Changing colours of an area in an image using opencv in python

Question

I have a picture were I want to change all white-ish pixels to grey, but only for a certain area of the image. Example picture, I just want to change the picture outside of the red rectangle, without changing the image within the red rectangle:

I already have the general code, which was part of someone elses Stackoverflow question, that changes the colour of every white pixel instead of only just the one outside of an area.

image = cv.imread("meme 2.jpg")
hsv = cv.cvtColor(image, cv.COLOR_BGR2HSV)

# Define lower and uppper limits of what we call "white-ish"
sensitivity = 19
lower_white = np.array([0, 0, 255 - sensitivity])
upper_white = np.array([255, sensitivity, 255])

# Mask image to only select white
mask = cv.inRange(hsv, lower_white, upper_white)

# Change image to grey where we found brown
image[mask > 0] = (170, 170, 170)
cv.imwrite(file, image)

user_7355741 · Answer

Here is one way to do that in Python/OpenCV.

Read the input
Convert to HSV color space
Threshold on desired color to make a mask
Use the mask to change the color of all corresponding pixels in the image
Draw a new rectangular mask for the region where you do not want to change
Invert the new mask for the region where you do want to change
Apply the new mask to the original image
Apply the inverted new mask to the color changed image
Add the two results together to form the final image
Save the results

Input:

import cv2
import numpy as np

# Read image
image = cv2.imread('4animals.jpg')

# Convert to HSV
hsv = cv2.cvtColor(image, cv2.COLOR_BGR2HSV)

# Define lower and uppper limits of what we call "white-ish"
sensitivity = 19
lower_white = np.array([0, 0, 255 - sensitivity])
upper_white = np.array([255, sensitivity, 255])

# Create mask to only select white
mask = cv2.inRange(hsv, lower_white, upper_white)

# Change image to grey where we found white
image2 = image.copy()
image2[mask > 0] = (170, 170, 170)

# Create new rectangular mask that is white on black background
x,y,w,h = 33,100,430,550
mask2 = np.zeros_like(image)
cv2.rectangle(mask2, (x,y), (x+w,y+h), (255, 255, 255), -1)

# invert mask
mask2_inv = 255 - mask2

# apply mask to image
image_masked = cv2.bitwise_and(image, mask2)

# apply inverted mask to image2
image2_masked = cv2.bitwise_and(image2, mask2_inv)

# add together
result = cv2.add(image_masked, image2_masked)

# save results
cv2.imwrite('4animals_mask.jpg', mask)
cv2.imwrite('4animals_modified.png', image2)
cv2.imwrite('4animals_mask2.jpg', mask2)
cv2.imwrite('4animals_mask2_inv.jpg', mask2_inv)
cv2.imwrite('4animals_masked.jpg', image_masked)
cv2.imwrite('4animals_modified_masked.jpg', image2_masked)
cv2.imwrite('4animals_result.jpg', result)

cv2.imshow('mask', mask)
cv2.imshow('image2', image2)
cv2.imshow('mask2', mask2 )
cv2.imshow('mask2_inv', mask2_inv)
cv2.imshow('image_masked', image_masked)
cv2.imshow('image2_masked', image2_masked)
cv2.imshow('result', result)
cv2.waitKey(0)
cv2.destroyAllWindows()

Color mask:

Rectangle mask:

Inverted rectangle mask:

Color changed image:

Masked input:

Masked color changed image:

Result:

user_7355741 · Answer

Here is another simpler method in Python/OpenCV. My previous answer was overly complicated.

Read the input
Convert to HSV color space
Create a mask image by color thresholding
Draw a black rectangle on the previous mask for where you do not want to change the color
Apply the new combined mask to the image to change the color in the desired region
Save the result

Input:

import cv2
import numpy as np

# Read image
image = cv2.imread('4animals.jpg')

# Convert to HSV
hsv = cv2.cvtColor(image, cv2.COLOR_BGR2HSV)

# Define lower and uppper limits of what we call "white-ish"
sensitivity = 19
lower_white = np.array([0, 0, 255 - sensitivity])
upper_white = np.array([255, sensitivity, 255])

# Create mask to only select white
mask = cv2.inRange(hsv, lower_white, upper_white)

# Draw new rectangular mask on old mask that is black inside the rectangle and white outside the rectangle
x,y,w,h = 33,100,430,550
mask2 = mask.copy()
cv2.rectangle(mask2, (x,y), (x+w,y+h), 0, -1)

# Change image to grey where we found white for combined mask
result = image.copy()
result[mask2 > 0] = (170, 170, 170)

# save results
cv2.imwrite('4animals_mask.jpg', mask)
cv2.imwrite('4animals_mask2.jpg', mask2)
cv2.imwrite('4animals_result.jpg', result)

cv2.imshow('mask', mask)
cv2.imshow('mask2', mask2 )
cv2.imshow('result', result)
cv2.waitKey(0)
cv2.destroyAllWindows()

Mask from color thresholding:

Modified mask with rectangle drawn over it:

Result:

Changing colours of an area in an image using opencv in python

2 Answers