Opcode: turning a ray collision into an intersection point

[sirSolarius]

I've been getting pretty frustrated with Opcode, but I think I'm close to getting what I need.

What I want to do is to load a walkmap .mesh into Ogre and perform ray collisions on it to determine if my character can walk to a certain place (and where to move him, ie up a ramp).

I basically copy-pasted the OgreOpcode code to load an Entity into Opcode, but now I don't understand how I can turn the results of a ray collision into an intersection point.

Here is what I have:

Code: Select all

bool CollisionManager::RayCollide(const Vector3 &start, const Vector3 &end, 
													MeshCollider *mesh, Vector3 &intersect)
{
	RayCollider RC;
	RC.SetFirstContact(true);
	RC.SetClosestHit(true);
	RC.SetCulling(true);
	RC.SetTemporalCoherence(false);
	RC.SetMaxDist( (end-start).length());

	CollisionFaces CF;
	RC.SetDestination(&CF);

	RC.SetCallback(collisionCallback, (udword)mesh);

	// We know that there is only one subentity per entity
	IceMaths::Matrix4x4 opWorld; 
	mesh->GetWorldMatrix(&opWorld);

	IceMaths::Ray ray(start, (end-start).normalisedCopy());
	RC.GetNbIntersections();

	bool isOK = collider.Collide(ray, mesh->mCollisionModel, opWorld, mColCache);
	if(!isOk)
	{
		ErrorManager::SendError(false, "Collision error!");
		return false;
	}

Thanks!

[jacmoe]

You should call RC.GetNbIntersections(); after your call to collider.Collide - and you should loop through CollisionFaces (through CF) to see the results.
You also need a function to extract the hit vertices from your mesh data from the returned index.
Take a closer look at OgreOpcode.

[sirSolarius]

Ok, well if I can find out the triangle then I can use ray-triangle collision to find the intersection point.

Looking at the SourceForge CVS, it seems that OgreOpcode just does a boolean test like "does A overlap B?" rather than "where does A intersect B?" So OgreOpcode doesn't seem to help with this question.

Looking through Opcode, a CollisionFaces object returns an array of CollisionFace objects. Each CollisionFace object then has a few details, one of them being faceID. Thanks to the kind folks at Opcode, there is absolutely no documentation on this. What do I do from here to return the 3 points of the stabbed triangle?

[sirSolarius]

Ok, well here's my function. Is this the correct usage of the faceid?

Also, excuse the loop that terminates after one iteration... I'm going to be expanding it but I need to make sure it works first =)

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bool CollisionManager::GetIntersectPoint(const Vector3 &start, const Vector3 &dir, 
							MeshCollider *m, Opcode::CollisionFaces *cf, Vector3 &intersect)
{
	int num=cf->GetNbFaces();
	const Opcode::CollisionFace *f=cf->GetFaces();

	for(int i=0; i < num; ++i)
	{
		int index=3*f[i].mFaceID;
		/******************************************************************************/
		/************* THIS IS GHETTO CODE- No idea what index is used for! ***********/
		IceMaths::Point p1=m->mIndexList[index], p2=m->mIndexList[index+1], 
																p3=m->mIndexList[index+2];
		/******************************************************************************/

		Vector3 v1(p1.x, p1.y, p1.z), v2(p2.x, p2.y, p2.z), v3(p3.x, p3.y, p3.z);
		
		Ogre::Plane pn(v1, v2, v3);
		Ogre::Ray r(start, dir);
		std::pair<bool, Real> info=r.intersects(pn);
		float t=info.second;

		intersect=r.getPoint(t);
		return true;
	}
	return false;
}

[jacmoe]

You retrieve the three vertices like this:

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/// Extract triangle coordinates from triangle index.
inline void CollisionShape::GetTriCoords(int index, Vector3& v0, Vector3& v1, Vector3& v2)
{
      int* indexPtr = &(faceData[3 * index]);
      float* vp0 = &(vertexData[3 * indexPtr[0]]);
      float* vp1 = &(vertexData[3 * indexPtr[1]]);
      float* vp2 = &(vertexData[3 * indexPtr[2]]);
      v0 = Vector3(vp0[0], vp0[1], vp0[2]);
      v1 = Vector3(vp1[0], vp1[1], vp1[2]);
      v2 = Vector3(vp2[0], vp2[1], vp2[2]);
}

In other words: the index from facedata is an index into the indexed mesh data.

[sirSolarius]

Sorry, but where do I get the faceData array from? Looking at your OgreCollisionEntity class, I only see mIndexList and mVertexList (here).

I basically just used your code to load an Entity into Opcode, so I am also missing this array.

[okki]

take a look at jacmoe's postings at https://developer.berlios.de/project/sh ... up_id=3423 and the OgreOpcode::OgreCollisionShape->load(). it refers to the getMeshInformation function
hth

[jacmoe]

@sirSolarius:
There's two versions of OgreOpcode: one from ogreaddons CVS (which works with Opcode 1.3 using my patch) and the WIP from Berlios, which I posted a link to in the other opcode thread in general discussion.
If you are using the ogreaddons version, which you are, the code for getting the tricoords is available from the callback:

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  udword* indexList = obj->getIndexList();
  Point* vertexList = obj->getVertexList();

  unsigned int index = triIndex * 3;

  triangle.Vertex[0] = &vertexList[indexList[index]];
  triangle.Vertex[1] = &vertexList[indexList[index + 1]];
  triangle.Vertex[2] = &vertexList[indexList[index + 2]];