[Numpy-discussion] Finding Unique Pixel Values

Charles R Harris charlesr.harris@gmail....
Fri Jul 23 00:05:55 CDT 2010


On Thu, Jul 22, 2010 at 9:59 PM, Ian Mallett <geometrian@gmail.com> wrote:

> Hi again,
>
> I've condensed the problem down a lot, because I both presented it in an
> overcomplicated way, and did not explain it particularly well.
>
> Condensed problem:
> a = np.zeros(num_patches)
> b = np.array(...) #created, and is size 512^512 = 262,144
> #Each value in "b" is an index into "a".
> #For each occurrence of a given index in "b", increment the corresponding
> value in "a" by +1.
> #How should I do that?
>
> Simpler, and more importantly, better explained problem.  Help?
>
>
Is that what you want, or do you just want to know how many unique indices
there are? As to encoding the RGB, unless there is a existing program your
best bet is probably to use a dot product, i.e., if pix_data is (n,3) uint8,
then

pix_index = np.dot(pix_data, array([1, 2**8, 2**16], dtype=uint32)

then check out the documentation for np.unique.

Chuck
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