Problem about Background transparent .png format OpenCV with Python

Question

I'm studying on OpenCV with Python. I tried to change a color of picture in PNG format, but I have got some problem with PNG background (The image has transparent background).

When I change it to grayscale, the background has changed to black -- my picture is not transparent anymore. What I desire is to keep the transparent background of picture.

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Original image:

My code:

img = cv2.imread('line.png',cv2.IMREAD_UNCHANGED)
cv2.imshow('line',img)
cv2.waitKey()

Output image:

Desired output:

The white color around the border image should be transparent. How I can do this?

Answer 1

First, let me mention that if you display an image with alpha transparency using cv2.imshow then the transparent areas are going to be black.

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Since your input image already contains an alpha channel, the solution is simple -- just reuse the alpha channel.

There's a slight problem -- even though PNG format allows to have grayscale with alpha channel, AFAIK there is no way to write such an image with OpenCV.

Therefore the solution is straightforward: Take the processed grayscale image, convert it back to BGR, add the original alpha channel, and save the result.

Since we're in Python, and therefore the image is represented as a numpy array, we can use array indexing to extract the channels we need. numpy.dstack allows us to add the alpha channel easily.

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Sample code:

import cv2
import numpy as np

src = cv2.imread('51IgH.png', cv2.IMREAD_UNCHANGED)

bgr = src[:,:,:3] # Channels 0..2
gray = cv2.cvtColor(bgr, cv2.COLOR_BGR2GRAY)

# Some sort of processing...

bgr = cv2.cvtColor(gray, cv2.COLOR_GRAY2BGR)
alpha = src[:,:,3] # Channel 3
result = np.dstack([bgr, alpha]) # Add the alpha channel

cv2.imwrite('51IgH_result.png', result)

Result:

Once more on different background, so you can see it's really transparent:

Answer 2

You have to add a fourth channel in your array so that the transparency information can be encoded in the alpha channel.

Here is a python code example:

import cv2
file_name = "source.png"

src = cv2.imread(file_name, cv2.IMREAD_UNCHANGED)

# Save the transparency channel alpha
*_, alpha = cv2.split(src)

gray_layer = cv2.cvtColor(src, cv2.COLOR_BGR2GRAY)

# ... Your image processing

# Duplicate the grayscale image to mimic the BGR image and finally add the transparency
dst = cv2.merge((gray_layer, gray_layer, gray_layer, alpha))
cv2.imwrite("result.png", dst)

Result for grey image:

Result for BGR image: