D3DXIntersect Question

Hi, ok I have a few minutes, this is how I do it:

I have mouse x and y positions as mx and my, to create the ray position and direction:

You need to translate the mouse position so it is the position in the world as if a ray is cast from your eye through the screen, so we need to take the projection matrix (matProj) and the backbuffer sizes into account (w and h):

// Compute the vector of the pick ray in screen space
D3DXVECTOR3 v;
v.x = ( ( ( 2.0f * sx ) / w ) - 1 ) / matProj._11;
v.y = -( ( ( 2.0f * sy ) / h ) - 1 ) / matProj._22;
v.z = 1.0f;

// Get the inverse view matrix
D3DXMATRIX matView;
m_camera->GetViewMatrix(matView);
D3DXMatrixInverse( &matView, NULL, &matView );

// Transform the screen space pick ray into 3D space
rayDir->x = v.x*m._11 + v.y*m._21 + v.z*m._31;
rayDir->y = v.x*m._12 + v.y*m._22 + v.z*m._32;
rayDir->z = v.x*m._13 + v.y*m._23 + v.z*m._33;
rayOrigin->x = m._41;
rayOrigin->y = m._42;
rayOrigin->z = m._43;

Ok now you have the pick ray position and direction. Note: I do not include the world matrix in my calculation as this is specific to each object being tested and so I apply that later. Remember D3DXIntersect works in the object space not world space hence the conversion is required.

So when I come to test I loop through all my objects checking for a collision and remember the closest one (that is not negative distance) and that will be the hit.

So we need a fn that given an object with a mesh and a world position will return the distance to a collision with it (if there is one).

We now need to transform the ray into the objects space, something like this (matWorld is objects world matrix):


// Use inverse of matrix
D3DXMATRIXA16 matInverse;
D3DXMatrixInverse(&matInverse,NULL,&matWorld);

// Transform ray origin and direction by inv matrix
D3DXVECTOR3 rayObjOrigin,rayObjDirection;

D3DXVec3TransformCoord(&rayObjOrigin,&rayOrigin,&matInverse);
D3DXVec3TransformNormal&rayObjDirection,&rayDirection,&matInverse);
D3DXVec3Normalize(&rayObjDirection,&rayObjDirection);

Right, nearly there. The next step is to actually call the intersect:

BOOL hasHit;
DWORD dwFace;
FLOAT fBary1, fBary2;

D3DXIntersect(m_mesh, &rayOrigin, &rayDirection, &hasHit, NULL, NULL, NULL, distanceToCollision, NULL, NULL);

and thats it - works perfectly. Bare in mind that you would probably want to do some bounding box intersects first to speed things up. Hope this helps

Thanx for the help that is ace!

regards, ace