[Numpy-discussion] "upsample" or scale an array

[Derek Homeier]

On 03.12.2011, at 6:22PM, Robin Kraft wrote:

> That does repeat the elements, but doesn't get them into the desired order.
> 
> In [4]: print a
> [[1 2]
>  [3 4]]
> 
> In [7]: np.tile(a, 4)
> Out[7]: 
> array([[1, 2, 1, 2, 1, 2, 1, 2],
>        [3, 4, 3, 4, 3, 4, 3, 4]])
> 
> In [8]: np.tile(a, 4).reshape(4,4)
> Out[8]: 
> array([[1, 2, 1, 2],
>        [1, 2, 1, 2],
>        [3, 4, 3, 4],
>        [3, 4, 3, 4]])
> 
> It's close, but I want to repeat the elements along the two axes, effectively stretching it by the lower right corner:
> 
> array([[1, 1, 2, 2],
>        [1, 1, 2, 2],
>        [3, 3, 4, 4],
>        [3, 3, 4, 4]])
> 
> It would take some more reshaping/axis rolling to get there, but it seems doable.
> 
> Anyone know what combination of manipulations would work with the result of np.tile?
> 
Rolling was the keyword:

np.rollaxis(np.tile(a, 4).reshape(2,2,-1), 2, 1).reshape(4,4))
[[1 1 2 2]
 [1 1 2 2]
 [3 3 4 4]
 [3 3 4 4]]

I leave the generalisation and timing up to you, but it seems for 
a = np.arange(M**2).reshape(M,-1)

np.rollaxis(np.tile(a, N**2).reshape(M,N,-1), 2, 1).reshape(M*N,-1) 

should do the trick.

Cheers,
						Derek