[Numpy-discussion] Specially Constructed Arrays

[Robert Kern]

On Wed, Apr 8, 2009 at 20:39, Ian Mallett <geometrian@gmail.com> wrote:
> Hello,
>
> I want to make an array of size sqrt(n) by sqrt(n) by 3, filled with special
> values.
>
> The values range from 0.0 to 3.0, starting with 0.0 at one corner and ending
> at 3.0 in the opposite, increasing going row by row.  The value is to be
> encoded in each color.  Because this is somewhat abstract, here's a small
> example (n=25), generated using the attached code (it also multiplies the
> number by 255 to obtain a RGB color and not messy floats) to show the
> concept.  The real version should be done by NumPy.  This is where I need
> help; I have no idea how to even approach the problem.
>
> [[  0,  0,  0],[ 32,  0,  0],[ 64,  0,  0],[ 96,  0,  0],[128,  0,  0],
>  [159,  0,  0],[191,  0,  0],[223,  0,  0],[255,  0,  0],[255, 32,  0],
>  [255, 64,  0],[255, 96,  0],[255,128,  0],[255,159,  0],[255,191,  0],
>  [255,223,  0],[255,255,  0],[255,255, 32],[255,255, 64],[255,255, 96],
>  [255,255,128],[255,255,159],[255,255,191],[255,255,223],[255,255,255]]
>
> Arrays like this need to be generated quite quickly, so the per-pixel method
> I presented will not work.  How should I do it with NumPy?

In [1]: from numpy import *

In [2]: sqrtn = 5

In [3]: n = sqrtn**2

In [4]: x = linspace(0.0, 3.0, n)

In [5]: y = column_stack([x, x-1, x-2]).clip(0, 1).reshape([sqrtn, sqrtn, 3])

In [6]: z = (y * 255).round().astype(uint8)

In [7]: z
Out[7]:
array([[[  0,   0,   0],
        [ 32,   0,   0],
        [ 64,   0,   0],
        [ 96,   0,   0],
        [128,   0,   0]],

       [[159,   0,   0],
        [191,   0,   0],
        [223,   0,   0],
        [255,   0,   0],
        [255,  32,   0]],

       [[255,  64,   0],
        [255,  96,   0],
        [255, 128,   0],
        [255, 159,   0],
        [255, 191,   0]],

       [[255, 223,   0],
        [255, 255,   0],
        [255, 255,  32],
        [255, 255,  64],
        [255, 255,  96]],

       [[255, 255, 128],
        [255, 255, 159],
        [255, 255, 191],
        [255, 255, 223],
        [255, 255, 255]]], dtype=uint8)

-- 
Robert Kern

"I have come to believe that the whole world is an enigma, a harmless
enigma that is made terrible by our own mad attempt to interpret it as
though it had an underlying truth."
  -- Umberto Eco