Simple approach to crop the margins of an image

kill_margins.py

import numpy as np
import cv2

# use the crop_image() function at the bottom. The rest are pretty much helpers.
# works well enough with William Blake illuminated books that I didn't have to 
# try anything more sophisticated

def read_image(path):
    return cv2.cvtColor(cv2.imread(path), cv2.COLOR_RGB2BGR)

def crop_line(img, x,y,w, h):
    return img[y:y+h, x:x+w]

def crop_horizontal_line(img, x, y, width, thickness=1):
    return crop_line(img, x, y, width, thickness)

def crop_vertical_line(img, x, y, height, thickness=1):
    return crop_line(img, x, y, thickness, height)

def crop_border(img, x1, y1, x2, y2, thickness=1):
    width = x2 - x1
    height = y2 - y1
    
    top = crop_horizontal_line(img, x1, y1, width, thickness=thickness)
    bottom = crop_horizontal_line(img, x1, y1 + height, width, thickness=thickness)
    left = crop_vertical_line(img, x1, y1, height, thickness=thickness)
    right = crop_vertical_line(img, x1 + width, y1, height, thickness=thickness)
    
    return {
        "top": top.std(),
        "bottom": bottom.std(),
        "left": left.std(),
        "right": right.std(),
        #"img": img[y1:y1+height, x1:x1+width]
    }

def find_image(img, thresh=20):
    w = img.shape[1]
    h = img.shape[0]
    
    x1 = int(w / 2) - 50
    x2 = int(w / 2) + 50
    
    y1 = int(h / 2) - 50
    y2 = int(h / 2) + 50
    
    
    while True:
        devs = crop_border(img, x1, y1, x2, y2)
        #print (x1, y1, x2, y2, w, h)
        #print (devs)
        changed = False
        if devs["top"] > thresh:
            if y1 - 1 >= 0:
                y1 -= 1
                changed = True
        if devs["bottom"] > thresh:
            if y2 + 1 < h:
                y2 = min(h, y2 + 1)
                changed = True
        if devs["left"] > thresh:
            if x1 - 1 >= 0:
                x1 = max(0, x1 - 1) 
                changed = True
        if devs["right"] > thresh:
            if x2 + 1 < w:
                x2 = min(w, x2 + 1)
                changed = True
        
        if not changed:
            break
        
        #print (x1,x2)
    
    return {"x":x1, "y":y1, "w":x2 - x1,"h": y2 - y1}
    
    
def crop_image(img, thresh=20):
    d = find_image(img, thresh=thresh)
    return img[d["y"]:d["y"]+d["h"],d["x"]:d["x"] + d["w"]]