Simple python script to generate polar checked stimuli patterns.

gen_polar_checks.py

import numpy as np
from scipy.misc import imsave

PIXELS_X = 256
PIXELS_Y = PIXELS_X

COLOR_SOLID_WHITE = (255,255,255,255)
COLOR_SOLID_BLACK = (0,0,0,255)
COLOR_FULLY_TRANSPARENT_BLACK = (0,0,0,0)

def draw_sector(A,r1,r2, a1, a2, 
                color = COLOR_SOLID_WHITE,
                ):
    xdim = A.shape[0]
    ydim = A.shape[1]
    x0 = xdim/2.0
    y0 = ydim/2.0
    x11 = r1*np.cos(a1) + x0
    y11 = r1*np.sin(a1) + y0
    x12 = r1*np.cos(a2) + x0
    y12 = r1*np.sin(a2) + y0
    x21 = r2*np.cos(a1) + x0
    y21 = r2*np.sin(a1) + y0
    x22 = r2*np.cos(a2) + x0
    y22 = r2*np.sin(a2) + y0
    xmin = int(min(x11,x12,x21,x22) - 0.5)
    xmax = int(max(x11,x12,x21,x22) + 0.5)
    ymin = int(min(y11,y12,y21,y22) - 0.5)
    ymax = int(max(y11,y12,y21,y22) + 0.5)
    for x in range(xmin,xmax):
        for y in range(ymin,ymax):
            x_c = x - x0
            y_c = y - y0
            a_c = np.arctan2(y_c,x_c)
            d_sq = x_c*x_c + y_c*y_c
            if d_sq >= r1*r1 and\
               d_sq <= r2*r2 and\
               a_c >= a1     and\
               a_c <= a2:
                #solid
                A[x,y,0] = color[0]
                A[x,y,1] = color[1]
                A[x,y,2] = color[2]
                A[x,y,3] = color[3]

if __name__ == "__main__":
    S = np.zeros((PIXELS_X, PIXELS_Y, 4), dtype=np.uint8)
    R_COUNT = 20
    Rsectors = np.linspace(0,1.0,R_COUNT)*PIXELS_X/2.0
    A_COUNT = 51
    Asectors = np.linspace(-np.pi,np.pi,A_COUNT)
    c1 = COLOR_SOLID_WHITE
    c2 = COLOR_SOLID_BLACK
    for i in range(len(Rsectors)-1):
        r1 = Rsectors[i]
        r2 = Rsectors[i+1]
        for j in range(len(Asectors)-1):
            a1 = Asectors[j]
            a2 = Asectors[j+1]
            draw_sector(S,r1=r1,r2=r2,a1=a1,a2=a2, color = c1)
            c1, c2 = (c2, c1) #swap color
        c1, c2 = (c2, c1) #swap color

    imsave("./stim.png", S)