BEPUphysics

Posted: **Thu Nov 21, 2013 4:26 pm**

Hello,

if got a Problem by the interesection checking of compound bodies.....

If I specify Tags in coumpound body and in Ini like this:

ini.CollisionInformation.Tag = “ini_1”;
compoundBody.CollisionInformation.Tag = “compound_payload”;

and if I register event handling to ini like like we correctly do (i hope so):
ini.CollisionInformation.Events.EndOfCollision += Event_EndOfCollision; // or start

I get both A and B collidables correctly, but TAG of compound collidable is NULL, even if I set it correctly before.
Tag of ini collidable is OK.

I tried to solve it thousand ways, I tried even compound body demo in bepu. And still the same result. But:
Here here is an interesting thing – when I register collision event handler not to INI, but to Compound body thing, like:

compoundBody.CollisionInformation.Events.EndOfCollision += Event_EndOfCollision;

then on collision event I get correctly both of Tags…. And strings from them!

Here the CompundBodieDemo changed to Show the Problem....

Code: Select all

using BEPUphysics.Entities.Prefabs;
using BEPUphysics.CollisionShapes;
using BEPUphysics.CollisionShapes.ConvexShapes;
using System.Collections.Generic;
using System.Diagnostics;
using BEPUutilities;

namespace BEPUphysicsDemos.Demos
{
    /// <summary>
    /// Compound bodies are created from other entities to make concave shapes.
    /// </summary>
    public class CompoundBodiesDemo : StandardDemo
    {
        /// <summary>
        /// Constructs a new demo.
        /// </summary>
        /// <param name="game">Game owning this demo.</param>
        public CompoundBodiesDemo(DemosGame game)
            : base(game)
        {

            //Build the first body
            var bodies = new List<CompoundShapeEntry>
            {
                new CompoundShapeEntry(new SphereShape(.5f), new Vector3(0, 1, 0), 1),
                new CompoundShapeEntry(new ConeShape(2, .5f), new Vector3(1, 1, 0), 1),
                new CompoundShapeEntry(new SphereShape(.5f), new Vector3(-1, 1, 0), 1)
            };
            var cb1 = new CompoundBody(bodies, 45);



            //Build the second body
            bodies = new List<CompoundShapeEntry>
            {
                new CompoundShapeEntry(new BoxShape(1,1,1), new Vector3(0, 3, 0), 1),
                new CompoundShapeEntry(new BoxShape(1,1,1), new Vector3(1, 3.5f, 0), 1),
            };
            var cb2 = new CompoundBody(bodies, 4);

            bodies = new List<CompoundShapeEntry>();
            //Build the third Braum's-fry style body
            for (int k = 0; k < 7; k++)
            {
                bodies.Add(new CompoundShapeEntry(new BoxShape(1, 1, 1), new Vector3(-4 + k * .7f, 2 + .7f * k, 2 + k * .2f), 1));
            }
            var cb3 = new CompoundBody(bodies, 7);

            //Add them all to the space
            cb3.CollisionInformation.Tag = "cb";


            Space.Add(cb3);
            //  Space.Add(cb2);
            //   Space.Add(cb1);

            Box ground = new Box(new Vector3(0, -.5f, 0), 10, 1, 10);
            ground.CollisionInformation.Tag = "ground";


            ground.CollisionInformation.Events.InitialCollisionDetected += Events_InitialCollisionDetected;
        //    cb3.CollisionInformation.Events.InitialCollisionDetected += Events_InitialCollisionDetected;

            Space.Add(ground);
            game.Camera.Position = new Vector3(0, 3, 15);

        }

        void Events_InitialCollisionDetected(BEPUphysics.BroadPhaseEntries.MobileCollidables.EntityCollidable sender, BEPUphysics.BroadPhaseEntries.Collidable other, BEPUphysics.NarrowPhaseSystems.Pairs.CollidablePairHandler pair)
        {
            string a = ((string)pair.CollidableA.Tag);
            string b = ((string)pair.CollidableB.Tag);
        }

        /// <summary>
        /// Gets the name of the simulation.
        /// </summary>
        public override string Name
        {
            get { return "Compound Bodies"; }
        }
    }
}

Posted: **Thu Nov 21, 2013 8:24 pm**

Let me preface with: the implementation of collision events is a giant tangled mess, and I'd like to delete/redo it all. It took me a while to figure out if this was intended behavior

This issue is based on the event reporting the child collidable associated with the collision instead of the parent collidable. Since the child collidable was not given a tag, it shows up as null.

-When an event is hooked on the top-level compound, it should fire an event that reports the top-level compound collidable as the sender.
-When an event is hooked on another object which collides with a compound, the 'other' is reported as the involved child collidable instead of the top-level compound.

This is technically intended behavior, since reversing the choices causes issues:
-In the former case, reporting the child compound would be unexpected and redundant with hooking the event directly to the relevant child collidable.
-In the latter case, reporting the top-level compound would make it difficult (in the current implementation) to know which child was hit.

Here's a few solutions:
1) Put the tag on the child collidables. For example:

Code: Select all

            foreach (var child in cb3.CollisionInformation.Children)
            {
                child.CollisionInformation.Tag = "meow meow meow meow";
            }

The tag could be the same tag, or it could be child-specific, or it could just point to the parent compound.

2) Walk up the hierarchy of pairs. For example, insert this at the beginning of the event handler:

Code: Select all

            while (pair.Parent != null)
                pair = (CollidablePairHandler)pair.Parent;

That will get you the top-level pair, which contains the top-level compound.

3) Instead of using events, just query state. For example, to know if an object is currently in collision:

Code: Select all

            foreach (var pair in cb3.CollisionInformation.Pairs)
            {
                if (pair.Colliding)
                {
                    //This pair is colliding!
                    //[dosomething]
                }
            }

I find that these sorts of checks tend to fit into program flow more easily than events, and they don't have any hidden complexities.

Posted: **Thu Nov 21, 2013 8:37 pm**

Norbo thx for you fast reply ...
I will use it in the way you wrote ...
so once again the problem is solved for me

YEHAAA weekend can come ...

Thanks and all a nice weekend ! ..

BEPUphysics

Problem with Compound Bodies intersection events

Problem with Compound Bodies intersection events

Re: Problem with Compound Bodies intersection events

Re: Problem with Compound Bodies intersection events