Quaternion-derived rotation matrix:

n = w * w + x * x + y * y + z * z
s = if n == 0 then 0 else 2 / n
wx = s * w * x, wy = s * w * y, wz = s * w * z
xx = s * x * x, xy = s * x * y, xz = s * x * z
yy = s * y * y, yz = s * y * z, zz = s * z * z

[ 1 - (yy + zz)         xy - wz          xz + wy  ]
[      xy + wz     1 - (xx + zz)         yz - wx  ]
[      xz - wy          yz + wx     1 - (xx + yy) ]