Hello, I'm a beginner. I have a problem about Matrix. I want to move and rotate a object e.g. car. How can i translate it in right direction after it rotated. I have read the post before which is asking the same problem. The answer is translate the object back to (0,0,0), then rotate it, then translate back to before coordinates. D3DXMATRIX matIdentity; D3DXMatrixIdentity(matIdentity); // set object back to (0,0,0) m_pD3DDevice->SetTransform(D3DTS_WORLD, &matIdentity); // rotate oject D3DXMatrixMultiply(&matWorld, &matRotation, &matScale); m_pD3DDevice->SetTransform(D3DTS_WORLD, &matWorld); // set oject before translation D3DXMatrixMultiply(&matWorld, &matTranslation, &matRotation); m_pD3DDevice->SetTransform(D3DTS_WORLD, &matWorld); The result is the car still rotate in world coordinates, not local coordinates. Is that my concept totally wrong? Pls help...Thanks for reading~

thank you...Skeleton_V@T

I have read andypike's tutorial which is very good, and i got more understand about matrix...

And i implement the code...

// Save Translation
D3DXMATRIX matTemp = matTranslation;

// Set to (0,0,0)
D3DXMATRIX matInverse;
D3DXMatrixInverse(&matInverse, NULL, &matTranslation);
D3DXMatrixMultiply(&matWorld, &matTranslation, &matInverse);

// rotate object and then restore Translation
D3DXMatrixMultiply(&matWorld, &matWorld, &matRotation);
D3DXMatrixMultiply(&matWorld, &matTemp, &matWorld );

But now i am facing the problem is the car will rotate at world coordinates(0,0,0) when it's not at the world coordinates(0,0,0)...

http://hk.geocities.com/ktfsince1981/pic.jpg

please Help again.

Normally one first scales the model (if needed), then rotates it into the correct orientation, then translates it to the correct position. This actually means to scale about the world center, and to rotate around the world center. If you e.g. first translate and then rotate, you simultanously do a translation (on an arc) with the rotation, what isn't what you want normally.

Your model is (hopefully) defined in its own local co-ordinate system. So you don't need to do the inverse translation I've seen from your code snippet. Instead, set-up a fresh matrix from the car's parameters, and set the transform to it:

Say, the origin of your model is left at the world's origin for rotation, and is translated after that to the target position.

EDIT: If you look at the code snippet above, you'll see this scheme (look especially at the MatrixMultiply):
M := R * T
where M denotes the resulting transformation matrix, R denotes the rotation matrix, and T denotes the translation matrix. D3D applies the transformation so that a new vertex position is computed by
v' := v * M = v * R * T
This could be read as follows: First R ist applied, then T is applied. Exactly what normally is wanted.