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i have two images, i need to use numpy and opencv to overlay foreground on top of background using numpy masks.

import numpy as np
import cv2
import matplotlib.pyplot as plt

background = cv2.imread("background.jpg")
foreground = cv2.imread("foreground.png")
foreground = cv2.cvtColor(foreground ,cv2.COLOR_BGR2GRAY)
background = cv2.cvtColor(background ,cv2.COLOR_BGR2GRAY)

arr = []
for i in range(foreground.shape[1]): #parse forground pixels
    temp_row = []
    for j in range(foreground.shape[0]):
        if((foreground [i][j] == 0)):#if pixel transperant draw background
            temp_row.append(background[i][j])
        else: # draw forground
            temp_row.append(foreground[i][j])
    arr.append(temp_row)

res_im = np.array(arr)
plt.figure()
plt.imshow(res_im, cmap='gray', vmin=0, vmax=255)
plt.show()

i used this solution but was told i needed to use masks.. help?

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1 Answer 1

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8

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

  • Read the transparent foreground image
  • Read the background image
  • Extract the alpha channel from the foreground image
  • Extract the BGR channels from the foreground image
  • Composite them together using np.where conditional
  • Save the result

Front:

enter image description here

Back:

enter image description here

import cv2
import numpy as np

# read foreground image
img = cv2.imread('front.png', cv2.IMREAD_UNCHANGED)

# read background image
back = cv2.imread('back.png')

# extract alpha channel from foreground image as mask and make 3 channels
alpha = img[:,:,3]
alpha = cv2.merge([alpha,alpha,alpha])

# extract bgr channels from foreground image
front = img[:,:,0:3]

# blend the two images using the alpha channel as controlling mask
result = np.where(alpha==(0,0,0), back, front)

# save result
cv2.imwrite("front_back.png", result)

# show result
cv2.imshow("RESULT", result)
cv2.waitKey(0)

Result:

enter image description here

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