How to use skimage glcm routines to create a glcm image.

glcm_image.py

import numpy as np
import skimage
from skimage.feature import greycomatrix, greycoprops

def glcm_image(img, measure="dissimilarity"):
    """TODO: allow different window sizes by parameterizing 3, 4. Also should
    parameterize direction vector [1] [0]"""
    texture = np.zeros_like(sub)

    # quadratic looping in python w/o vectorized routine, yuck!
    for i in range(img.shape[0] ):  
        for j in range(sub.shape[1] ):  
          
            # don't calculate at edges
            if (i < 3) or \
               (i > (img.shape[0])) or \
               (j < 3) or \
               (j > (img.shape[0] - 4)):          
                continue  
        
            # calculate glcm matrix for 7 x 7 window, use dissimilarity (can swap in
            # contrast, etc.)
            glcm_window = img[i-3: i+4, j-3 : j+4]  
            glcm = greycomatrix(glcm_window, [1], [0],  symmetric = True, normed = True )   
            texture[i,j] = greycoprops(glcm, measure)  
    return texture

What is the value of sub of this code?

What is the value of (sub)?