Calculate 3D distance with Bullet Engine

[MrOizo]

Hi!

I'd like to use bullet engine to calculate distances between convex shapes. The problem is that I'd like to use it under C#.

Could you give me some idea how to use Bullet with WPF?

Thanks in advance!

Regards,
Zoli

[Erwin Coumans]

The btCollisionWorld::contactTest / contactPairTest can be modified to compute the closest distance for convex objects (using GJK for positive distance and EPA for penetration depth internally).

This can be wrapped for C# usage with C++/CLI.

We'll try to provide a code sample soon,
Thanks,
Erwin

[MrOizo]

The btCollisionWorld::contactTest / contactPairTest can be modified to compute the closest distance for convex objects (using GJK for positive distance and EPA for penetration depth internally).

This can be wrapped for C# usage with C++/CLI.

We'll try to provide a code sample soon,
Thanks,
Erwin

Thanks, for the name of the function.

A sample code will be a very big help for me, how to use the wrapper.

Thanks in advance!

Regards,
Zoli

[Erwin Coumans]

See the latest trunk version (revision r2184 or later) in bullet-2.77\Demos\ConvexHullDistance\ConvexHullDistanceDemo.cpp for two different ways to compute the closest points between two convex objects:

Directly using GJK:

Code: Select all

btGjkEpaPenetrationDepthSolver epa;
	btGjkPairDetector	convexConvex(shapePtr[0],shapePtr[1],&sGjkSimplexSolver,&epa);

	btVector3 seperatingAxis(0.00000000f,0.059727669f,0.29259586f);
	convexConvex.setCachedSeperatingAxis(seperatingAxis);

	btPointCollector gjkOutput;
	btGjkPairDetector::ClosestPointInput input;
	input.m_transformA = tr[0];
	input.m_transformB = tr[1];

	convexConvex.getClosestPoints(input ,gjkOutput,0);

	if (gjkOutput.m_hasResult)
	{
		btVector3 endPt = gjkOutput.m_pointInWorld +
			gjkOutput.m_normalOnBInWorld*gjkOutput.m_distance;

		 glBegin(GL_LINES);
		glColor3f(1, 0, 0);
		glVertex3d(gjkOutput.m_pointInWorld.x(), gjkOutput.m_pointInWorld.y(),gjkOutput.m_pointInWorld.z());
		glVertex3d(endPt.x(),endPt.y(),endPt.z());
		glEnd();

	}

or using the btCollisionWorld:

Code: Select all

struct	MyContactResultCallback : public btCollisionWorld::ContactResultCallback
	{
		virtual	btScalar	addSingleResult(btManifoldPoint& cp,	const btCollisionObject* colObj0,int partId0,int index0,const btCollisionObject* colObj1,int partId1,int index1)
		{
			 glBegin(GL_LINES);
			glColor3f(1, 0, 0);
			glVertex3d(cp.m_positionWorldOnA.getX(),cp.m_positionWorldOnA.getY(),cp.m_positionWorldOnA.getZ());
			glVertex3d(cp.m_positionWorldOnB.getX(),cp.m_positionWorldOnB.getY(),cp.m_positionWorldOnB.getZ());
			glEnd();

			return 1.f;
		}
	};

	btDefaultCollisionConfiguration collisionConfiguration;
	btCollisionDispatcher				dispatcher(&collisionConfiguration);
	btDbvtBroadphase pairCache;
	btCollisionWorld world (&dispatcher,&pairCache,&collisionConfiguration);
	world.getDispatchInfo().m_convexMaxDistanceUseCPT = true;
	MyContactResultCallback result;
	btCollisionObject obA;
	obA.setCollisionShape(shapePtr[0]);
	obA.setWorldTransform(tr[0]);
	btCollisionObject obB;
	obB.setCollisionShape(shapePtr[1]);
	obB.setWorldTransform(tr[1]);
	world.contactPairTest(&obA,&obB,result);

This needs to be wrapped in an C++/CLI interface.

Thanks,
Erwin

[MrOizo]

Thanks for the example!

I found your examples for XNA, and I tried them, they work fine with the bulletsharp.dll .

By right of your examples I tried to calculate the distance between two boxshapes.

Here is the code:

Code: Select all

ConvexShape closestShape1 = new BoxShape(1, 1, 1);
ConvexShape closestShape2 = new BoxShape(1, 1, 1);

VoronoiSimplexSolver vSimpSolver = new VoronoiSimplexSolver();
ConvexPenetrationDepthSolver epa = new GjkEpaPenetrationDepthSolver();
GjkPairDetector gjkPairDetector = new GjkPairDetector(closestShape1, closestShape2, vSimpSolver, epa);
Vector3 sepAxes = new Vector3(0f, 0.059727669f, 0.29259586f);
gjkPairDetector.CachedSeparatingAxis = sepAxes;

PointCollector output = new PointCollector();
GjkPairDetector.ClosestPointInput closestPointInput = new DiscreteCollisionDetectorInterface.ClosestPointInput();
closestPointInput.TransformA = Matrix.Identity;
closestPointInput.TransformB = Matrix.CreateTranslation(new Vector3(0, 5, 0));

gjkPairDetector.GetClosestPoints(closestPointInput, output, DebugDrawer);

In the output I found a distance field, but the result is wrong.

Could you explain me, what I'm doing wrong?

Anyway, what is this Separating Axes, and why is its value Vector3(0f, 0.059727669f, 0.29259586f) ?

EDIT: I tried to calculate with CollisionWorld, but after this function

Code: Select all

world.contactPairTest(obA,obB,result);

in the result I couldn't find the distance between the shapes. Am I doing something wrong?

I reckon that I'm so close to the solution .

Thanks in advance!

Best Regards,
Zoli

[MrOizo]

I found something which is not correctly works:

Code: Select all

GjkPairDetector.ClosestPointInput closestPointInput = new DiscreteCollisionDetectorInterface.ClosestPointInput();

closestPointInput.TransformA = Matrix.Identity;
closestPointInput.TransformB = Matrix.CreateTranslation(new Vector3(0, 20, 0));

I debugged the last two lines, and after I set the TransformA or the TransformB, the values of them is the same, before I set them.

So I think, this is the reason, why I get wrong distance in the output, because this transforms my shapes into wrong places, instead of using my transform matrices.

Could You give me some idea, why is it happens?

Thanks,
Zoli

[Erwin Coumans]

It is not clear what XNA version you are talking about, there are plenty of ports of the C++ version of Bullet to various languages, most are incomplete or broken.

Only the C++ version of Bullet is supported here.
Thanks,
Erwin

[MrOizo]

It is not clear what XNA version you are talking about, there are plenty of ports of the C++ version of Bullet to various languages, most are incomplete or broken.

Only the C++ version of Bullet is supported here.
Thanks,
Erwin

I'm talking about bulletsharp. It may be unfortunately broken .

I'll try to calculate it with another wrapper.

Regards,

Zoli

[MrOizo]

I calculated the distance with BulletX with the same way like I wrote before with BulletSharp, and it works well.

So what I wrote before is a bug in BulletSharp, that is the reason, why I couldn't set the TransfromA and TransformB value.
I notify who created BulletSharp wrapper, and he wrote me he had fixed it.

Thanks for the helps Erwin!

Best Regards,
Zoli