DirectX 9 rotating around "joints" - animation

Question

I'm having trouble figuring out how to generate matrixes.
Hopefully that picture explains it, but basically I have an initial position, and I'm trying to rotate the main joint, 90 degrees, then following that, rotate the last joint, by 90 degrees. I then apply translation afterwards to get a final matrix (see code). That is applied to a set of points, that are relative to its joint.
The last rotation doesn't seem to work, it is ok if I don't put in the line: matrixPositions[2].appliedRotationMatrix *= (matrixRotX * matrixRotY * matrixRotZ); (the leg is straight down). I must be missing something obvious? Can you not do matrix multiplication this way for rotations?
D3DXMATRIX matrixRotX, matrixRotY, matrixRotZ;
D3DXMatrixRotationX(&matrixRotX, 0);
D3DXMatrixRotationY(&matrixRotY, 0);
D3DXMatrixRotationZ(&matrixRotZ, -PI/2);
matrixPositions[0].appliedRotationMatrix *= (matrixRotX * matrixRotY * matrixRotZ);
D3DXMATRIX matTranslationIn1;
D3DXMatrixTranslation(&matTranslationIn1, (matrixPositions[0].position.x-matrixPositions[1].position.x), (matrixPositions[0].position.y-matrixPositions[1].position.y), (matrixPositions[0].position.z-matrixPositions[1].position.z));
D3DXMATRIX matTranslationOut1;
D3DXMatrixTranslation(&matTranslationOut1, -(matrixPositions[0].position.x-matrixPositions[1].position.x), -(matrixPositions[0].position.y-matrixPositions[1].position.y), -(matrixPositions[0].position.z-matrixPositions[1].position.z));
matrixPositions[1].appliedRotationMatrix *= (matTranslationIn1 * (matrixRotX * matrixRotY * matrixRotZ) * matTranslationOut1);
D3DXMatrixTranslation(&matTranslationIn1, (matrixPositions[0].position.x-matrixPositions[2].position.x), (matrixPositions[0].position.y-matrixPositions[2].position.y), (matrixPositions[0].position.z-matrixPositions[2].position.z));
D3DXMatrixTranslation(&matTranslationOut1, -(matrixPositions[0].position.x-matrixPositions[2].position.x), -(matrixPositions[0].position.y-matrixPositions[2].position.y), -(matrixPositions[0].position.z-matrixPositions[2].position.z));
matrixPositions[2].appliedRotationMatrix *= (matTranslationIn1 * (matrixRotX * matrixRotY * matrixRotZ) * matTranslationOut1);
matrixPositions[2].appliedRotationMatrix *= (matrixRotX * matrixRotY * matrixRotZ);
D3DXMATRIX matrix[3];
for (int x = 0; x < 3; x++)
{
D3DXMatrixIdentity( &matrix[x]);
D3DXMATRIX matTranslation;
D3DXMatrixTranslation(&matTranslation, matrixPositions[x].position.x, matrixPositions[x].position.y, matrixPositions[x].position.z);
matrix[x] = matrix[x] * matrixPositions[x].appliedRotationMatrix * matTranslation;
}

Answer 1

There are two main steps for your requirements.
Rotate joints 0, 1 and 2 around the origin by 90 degrees.
Rotate joint 2 around joint 1 by 90 degrees.
I write some pseudo code, it almost done, but you still need some updates to use it. see comments in the code for details.
void Rotatation()
{
// Build up the rotation matrix for step 1
D3DXVECTOR3 rotAxis(0, 0, 1);
float angle = -(D3DX_PI / 2);
D3DXMATRIX rotMatrix;
D3DXMatrixRotationAxis(&rotMatrix, &rotAxis, angle);
// rotate joints 0, 1 and 2 by apply the matrix above
for (int i = 0; i < 3; i++)
{
joints[i].matrix *= rotMatrix;
}
// Build up the rotation matrix for joint 2
// Since joint 2 was not rotate around the origin(I mean the axis should pass the origin), so first you need to translate the rotation center to origin
// then rotate joint 2, and last move back
// After the rotation in step 1, joint 1 now locate at (0, 2, 0)
// to translate it to the origin.
D3DXMATRIX transMat;
D3DXMatrixTranslation(&transMat, 0, 2, 0);
// Now joint 2 can rotate around z-axis, so the rotate matrix is same as step 1
// after rotation, move back, this matrix is the inverse of transMat
D3DXMATRIX inverseTransMat;
D3DXMatrixTranslation(&transMat, 0, -2, 0);
// Combine the 3 matrix above
D3DXMATRIX rotMatjoin2 = transMat * rotMatjoin2 * inverseTransMat;
// rotate jonit 2
joints[2].matrix *= rotMatjoin2;
}