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Mathematically equivalent to:
ret = llVecNorm(<0., 0., 1.> * q);
If q is known to be a unit quaternion then it can be simplified as:
ret = <0., 0., 1.> * q;
Keep in mind that object and agent rotations will always be unit quaternions. For example, <0.0, 0.0, 1.0>*llGetRot() is about 25-30% faster than llRot2Up(llGetRot()) depending on the VM used. If done often and at extremely fast rates, it can be advantageous to even save <0.0, 0.0, 1.0> to a local/global variable and reuse it.
Can be useful to identify the orientation of the local horizontal-plane of the prim, since it's z-axis is always perpendicular to this local horizontal plane.
function vector llRot2Up( rotation q );