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

Travis Oliphant oliphant.travis at ieee.org
Fri Feb 17 11:25:59 CST 2006

Charles R Harris wrote:

>On 2/17/06, Bill Baxter <wbaxter at gmail.com> wrote:
>>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.
>On a related note, indexing in numarray is some 3x faster than in
>NumPy and I'm wondering what needs to be done to speed that up.
Please explain with a benchmark.  This is not true for all indexing 
operations.  But, it is possible that certain use cases are faster.    
We can't do anything without knowing what you are talking about exactly.


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