them computationally (although with the rotation of the tetrahedra you need to be sure not to get stuck in a local minimum). Another option might be to plug it into something like Mathematica to do the cranking for you. (Not all problems have easy answers.)

0
votes

Sorry I'm not a math boff and have no idea what the correct terminology is. Hope my poor terms don't hide my meaning too much.

Pick some arbitrary timestep.

Compute the bounds of each shape in two dimensions perpendicular to the axis it is moving on for the timestep.

For a timestep: If the shaft of those bounds for any two objects intersect, half timestep and start recurse in.

A kind of binary search of increasingly fine precision to discover the point at which a finite intersection occurs.

0
votes

Your problem can be cast into a linear programming problem and solved exactly.

First, suppose (p0,p1,p2,p3) are the vertexes at time t0, and (q0,q1,q2,q3) are the vertexes at time t1 for the first tetrahedron, then in 4d space-time, they fill the following 4d closed volume

V = { (r,t) | (r,t) = a0 (p0,t0) + … + a3 (p3,t0) + b0 (q0,t1) + … + b3 (q3,t1) }

Here the a0...a3 and b0…b3 parameters are in the interval [0,1] and sum to 1:

a0+a1+a2+a3+b0+b1+b2+b3=1

The second tetrahedron is similarly a convex polygon (add a ‘ to everything above to define V’ the 4d volume for that moving tetrahedron.

Now the intersection of two convex polygon is a convex polygon. The first time this happens would satisfy the following linear programming problem:

If (p0,p1,p2,p3) moves to (q0,q1,q2,q3) and (p0’,p1’,p2’,p3’) moves to (q0’,q1’,q2’,q3’) then the first time of intersection happens at points/times (r,t):

Minimize t0*(a0+a1+a2+a3)+t1*(b0+b1+b2+b3) subject to

0 <= ak <=1, 0<=bk <=1, 0 <= ak’ <=1, 0<=bk’ <=1, k=0..4
a0*(p0,t0) + … + a3*(p3,t0) + b0*(q0,t1) + … + b3*(q3,t1) 
  = a0’*(p0’,t0) + … + a3’*(p3’,t0) + b0’*(q0’,t1) + … + b3’*(q3’,t1)

The last is actually 4 equations, one for each dimension of (r,t). This is a total of 20 linear constraints of the 16 values ak,bk,ak', and bk'. If there is a solution, then 

(r,t)= a0*(p0,t0) + … + a3*(p3,t0) + b0*(q0,t1) + … + b3*(q3,t1)

Is a point of first intersection. Otherwise they do not intersect.

0
votes

Thought about this in the past but lost interest... The best way to go about solving it would be to abstract out one object. Make a coordinate system where the first tetrahedron is the center (barycentric coords or a skewed system with one point as the origin) and abstract out the rotation by making the other tetrahedron rotate around the center. This should give you parametric equations if you make the rotation times time. Add the movement of the center of mass towards the first and its spin and you have a set of equations for movement relative to the first (distance). Solve for t where the distance equals zero.

Obviously with this method the more effects you add (like wind resistance) the messier the equations get buts its still probably the simplest (almost every other collision technique uses this method of abstraction). The biggest problem is if you add any effects that have feedback with no analytical solution the whole equation becomes unsolvable.

Note: If you go the route of of a skewed system watch out for pitfalls with distance. You must be in the right octant! This method favors vectors and quaternions though, while the barycentric coords favors matrices. So pick whichever your system uses most effectively.