[Numpy-discussion] efficient 3d histogram creation
Mon May 4 14:18:20 CDT 2009
On Mon, May 4, 2009 at 7:00 AM, <firstname.lastname@example.org> wrote:
> On Mon, May 4, 2009 at 12:31 AM, Chris Colbert <email@example.com>
> > 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
> > bins per component color.
> > on a 640x480 image, it executes in 0.3 seconds vs 4.5 seconds for a for
> > loop.
> > 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
Indeed, the strategy used in the histogram function is faster than the one
used in the histogramdd case, so porting one to the other should speed
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