Learn OpenCV (Python): Basic image manipulations / Operations

System information:

• OS: Ubuntu 16.04
• OpenCV: 3.1.0

• Convert Array to Image

import numpy
import os
import cv2

random_byte_array = bytearray(os.urandom(120000))
# or random_byte_array = numpy.random.randint(0, 256, 120000)
flat_numpy_array = numpy.array(random_byte_array)

# reshape to an grayscale image with 300px in height, 400px in width
# which is a 2D array
gray_image = flat_numpy_array.reshape(300, 400)
cv2.imwrite('../data/random_gray.png', gray_image)

# reshape to an BGR image with  100px in heigth, 400px in width
# and 3 channels (BGR)
# which is a 3D array
bgr_image = flat_numpy_array.reshape(100, 400, 3)
cv2.imwrite('../data/random_bgr.png', bgr_image)

# *Note: this is only an example, the paths is not necessarily to be
# the same on your system

• Change pixel values in gray-scale image

import numpy as np
import cv2
import random
import datetime

# seed the current time for random every time program starts
random.seed(datetime.datetime.now())
# help code more readable
ESCAPE_KEY = 27

# read image and change to gray scale
img = cv2.imread('../../data/planet_glow.jpg', cv2.IMREAD_GRAYSCALE)
# img.shapre on a gray-scale image return (row, col)
row_count, column_count = img.shape

# change all pixels value to random value from 0-255
for i in xrange(row_count):
	for j in xrange(column_count):
		img[i, j] = random.randint(0, 255)

# show image with title "show"
cv2.imshow("show", img)

# pause the program
# if you remove this if block
# the window will close immediately after the program starts
# causing you see nothing
if cv2.waitKey() == ESCAPE_KEY:
	cv2.destroyAllWindows()

• Change pixel values in BGR image (in open cv, BGR = RGB, weird!)

import numpy as np
import cv2
import random
import datetime

# help code more readable
ESCAPE_KEY = 27
# seed every time start a new program
random.seed(datetime.datetime.now())

# read image
img = cv2.imread('../../data/planet_glow.jpg', cv2.IMREAD_COLOR)
# with color image, img.shape returns (row, column, channels)
row, column, channels = img.shape

# change all pixels to random value
for i in xrange(row):
	for j in xrange(column):
		B_random = random.randint(0, 255)
		G_random = random.randint(0, 255)
		R_random = random.randint(0, 255)
		img[i, j] = [B_random, G_random, R_random]

# show image
cv2.imshow("Image", img)

# pause program, to help you see the image
# press ESC key to exit
if cv2.waitKey() == ESCAPE_KEY:
	cv2.destroyAllWindows()

But changing pixel values by indexing is not an appropriate way to do, instead using item (to retrieve value) and itemset (to change pixel value) method.

import numpy as np
import cv2

ESCAPE_KEY = 27
# B, G, R = 0, 1, 2 respectively
B, G, R = 0, 1, 2 

# read image
img = cv2.imread('../../data/planet_glow.jpg', cv2.IMREAD_COLOR)
# img.shape return (row, col, channels)
row, col, channels = img.shape

# for example
# we want to set all B values of each pixel to 0
# itemset((i, j, channel), new_val), channel could be B, G, R (or 0, 1, 2)
for i in xrange(row):
	for j in xrange(col):
		new_val = 0
		img.itemset((i, j, B), new_val)

# show image
cv2.imshow("Tittle", img)

if cv2.waitKey() == ESCAPE_KEY:
	cv2.destroyAllWindows()

Here’s how you can change all pixel values without for loop, using numpy array syntax.

import cv2
import numpy as np
import random
import datetime

ESCAPE_KEYS = 27
random.seed(datetime.datetime.now())
B, G, R = 0, 1, 2

img = cv2.imread('../../data/planet_glow.jpg', cv2.IMREAD_COLOR)

# change all pixels in original image to a random color
# create a solid-random-color image
B_random = random.randint(0, 255)
G_random = random.randint(0, 255)
R_random = random.randint(0, 255)
img[:, :] = [B_random, G_random, R_random]

# or change only channel B to random value
img[:, :, B] = B_random
# or change channels B, G to random value
img[:, :, (B, G)] = (B_random, G_random)
# or change all three channels
# this have the same effect with line 17
img[:, :, (B, G, R)] = (B_random, G_random, R_random)
cv2.imshow("show", img)
if cv2.waitKey() == ESCAPE_KEYS:
	cd2.destroyAllWindows()

• Copy a portion of an image, and place it overlap to other portion

The code below copy all pixel values in a 100×100 portion, starting index (0,0) and place overlap it on portion with starting index (300, 300)

import numpy as np
import cv2

ESCAPE_KEY = 27

img = cv2.imread('../../data/planet_glow.jpg', cv2.IMREAD_COLOR)
# get 100x100 portion with starting index at (0, 0)
# and place it overlap portion with starting index at (300, 300)
my_roi = img[0:100, 0:100]
img[300:400, 300:400] = my_roi

cv2.imshow("show", img)
if cv2.waitKey() == ESCAPE_KEY:
	cv2.destroyAllWindows()

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