Collision points

Discuss any questions about BEPUphysics or problems encountered.
Post Reply
AlTerz93
Posts: 2
Joined: Tue Jan 21, 2020 8:27 am

Collision points

Post by AlTerz93 » Wed Jan 22, 2020 10:35 am

Hi everybody!
I've a question about collision points. Where can I find them? I want to show them in my simulation but I'm not able to find where their positions are stored.

Thanks in advance

Norbo
Site Admin
Posts: 4888
Joined: Tue Jul 04, 2006 4:45 am

Re: Collision points

Post by Norbo » Wed Jan 22, 2020 11:54 pm

The engine doesn't store anything it doesn't use internally, so the only place contacts are 'stored' is a processed version in the solver's prestep data.

Custom INarrowPhaseCallbacks can be used to collect contact information as it is generated by the narrow phase. Some demos, like the ContactEventsDemo and TankDemo, use the callbacks to collect information about contacts.

The ContactLineExtractor in the DemoRenderer takes the processed version of constraint prestep data and uses it to visualize contacts and joints. It works, but it's pretty much the lowest level option possible.

More recently, IDirectContactDataExtractor was added for extracting contact data from the solver. It's still very low level- it was added with the intention that other abstractions could be built on top of it without introducing intermediate overhead- but it is specialized for contact data, unlike the DemoRenderer utility.

An example of its use taken from one of my projects, where contactExtractor is an IDirectContactDataExtractor implementation:

Code: Select all

                var body = simulation.Bodies.GetBodyReference(Sensors[sensorIndex]);
                contactExtractor.Reset(body.Handle);
                for (int constraintIndex = 0; constraintIndex < body.Constraints.Count; ++constraintIndex)
                {
                    ref var constraint = ref body.Constraints[constraintIndex];
                    ref var location = ref simulation.Solver.HandleToConstraint[constraint.ConnectingConstraintHandle];
                    if (simulation.NarrowPhase.TryGetContactConstraintAccessor(location.TypeId, out var accessor))
                    {
                        accessor.ExtractContactData(constraint.ConnectingConstraintHandle, simulation.Solver, ref contactExtractor);
                    }
                }
The basic idea is enumerating all constraints associated with a body, and using a type-specific contact accessor to parse the AOSOA solver data into a more convenient representation. The IDirectContactDataExtractor itself specifies what data it is looking for as properties (which, in combination with a struct type and inlining, will be considered JIT-time constants within the contact accessor, avoiding unnecessary work) and then has a bunch of callbacks for different types of data.

It's not the easiest thing to use, but it is pretty close to zero overhead and gives you direct access to the solver data in a type-aware way. I've been intending to create a demo showing how to wrap it in a more easily consumed abstraction, but I haven't gotten around to it.

AlTerz93
Posts: 2
Joined: Tue Jan 21, 2020 8:27 am

Re: Collision points

Post by AlTerz93 » Thu Jan 30, 2020 10:11 am

Thanks for the reply!

I tried to implement it but nothing works. It is probably due to my lack of knowledge on the topic.
In particular I had difficulties creating the contactExtractor.

How should I proceed to implement it?

Thanks in advance

Norbo
Site Admin
Posts: 4888
Joined: Tue Jul 04, 2006 4:45 am

Re: Collision points

Post by Norbo » Thu Jan 30, 2020 11:05 pm

I just simplified the extractor a bit in the latest commit. There's a demo showing how to use it now too: https://github.com/bepu/bepuphysics2/bl ... ionDemo.cs

Post Reply