How to overlay outline Image with transparent background and filling?

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I want to take one image, and overlay it as its outline only without background/filling. I have one image that is an outline in PNG format, that has had its background, as well as the contents within the outline removed, so that when opened, all is transparent except the outline, similar to this image:

enter image description here

However, when I open the image and try to overlay it in OpenCV, the background and area within the outline shows as all-white, showing the full rectangle of the image's dimensions and obscuring the background image.

However, what I want to do is the following, where only the outline is overlayed on the background image, like so:

enter image description here

Bonus points if you can help me with changing the color of the outline as well.

I don't want to deal with any blending with alphas, as I need the background to appear in full, and want the outline very clear.

blending computer-vision mask opencv python
1 Answers

In this special case, your image has some alpha channel you can use. Using Boolean array indexing, you can access all values 255 in the alpha channel. What's left to do, is setting up some region of interest (ROI) in the "background" image w.r.t. some position, and in that ROI, you again use Boolean array indexing to set all pixels to some color, i.e. red.

Here's some code:

import cv2

# Open overlay image, and its dimensions
overlay_img = cv2.imread('1W7HZ.png', cv2.IMREAD_UNCHANGED)
h, w = overlay_img.shape[:2]

# In this special case, take the alpha channel of the overlay image, and
# check for value 255; idx is a Boolean array
idx = overlay_img[:, :, 3] == 255

# Open image to work on
img = cv2.imread('path/to/your/image.jpg')

# Position for overlay image
top, left = (50, 50)

# Access region of interest with overlay image's dimensions at position
#   img[top:top+h, left:left+w]   and there, use Boolean array indexing
# to set the color to red (for example)
img[top:top+h, left:left+w, :][idx] = (0, 0, 255)

# Save image
cv2.imwrite('output.png', img)

That's the output for some random "background" image:

Output

For the general case, i.e. without a proper alpha channel, you could threshold the overlay image to set up a proper mask for the Boolean array indexing.

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System information
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Platform:    Windows-10-10.0.16299-SP0
Python:      3.8.5
OpenCV:      4.5.1
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user_110899320
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