[Numpy-discussion] Fastest binary threshold?
Stefan van der Walt
stefan at sun.ac.za
Wed Aug 2 19:45:22 CDT 2006
On Wed, Aug 02, 2006 at 04:51:07PM -0400, Mark Heslep wrote:
> I need a binary threshold and numpy.where() seems very slow on numpy
> 0.9.9.2800:
>
> python -m timeit -n 10 -s "import numpy as n;a=n.ones((512,512),
> n.uint8)*129"
> "a_bin=n.where( a>128, 128,0)"
> 10 loops, best of 3: 37.9 msec per loop
Using numpy indexing brings the time down by a factor of 10 or so:
In [46]: timeit b = N.where(a>128,128,0)
10 loops, best of 3: 27.1 ms per loop
In [47]: timeit b = (a > 128).astype(N.uint8) * 128
100 loops, best of 3: 3.45 ms per loop
Binary thresholding can be added to ndimage easily, if further speed
improvement is needed.
Regards
Stéfan
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