overlay one part of image onto another image

Question

There are two corresponding images, the second one reflects the mask area of the first one.

How to over lay the red area in the second image onto the first image?

Answer 1

You can do it with OpenCV like this:

#!/usr/local/bin/python3

import numpy as np
import cv2

# Load base image and overlay
base = cv2.imread("image.jpg",   cv2.IMREAD_UNCHANGED)
over = cv2.imread("overlay.jpg", cv2.IMREAD_UNCHANGED)

# Anywhere the red channel of overlay image exceeds 127, make base image red
# Remember OpenCV uses BGR ordering, not RGB
base[over[...,2]>127] = [0,0,255]

# Save result
cv2.imwrite('result.jpg',base)

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If you wanted to blend a small percentage of red (say 20%) while retaining the structure of the underlying image, you could do this:

#!/usr/local/bin/python3

import numpy as np
import cv2

# Load base image and overlay
base = cv2.imread("image.jpg",   cv2.IMREAD_UNCHANGED)
over = cv2.imread("overlay.jpg", cv2.IMREAD_UNCHANGED)

# Blend 80% of the base layer with 20% red
blended = cv2.addWeighted(base,0.8,(np.zeros_like(base)+[0,0,255]).astype(np.uint8),0.2,0)

# Anywhere the red channel of overlay image exceeds 127, use blended image, elsewhere use base
result = np.where((over[...,2]>127)[...,None], blended, base)

# Save result
cv2.imwrite('result.jpg',result)

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By the way, you don't actually need any Python, you can just do it in Terminal with ImageMagick like this:

magick image.jpg \( overlay.jpg -fuzz 30% -transparent blue \) -composite result.png

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Keywords: Python, image processing, overlay, mask.