[Numpy-discussion] Fast histogram

[Sebastian Haase]

On Thu, Apr 17, 2008 at 6:54 PM, Zachary Pincus <zachary.pincus at yale.edu> wrote:
> Hi, and thanks for the suggestion!
>
>
>  > How many bits per pixel does your camera actually generate !?
>  > If its for example a 12 bit camera, you could just fill in directly
>  > into 4096 preallocated bins.
>  > You would not need any sorting !!
>  > That's what I did for a 16 bit camera -- but I wrote it in C and I had
>  > 4 cameras at 30 Hz.
>
>  That approach avoids the bin-index calculation line:
>  indices = numpy.clip(((array.astype(float) - min) * bins / (max -
>
> min)).astype(int), 0, bins-1)
>
>  But even if indices = array, one still needs to do something like:
>  for index in indices: histogram[index] += 1
>
>  Which is slow in python and fast in C.
>
>  I'm guessing that there's no utility function in numpy that does a
>  loop like this? If so, that would be handy, but if now, I think I need
>  to dig out the numpy book and write a little extension...
>
>
I thought of a broadcasting approach... what are the chances that a simple

bins[:] = 0
bins[ img.flat ] += 1

works !?  Or something along those lines ....
-Sebastian