Direct3D (v10) Multiple World Transformations

Question

In my code I am trying to run two (at the moment, probably more in the future) matrix transformations on my world matrix. Like so:

D3DXMatrixRotationY(&worldMatrix, rotation);
D3DXMatrixTranslation(&worldMatrix, 0.0f, -1.0f, 0.0f);

where rotation is a changing float and worldMatrix is a D3DXMATRIX. My problem is that only the last line of code in the transformation statements works. So in the above code, the worldMatrix would get translated, but not rotated. But if I switch the order of the two statements, the worldMatrix will get rotated, but not translated. However, I played around with it, and this code works just fine:

D3DXMatrixRotationY(&worldMatrix, rotation);
D3DXMATRIX temp = worldMatrix;
D3DXMatrixTranslation(&worldMatrix, 0.0f, -1.0f, 0.0f);
worldMatrix *= temp;

After this, the worldMatrix is translated and rotated. Why doesn't it work if I only use the variables and don't include the temp matrix? Thank you!!

score 1 · Accepted Answer · answered Nov 08 '13 at 08:20

D3DXMatrixTranslation takes an output parameter as 1st parameter. The created matrix is written to that matrix, overriding the already present elements in that matrix. The matrices are not automatically multiplied by that call.
Your new code is fine; you could also write it like this:

D3DXMatrix rot;
D3DXMatrix trans;
D3DXMatrixRotationY(&rot, rotation);
D3DXMatrixTranslation(&trans, 0.0f, -1.0f, 0.0f);
D3DXMatrix world = rot * trans;

Direct3D (v10) Multiple World Transformations

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