Theoretical implementation of Erosion and Dilation using OpenCV

erode_dilate.py

__author__ = "K.M. Tahsin Hassan Rahit"
__email__ = "tahsin.rahit@gmail.com"

import cv2
import numpy as np

def dilate(img, struct_element, origin):
	h, w = img.shape
	result = img[:,:]
	add_y = struct_element.shape[0] - origin[0]
	add_x = struct_element.shape[1] - origin[1]
	for i in range (h):
		for j in range (w):
			if bool(img[i][j]) and bool(struct_element[origin[0]][origin[1]]):
				x1 = j-origin[1]
				x2 = j+add_x
				y1 = i-origin[0]
				y2 = i+add_y
				# structure modification
				s_x1 = 0
				s_x2 = struct_element.shape[1]
				s_y1 = 0
				s_y2 = struct_element.shape[0]

				if x1 < 0:
					s_x1 = s_x1 + (0 - x1)
					x1 = 0
				if x2 > w:
					s_x2 = s_x2 - (x2 - (w-1))
					x2 = w-1
				if y1 < 0:
					s_y1 = s_y1 + (0 - y1)
					y1 = 0
				if y2 > h-1:
					s_y2 = s_y2 - (y2 - (h-1))
					y2 = h-1

				window = img[y1:y2, x1:x2] | struct_element[s_y1:s_y2, s_x1:s_x2]
				result[y1:y2, x1:x2] = window
	return result


def erode(img, struct_element, origin):
	h, w = img.shape
	result = img[:,:]
	add_y = struct_element.shape[0] - origin[0] -1
	add_x = struct_element.shape[1] - origin[1] -1

	for i in range (h):
		for j in range (w):
			if bool(img[i][j]) and bool(struct_element[origin[0]][origin[1]]):
				x1 = j-origin[1]
				x2 = j+add_x
				y1 = i-origin[0]
				y2 = i+add_y
				# structure modification
				s_x1 = 0
				s_x2 = struct_element.shape[1]
				s_y1 = 0
				s_y2 = struct_element.shape[0]

				if x1 < 0 or x2 > w-1 or y1 < 0 or y2 > h-1:
					result[i][j] = 0
				else:
					window = img[y1:y2, x1:x2]
					zero_flag = False
					for x in range(window.shape[0]):
						for y in range(window.shape[1]):
							if img[i][j] is 0:
								zero_flag = True
					if zero_flag:
						result[i][j] = 0		
	return result


# To Dilate:
img = np.array([
		[0,0,0,0],
		[0,1,1,0],
		[0,0,0,0]
	])
print("Image:")
print(img)
struct_element = np.array([
		[1, 0],
		[1, 1],
	])
print("Structure Element:")
print(struct_element)
origin = (1,0)	# (y,x) or origin
print("After Dilation:")
print(dilate(img, struct_element, origin))

# To Erode:
img =  np.array([
		[0,0,1,1,0],
		[0,0,1,1,0],
		[0,0,1,1,0],
		[1,1,1,1,1]
	])
struct_element =  np.array([[1],[1],[1]])
print("Image:")
print(img)
print("Structure Element:")
print(struct_element)
print("After Erosion:")
print(erode(img, struct_element, origin))