Looking for an algorithm for a perfect "snake" from the center of a field?

Question

I'm looking for a piece of code:
From the middle, in a "circle"-way, slowly to the ends of the edges of a rectangle. And when it reaches the boundaries on one side, just skip the pixels.

I tried already some crazy for-adventures, but that was to much code.
Does anyone have any idea for a simple/ingenious way?

It's like to start the game snake from the center until the full field is used. I'll use this way to scan a picture (from the middle to find the first pixel next to center in a other color).

Maybe a picture could describe it better:

Answer 1

From this link requires numpy and python of course.

import numpy as np
a = np.arange(7*7).reshape(7,7)

def spiral_ccw(A):
    A = np.array(A)
    out = []
    while(A.size):
        out.append(A[0][::-1])    # first row reversed
        A = A[1:][::-1].T         # cut off first row and rotate clockwise
    return np.concatenate(out)


def base_spiral(nrow, ncol):
    return spiral_ccw(np.arange(nrow*ncol).reshape(nrow, ncol))[::-1]


def to_spiral(A):
    A = np.array(A)
    B = np.empty_like(A)
    B.flat[base_spiral(*A.shape)] = A.flat
    return B


to_spiral(a)

array([[42, 43, 44, 45, 46, 47, 48],
       [41, 20, 21, 22, 23, 24, 25],
       [40, 19,  6,  7,  8,  9, 26],
       [39, 18,  5,  0,  1, 10, 27],
       [38, 17,  4,  3,  2, 11, 28],
       [37, 16, 15, 14, 13, 12, 29],
       [36, 35, 34, 33, 32, 31, 30]])