Thread: Math Question

First, I want to say that math is not exactly my strong point. In fact, I am terrible at anything above basic algebra.

I am trying to create a collision detection routine that will compute the angle of deflection. i.e. a ball hitting a wall. I can do this simply by reversing the sign of the direction vector, but this only works for a flat surface. I will have some curved / angled walls that I will need to calculate for. I know I need to start off by finding the angle between the ball's direction vector and the plane normal but I am not exactly sure how to go about this.

If anyone can explain or point me to a good tutorial??

Well, I came up with a function to calculate the reflection angle based on the objects velocity vector and the normal of the intersected face of the mesh. I pluged it into a small console app to test it and the numbers seem to work.

Code:

int main() {
    D3DXVECTOR3 OldVelocity(4.0f, -2.0f, 1.0f), // Object velocity vector before collision
                PlaneNormal(0.0f,  1.0f, 0.0f), // Direction plane is facing
                NewVelocity;                    // Object velocity vector after collision
    
    std::cout<<OldVelocity.x <<"," <<OldVelocity.y <<"," <<OldVelocity.z <<"\n";
    
    float dot = (D3DXVec3Dot(&-OldVelocity, &PlaneNormal) );    
    NewVelocity = (2 * dot) * PlaneNormal + OldVelocity;
    
    std::cout<<NewVelocity.x <<"," <<NewVelocity.y <<"," <<NewVelocity.z <<"\n";
    
    std::cin.get();
    return 0;
}

Whatever numbers i assign to OldVelocity get mirrored over whatever numbers I assign to PlaneNormal, which is exactly what I need.

My only problem now is how to get the normal of the face that was intersected. I know I can use D3DXIntersect to get the index value of the face, but how do I get the face normal from this?

Here's what I have managed to come up with

Code:

bool CCollisionManager::FindNewVelocity(LPDIRECT3DDEVICE9 pDevice,
                                        CMeshInstance* pMesh,
                                        D3DXVECTOR3* pObjectPosition,
                                        D3DXVECTOR3* pOldVelocity, 
                                        D3DXVECTOR3* pNewVelocity) {
    BOOL Hit = FALSE;
    float u, v, dist;
    DWORD FaceIndex;
    D3DXVECTOR3 ObjectDirection,
                PrevObjectDirection,
                Velocity = -(*pOldVelocity);

    D3DXVECTOR3 PrevObjectPosition = *pObjectPosition - *pOldVelocity;
    D3DXVec3Normalize(&ObjectDirection, &PrevObjectDirection);
    
    // Get the normal of the intersected mesh face
    // First, Get the vertex and index buffers for the mesh
    LPDIRECT3DVERTEXBUFFER9 pVB;
    LPDIRECT3DINDEXBUFFER9  pIB;

    pMesh->GetMesh()->GetVertexBuffer(&pVB);
    pMesh->GetMesh()->GetIndexBuffer(&pIB);

    WORD                                *pIndices               = NULL;
    WORD                                *pIntersectedFaceIndex  = NULL;
    cstmVertex::PositionNormalTextured  *pVertices              = NULL;
    cstmVertex::PositionNormalTextured  *pIntersectedFaceVertex = NULL;

    pIB->Lock(0, 0, (void**)&pIndices, 0);
    pVB->Lock(0, 0, (void**)&pVertices, 0);

    // Now find which mesh face was intersected
    pDevice->SetTransform(D3DTS_WORLD, pMesh->GetTransform() ); 
    D3DXIntersect(pMesh->GetMesh(),
                  &PrevObjectPosition,
                  &ObjectDirection,
                  &Hit,
                  &FaceIndex,
                  &u, &v, &dist,
                  NULL, NULL);

    pIntersectedFaceIndex  = &pIndices[3 * FaceIndex];
    pIntersectedFaceVertex = &pVertices[*pIntersectedFaceIndex];
    
    pVB->Unlock();
    pIB->Unlock();
    
    // Get the normal for the face 
    D3DXVECTOR3 PlaneNormal(pIntersectedFaceVertex->Nx,
                            pIntersectedFaceVertex->Ny,
                            pIntersectedFaceVertex->Nz);
    
    // compute new velocity vector
    if (Hit == TRUE) {
        float dot = (D3DXVec3Dot(&Velocity, &PlaneNormal) );
        *pNewVelocity = (2 * dot) * PlaneNormal + *pOldVelocity;
    } else {
        *pNewVelocity = *pOldVelocity;
    }
    
    return Hit;
}

Everything seems to work fine except for two issues :

• If there is a collision on any axis other than X I get a seg fault

• no mater which face was intersected, I allways get a face normal of (0.0, -1.0, 0.0)

I adapted this from the DX9 SDK sample 'Pick.cpp'

Anyone have any ideas as to what is going on? I've tried everything I can think of and am still at a loss.

Am I even on the right track here? Or is there a better way of getting the face normal?