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Mouse coords to world coords -- Attaching a 3d object to cursor

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I know it's not a difficult problem but I'm stuck. I'm banging my head against the wall trying to figure this out. In my game, clicking on an object in the inventory will attach it to the mouse cursor so you can drop it somewhere else. Unfortunately the engine I'm using doesn't have a worldToScreen (and vice versa) function. The "picking" works (ie, I can select/deselect the objects in the inventory with no problem, but while my erroneous mouse following code runs as expected, it just doesn't look right). Can someone give me some psuedo-code or a description of what I need to do? I'll give any additional information that is needed. Thanks in advance!

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Ok, I figured this one out (thanks google):

vPickRay.x = (((2.0f * fScreenX) / fWindowWidth) - 1.0f) / m_ProjectionMatrix._11;
vPickRay.y = -(((2.0f * fScreenY) / fWindowHeight) - 1.0f) / m_ProjectionMatrix._22;
vPickRay.z = 1.0f;

D3DXMatrixInverse( &mInvView, NULL, &m_ViewMatrix );

// Transform the screen space pick ray into 3D space
vRayDir.x = (vPickRay.x * mInvView._11) + (vPickRay.y * mInvView._21) + (vPickRay.z * mInvView._31);
vRayDir.y = (vPickRay.x * mInvView._12) + (vPickRay.y * mInvView._22) + (vPickRay.z * mInvView._32);
vRayDir.z = (vPickRay.x * mInvView._13) + (vPickRay.y * mInvView._23) + (vPickRay.z * mInvView._33);

D3DXVECTOR3 vRayOrigin;
vRayOrigin.x = mInvView._41;
vRayOrigin.y = mInvView._42;
vRayOrigin.z = mInvView._43;

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