Collision detection

PLEASE learn to RESPECT the forums! You have posted this same question TEN times.

¬_¬

Hi click,

I use a hierarchical bounding sphere system for object culling. Transforming the center of the sphere is simple, because it is like transforming any other point in space:

D3DXMATRIX mtrxTransform; // transformation matrix of the object
D3DXVECTOR3 vct3Center; // sphere center in object space
D3DXVECTOR3 vct3AbsoluteCenter; // sphere center in world space

D3DXVec3TransformCoord( &vct3AbsoluteCenter, &vct3Center, &mtrxTransform );

Because a matrix can change the size of an object by scaling, you should also transform the radius. But it is not so easy when you want to get the correct value in a fast way. You have to transform some good points from the surface of the sphere. From the resulting points you should use the maximum to get the transform radius value.
If you only use scaling with equal values in all three dimensions, then you are able to use the following formula:
fAbsoluteRadius = D3DXVec3Length( D3DXVec3TransformNormal( &vct3Radius, &D3DXVECTOR3( D3DXVECTOR3( 0.57735f, 0.57735f, 0.57735f ) * fRadius ), &mtrxTransform ) );

It''s not that hard.

You move your object X units.

newCenter = oldCenter + X;

I really hope I''m misunderstanding your question. If I''m not, then maybe you need more math before attempting graphics programming.

If moving means the translation of the object then lucid999 is correct, but when the motion by transformation is meant then you have to transform the center, too.