[Numpy-discussion] cross product of two 3xn arrays

[Bill Baxter]

For folks using quats to represent rotations (which is all I use them for,
anyway), if you're batch transforming a bunch of vectors by one quaternion,
it's a lot more efficient to convert the quat to a 3x3 matrix first and
transform using matrix multiply (9 mults per transform that way vs 21 or so
depending on the implementation of q*v*q^-1).  Given that, I can't see many
situations when I'd need a super speedy C version of quaternion multiply.

--Bill
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