[Numpy-discussion] efficient 3d histogram creation
Mon May 4 06:00:13 CDT 2009
On Mon, May 4, 2009 at 12:31 AM, Chris Colbert <firstname.lastname@example.org> wrote:
> this actually sort of worked. Thanks for putting me on the right track.
> Here is what I ended up with.
> this is what I ended up with:
> def hist3d(imgarray):
> histarray = N.zeros((16, 16, 16))
> temp = imgarray.copy()
> bins = N.arange(0, 257, 16)
> histarray = N.histogramdd((temp[:,:,0].ravel(), temp[:,:,1].ravel(),
> temp[:,:,2].ravel()), bins=(bins, bins, bins))
> return histarray
> this creates a 3d histogram of rgb image values in the range 0,255 using 16
> bins per component color.
> on a 640x480 image, it executes in 0.3 seconds vs 4.5 seconds for a for
> not quite framerate, but good enough for prototyping.
I don't think your copy to temp is necessary, and use reshape(-1,3) as
in the example of Stefan, which will avoid copying the array 3 times.
If you need to gain some more speed, then rewriting histogramdd and
removing some of the unnecessary checks and calculations looks
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